wdiff rfc9421v1.txt rfc9421.txt

Internet Engineering Task Force (IETF) A. Backman, Ed.
Request for Comments: 9421 Amazon
Category: Standards Track J. Richer, Ed.
ISSN: 2070-1721 Bespoke Engineering
M. Sporny
Digital Bazaar
January
February 2024

HTTP Message Signatures

Abstract

This document describes a mechanism for creating, encoding, and
verifying digital signatures or message authentication codes over
components of an HTTP message. This mechanism supports use cases
where the full HTTP message may not be known to the signer and where
the message may be transformed (e.g., by intermediaries) before
reaching the verifier. This document also describes a means for
requesting that a signature be applied to a subsequent HTTP message
in an ongoing HTTP exchange.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9421.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.

Table of Contents

1. Introduction
1.1. Conventions and Terminology
1.2. Requirements
1.3. HTTP Message Transformations
1.4. Application of HTTP Message Signatures
2. HTTP Message Components
2.1. HTTP Fields
2.1.1. Strict Serialization of HTTP Structured Fields
2.1.2. Dictionary Structured Field Members
2.1.3. Binary-Wrapped HTTP Fields
2.1.4. Trailer Fields
2.2. Derived Components
2.2.1. Method
2.2.2. Target URI
2.2.3. Authority
2.2.4. Scheme
2.2.5. Request Target
2.2.6. Path
2.2.7. Query
2.2.8. Query Parameters
2.2.9. Status Code
2.3. Signature Parameters
2.4. Signing Request Components in a Response Message
2.5. Creating the Signature Base
3. HTTP Message Signatures
3.1. Creating a Signature
3.2. Verifying a Signature
3.2.1. Enforcing Application Requirements
3.3. Signature Algorithms
3.3.1. RSASSA-PSS Using SHA-512
3.3.2. RSASSA-PKCS1-v1_5 Using SHA-256
3.3.3. HMAC Using SHA-256
3.3.4. ECDSA Using Curve P-256 DSS and SHA-256
3.3.5. ECDSA Using Curve P-384 DSS and SHA-384
3.3.6. EdDSA Using Curve edwards25519
3.3.7. JSON Web Signature (JWS) Algorithms
4. Including a Message Signature in a Message
4.1. The Signature-Input HTTP Field
4.2. The Signature HTTP Field
4.3. Multiple Signatures
5. Requesting Signatures
5.1. The Accept-Signature Field
5.2. Processing an Accept-Signature
6. IANA Considerations
6.1. HTTP Field Name Registration
6.2. HTTP Signature Algorithms Registry
6.2.1. Registration Template
6.2.2. Initial Contents
6.3. HTTP Signature Metadata Parameters Registry
6.3.1. Registration Template
6.3.2. Initial Contents
6.4. HTTP Signature Derived Component Names Registry
6.4.1. Registration Template
6.4.2. Initial Contents
6.5. HTTP Signature Component Parameters Registry
6.5.1. Registration Template
6.5.2. Initial Contents
7. Security Considerations
7.1. General Considerations
7.1.1. Skipping Signature Verification
7.1.2. Use of TLS
7.2. Message Processing and Selection
7.2.1. Insufficient Coverage
7.2.2. Signature Replay
7.2.3. Choosing Message Components
7.2.4. Choosing Signature Parameters and Derived Components
over HTTP Fields
7.2.5. Signature Labels
7.2.6. Multiple Signature Confusion
7.2.7. Collision of Application-Specific Signature Tag
7.2.8. Message Content
7.3. Cryptographic Considerations
7.3.1. Cryptography and Signature Collision
7.3.2. Key Theft
7.3.3. Symmetric Cryptography
7.3.4. Key Specification Mixup
7.3.5. Non-deterministic Signature Primitives
7.3.6. Key and Algorithm Specification Downgrades
7.3.7. Signing Signature Values
7.4. Matching Signature Parameters to the Target Message
7.4.1. Modification of Required Message Parameters
7.4.2. Matching Values of Covered Components to Values in the
Target Message
7.4.3. Message Component Source and Context
7.4.4. Multiple Message Component Contexts
7.5. HTTP Processing
7.5.1. Processing Invalid HTTP Field Names as Derived
Component Names
7.5.2. Semantically Equivalent Field Values
7.5.3. Parsing Structured Field Values
7.5.4. HTTP Versions and Component Ambiguity
7.5.5. Canonicalization Attacks
7.5.6. Non-List Field Values
7.5.7. Padding Attacks with Multiple Field Values
7.5.8. Ambiguous Handling of Query Elements
8. Privacy Considerations
8.1. Identification through Keys
8.2. Signatures do not provide confidentiality
8.3. Oracles
8.4. Required Content
9. References
9.1. Normative References
9.2. Informative References
Appendix A. Detecting HTTP Message Signatures
Appendix B. Examples
B.1. Example Keys
B.1.1. Example Key RSA Test Key
B.1.2. Example RSA-PSS Key
B.1.3. Example ECC P-256 Test Key
B.1.4. Example Ed25519 Test Key
B.1.5. Example Shared Secret
B.2. Test Cases
B.2.1. Minimal Signature Using rsa-pss-sha512
B.2.2. Selective Covered Components Using rsa-pss-sha512
B.2.3. Full Coverage Using rsa-pss-sha512
B.2.4. Signing a Response Using ecdsa-p256-sha256
B.2.5. Signing a Request Using hmac-sha256
B.2.6. Signing a Request Using ed25519
B.3. TLS-Terminating Proxies
B.4. HTTP Message Transformations
Acknowledgements
Authors' Addresses

1. Introduction

Message integrity and authenticity are security properties that are
critical to the secure operation of many HTTP applications.
Application developers typically rely on the transport layer to
provide these properties, by operating their application over TLS
[TLS]. However, TLS only guarantees these properties over a single
TLS connection, and the path between the client and application may
be composed of multiple independent TLS connections (for example, if
the application is hosted behind a TLS-terminating gateway or if the
client is behind a TLS Inspection appliance). In such cases, TLS
cannot guarantee end-to-end message integrity or authenticity between
the client and application. Additionally, some operating
environments present obstacles that make it impractical to use TLS
(such as the presentation of client certificates from a browser) or
to use features necessary to provide message authenticity.
Furthermore, some applications require the binding of a higher-level
application-specific key to the HTTP message, separate from any TLS
certificates in use. Consequently, while TLS can meet message
integrity and authenticity needs for many HTTP-based applications, it
is not a universal solution.

Additionally, many applications need to be able to generate and
verify signatures despite incomplete knowledge of the HTTP message as
seen on the wire, due to the use of libraries, proxies, or
application frameworks that alter or hide portions of the message
from the application at the time of signing or verification. These
applications need a means to protect the parts of the message that
are most relevant to the application without having to violate
layering and abstraction.

Finally, object-based signature mechanisms such as JSON Web Signature
[JWS] require the intact conveyance of the exact information that was
signed. When applying such technologies to an HTTP message, elements
of the HTTP message need to be duplicated in the object payload
either directly or through the inclusion of a hash. This practice
introduces complexity, since the repeated information needs to be
carefully checked for consistency when the signature is verified.

This document defines a mechanism for providing end-to-end integrity
and authenticity for components of an HTTP message by using a
detached signature on HTTP messages. The mechanism allows
applications to create digital signatures or message authentication
codes (MACs) over only the components of the message that are
meaningful and appropriate for the application. Strict
canonicalization rules ensure that the verifier can verify the
signature even if the message has been transformed in many of the
ways permitted by HTTP.

The signing mechanism described in this document consists of three
parts:

* A common nomenclature and canonicalization rule set for the
different protocol elements and other components of HTTP messages,
used to create the signature base (Section 2).

* Algorithms for generating and verifying signatures over HTTP
message components using this signature base through the
application of cryptographic primitives (Section 3).

* A mechanism for attaching a signature and related metadata to an
HTTP message and for parsing attached signatures and metadata from
HTTP messages. To facilitate this, this document defines the
"Signature-Input" and "Signature" fields (Section 4).

This document also provides a mechanism for negotiating the use of
signatures in one or more subsequent messages via the "Accept-
Signature" field (Section 5). This optional negotiation mechanism
can be used along with opportunistic or application-driven message
signatures by either party.

The mechanisms defined in this document are important tools that can
be used to build an overall security mechanism for an application.
This toolkit provides some powerful capabilities but is not
sufficient in creating an overall security story. In particular, the
requirements listed in Section 1.4 and the security considerations
discussed in Section 7 are of high importance to all implementors of
this specification. For example, this specification does not define
a means to directly cover HTTP message content (defined in
Section 6.4 of [HTTP]); rather, it relies on the Digest specification
[DIGEST] to provide a hash of the message content, as discussed in
Section 7.2.8.

1.1. Conventions and Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

The terms "HTTP message", "HTTP request", "HTTP response", "target
URI", "gateway", "header field", "intermediary", "request target",
"trailer field", "sender", "method", and "recipient" are used as
defined in [HTTP].

For brevity, the term "signature" on its own is used in this document
to refer to both digital signatures (which use asymmetric
cryptography) and keyed MACs (which use symmetric cryptography).
Similarly, the verb "sign" refers to the generation of either a
digital signature or keyed MAC over a given signature base. The
qualified term "digital signature" refers specifically to the output
of an asymmetric cryptographic signing operation.

This document uses the following terminology from Section 3 of
[STRUCTURED-FIELDS] to specify data types: List, Inner List,
Dictionary, Item, String, Integer, Byte Sequence, and Boolean.

This document defines several string constructions using ABNF [ABNF]
and uses the following ABNF rules: VCHAR, SP, DQUOTE, and LF. This
document uses the following ABNF rules from [STRUCTURED-FIELDS]: sf-
string, inner-list, and parameters. This document uses the following
ABNF rules from [HTTP] and [HTTP/1.1]: field-content, obs-fold, and
obs-text.

In addition to those listed above, this document uses the following
terms:

HTTP Message Signature:
A digital signature or keyed MAC that covers one or more portions
of an HTTP message. Note that a given HTTP Message message can contain
multiple HTTP Message Signatures. message signatures.

Signer:
The entity that is generating or has generated an HTTP Message
Signature. message
signature. Note that multiple entities can act as signers and
apply separate HTTP Message Signatures message signatures to a given HTTP Message. message.

Verifier:
An entity that is verifying or has verified an HTTP Message
Signature message
signature against an HTTP Message. message. Note that an HTTP Message
Signature message
signature may be verified multiple times, potentially by different
entities.

HTTP Message Component:
A portion of an HTTP message that is capable of being covered by
an HTTP Message Signature. message signature.

Derived Component:
An HTTP Message Component message component derived from the HTTP message through
the use of a specified algorithm or process. See Section 2.2.

HTTP Message Component Name:
A string String that identifies an HTTP Message Component's message component's source, such
as a field name or derived component name.

HTTP Message Component Identifier:
The combination of an HTTP Message Component Name message component name and any
parameters. This combination uniquely identifies a specific HTTP
Message Component
message component with respect to a particular HTTP Message
Signature message
signature and the HTTP Message message it applies to.

HTTP Message Component Value:
The value associated with a given component identifier within the
context of a particular HTTP Message. message. Component values are
derived from the HTTP Message message and are usually subject to a
canonicalization process.

Covered Components:
An ordered set of HTTP message component identifiers for fields
(Section 2.1) and derived components (Section 2.2) that indicates
the set of message components covered by the signature, never
including the @signature-params identifier itself. The order of
this set is preserved and communicated between the signer and
verifier to facilitate reconstruction of the signature base.

Signature Base:
The sequence of bytes generated by the signer and verifier using
the covered components set and the HTTP Message. message. The signature
base is processed by the cryptographic algorithm to produce or
verify the HTTP Message Signature. message signature.

HTTP Message Signature Algorithm:
A cryptographic algorithm that describes the signing and
verification process for the signature, defined in terms of the
HTTP_SIGN and HTTP_VERIFY primitives described in Section 3.3.

Key Material:
The key material required to create or verify the signature. The
key material is often identified with an explicit key identifier,
allowing the signer to indicate to the verifier which key was
used.

Creation Time:
A timestamp representing the point in time that the signature was
generated, as asserted by the signer.

Expiration Time:
A timestamp representing the point in time after which the
signature should no longer be accepted by the verifier, as
asserted by the signer.

Target Message:
The HTTP message to which an HTTP Message Signature message signature is applied.

Signature Context:
The data source from which the HTTP Message Component Values message component values are
drawn. The context includes the target message and any additional
information the signer or verifier might have, such as the full
target URI of a request or the related request message for a
response.

The term "UNIX timestamp" refers to what Section 4.16 of [POSIX.1]
calls "seconds since the Epoch".

This document contains non-normative examples of partial and complete
HTTP messages. Some examples use a single trailing backslash (\) to
indicate line wrapping for long values, as per [RFC8792]. The \
character and leading spaces on wrapped lines are not part of the
value.

1.2. Requirements

HTTP permits, and sometimes requires, intermediaries to transform
messages in a variety of ways. This can result in a recipient
receiving a message that is not bitwise-equivalent to the message
that was originally sent. In such a case, the recipient will be
unable to verify integrity protections over the raw bytes of the
sender's HTTP message, as verifying digital signatures or MACs
requires both signer and verifier to have the exact same signature
base. Since the exact raw bytes of the message cannot be relied upon
as a reliable source for a signature base, the signer and verifier
have to independently create the signature base from their respective
versions of the message, via a mechanism that is resilient to safe
changes that do not alter the meaning of the message.

For a variety of reasons, it is impractical to strictly define what
constitutes a safe change versus an unsafe one. Applications use
HTTP in a wide variety of ways and may disagree on whether a
particular piece of information in a message (e.g., the message
content, the method, or a particular header field) is relevant.
Thus, a general-purpose solution needs to provide signers with some
degree of control over which message components are signed.

HTTP applications may be running in environments that do not provide
complete access to or control over HTTP messages (such as a web
browser's JavaScript environment) or may be using libraries that
abstract away the details of the protocol (such as the Java HTTP
Client (HttpClient) library
(https://openjdk.java.net/groups/net/httpclient/intro.html)). These
applications need to be able to generate and verify signatures
despite incomplete knowledge of the HTTP message.

1.3. HTTP Message Transformations

As mentioned earlier, HTTP explicitly permits, and in some cases
requires, implementations to transform messages in a variety of ways.
Implementations are required to tolerate many of these
transformations. What follows is a non-normative and non-exhaustive
list of transformations that could occur under HTTP, provided as
context:

* Reordering of fields with different field names (Section 5.3 of
[HTTP]).

* A combination Combination of fields with the same field name (Section 5.2 of
[HTTP]).

* Removal of fields listed in the Connection header field
(Section 7.6.1 of [HTTP]).

* Addition of fields that indicate control options (Section 7.6.1 of
[HTTP]).

* Addition or removal of a transfer coding (Section 7.7 of [HTTP]).

* Addition of fields such as Via (Section 7.6.3 of [HTTP]) and
Forwarded (Section 4 of [RFC7239]).

* Conversion between different versions of HTTP (e.g., HTTP/1.x to
HTTP/2, or vice versa).

* Changes in case (e.g., "Origin" to "origin") of any case-
insensitive components such as field names, request URI scheme, or
host.

* Changes to the request target and authority that, when applied
together, do not result in a change to the message's target URI,
as defined in Section 7.1 of [HTTP].

Additionally, there are some transformations that are either
deprecated or otherwise not allowed but that could still occur in the
wild. These transformations can still be handled without breaking
the signature; they include such actions as:

* Use, addition, or removal of leading or trailing whitespace in a
field value.

* Use, addition, or removal of obs-fold in field values (Section 5.2
of [HTTP/1.1]).

We can identify these types of transformations as transformations
that should not prevent signature verification, even when performed
on message components covered by the signature. Additionally, all
changes to components not covered by the signature should not prevent
signature verification.

Some examples of these kinds of transformations, and the effect they
have on the message signature, are found in Appendix B.4.

Other transformations, such as parsing and reserializing the field
values of a covered component or changing the value of a derived
component, can cause a signature to no longer validate against a
target message. Applications of this specification need to take care
to ensure that the transformations expected by the application are
adequately handled by the choice of covered components.

1.4. Application of HTTP Message Signatures

HTTP Message Signatures message signatures are designed to be a general-purpose tool
applicable in a wide variety of circumstances and applications. In
order to properly and safely apply HTTP Message Signatures, message signatures, an
application or profile of this specification MUST specify, at a
minimum, all of the following items:

* The set of component identifiers (Section 2) and signature
parameters (Section 2.3) that are expected and required to be
included in the covered components list. For example, an
authorization protocol could mandate that the Authorization field
be covered to protect the authorization credentials and mandate
that the signature parameters contain a created parameter
(Section 2.3), while an API expecting semantically relevant HTTP
message content could require the Content-Digest field defined in
[DIGEST] to be present and covered as well as mandate a value for
the tag parameter (Section 2.3) that is specific to the API being
protected.

* The expected Structured Field types [STRUCTURED-FIELDS] of any
required or expected covered component fields or parameters.

* A means of retrieving the key material used to verify the
signature. An application will usually use the keyid parameter of
the signature parameters (Section 2.3) and define rules for
resolving a key from there, though the appropriate key could be
known from other means such as preregistration of a signer's key.

* The set of allowable signature algorithms to be used by signers
and accepted by verifiers.

* A means of determining that the signature algorithm used to verify
the signature is appropriate for the key material and context of
the message. For example, the process could use the alg parameter
of the signature parameters (Section 2.3) to state the algorithm
explicitly, derive the algorithm from the key material, or use
some preconfigured algorithm agreed upon by the signer and
verifier.

* A means of determining that a given key and algorithm used for a
signature are appropriate for the context of the message. For
example, a server expecting only ECDSA signatures should know to
reject any RSA signatures, or a server expecting asymmetric
cryptography should know to reject any symmetric cryptography.

* A means of determining the context for derivation of message
components from an HTTP message and its application context.
While this is normally the target HTTP message itself, the context
could include additional information known to the application
through configuration, such as an external hostname.

* If binding between a request and response is needed using the
mechanism provided in Section 2.4, all elements of the request
message and the response message that would be required to provide
properties of such a binding.

* The error messages and codes that are returned from the verifier
to the signer when the signature is invalid, the key material is
inappropriate, the validity time window is out of specification, a
component value cannot be calculated, or any other errors occur
during the signature verification process. For example, if a
signature is being used as an authentication mechanism, an HTTP
status code of 401 (Unauthorized) or 403 (Forbidden) could be
appropriate. If the response is from an HTTP API, a response with
an HTTP status code such as 400 (Bad Request) could include more
details [RFC7807] [RFC9457], such as an indicator that the wrong
key material was used.

When choosing these parameters, an application of HTTP message
signatures has to ensure that the verifier will have access to all
required information needed to recreate the signature base. For
example, a server behind a reverse proxy would need to know the
original request URI to make use of the derived component @target-
uri, even though the apparent target URI would be changed by the
reverse proxy (see also Section 7.4.3). Additionally, an application
using signatures in responses would need to ensure that clients
receiving signed responses have access to all the signed portions of
the message, including any portions of the request that were signed
by the server using the req ("request-response") parameter
(Section 2.4).

Details regarding this kind of profiling are within the purview of
the application and outside the scope of this specification; however,
some additional considerations are discussed in Section 7. In
particular, when choosing the required set of component identifiers,
care has to be taken to make sure that the coverage is sufficient for
the application, as discussed in Sections 7.2.1 and 7.2.8. This
specification defines only part of a full security system for an
application. When building a complete security system based on this
tool, it is important to perform a security analysis of the entire
system, of which HTTP Message Signatures message signatures is a part. Historical
systems, such as AWS Signature Version 4 [AWS-SIGv4], can provide
inspiration and examples of how to apply similar mechanisms to an
application, though review of such historical systems does not negate
the need for a security analysis of an application of HTTP Message
Signatures. message
signatures.

2. HTTP Message Components

In order to allow signers and verifiers to establish which components
are covered by a signature, this document defines component
identifiers for components covered by an HTTP Message Signature, message signature, a
set of rules for deriving and canonicalizing the values associated
with these component identifiers from the HTTP Message, message, and the means
for combining these canonicalized values into a signature base.

The signature context for deriving these values MUST be accessible to
both the signer and the verifier of the message. The context MUST be
the same across all components in a given signature. For example, it
would be an error to use the raw query string for the @query derived
component but combined query and form parameters for the @query-param
derived component. For more considerations regarding the message
component context, see Section 7.4.3.

A component identifier is composed of a component name and any
parameters associated with that name. Each component name is either
an HTTP field name (Section 2.1) or a registered derived component
name (Section 2.2). The possible parameters for a component
identifier are dependent on the component identifier. The "HTTP
Signature Component Parameters" registry, which catalogs all possible
parameters, is defined in Section 6.5.

Within a single list of covered components, each component identifier
MUST occur only once. One component identifier is distinct from
another if the component name differs or if any of the parameters
differ for the same component name. Multiple component identifiers
having the same component name MAY be included if they have
parameters that make them distinct, such as "foo";bar and "foo";baz.
The order of parameters MUST be preserved when processing a component
identifier (such as when parsing during verification), but the order
of parameters is not significant when comparing two component
identifiers for equality checks. That is to say, "foo";bar;baz
cannot be in the same message as "foo";baz;bar, since these two
component identifiers are equivalent, but a system processing one
form is not allowed to transform it into the other form.

The component value associated with a component identifier is defined
by the identifier itself. Component values MUST NOT contain newline
(\n) characters. Some HTTP message components can undergo
transformations that change the bitwise value without altering the
meaning of the component's value (for example, when combining field
values). Message component values therefore need to be canonicalized
before they are signed, to ensure that a signature can be verified
despite such intermediary transformations. This document defines
rules for each component identifier that transform the identifier's
associated component value into such a canonical form.

The following sections define component identifier names, their
parameters, their associated values, and the canonicalization rules
for their values. The method for combining message components into
the signature base is defined in Section 2.5.

2.1. HTTP Fields

The component name for an HTTP field is the lowercased form of its
field name as defined in Section 5.1 of [HTTP]. While HTTP field
names are case insensitive, implementations MUST use lowercased field
names (e.g., content-type, date, etag) when using them as component
names.

The component value for an HTTP field is the field value for the
named field as defined in Section 5.5 of [HTTP]. The field value
MUST be taken from the named header field of the target message
unless this behavior is overridden by additional parameters and
rules, such as the req and tr flags, below. For most fields, the
field value is an ASCII string as recommended by [HTTP], and the
component value is exactly that string. Other encodings could exist
in some implementations, and all non-ASCII field values MUST be
encoded to ASCII before being added to the signature base. The bs
parameter, as described in Section 2.1.3, provides a method for
wrapping such problematic field values.

Unless overridden by additional parameters and rules, HTTP field
values MUST be combined into a single value as defined in Section 5.2
of [HTTP] to create the component value. Specifically, HTTP fields
sent as multiple fields MUST be combined by concatenating the values
using a single comma and a single space as a separator ("," + " ").
Note that intermediaries are allowed to combine values of HTTP fields
with any amount of whitespace between the commas, and if this
behavior is not accounted for by the verifier, the signature can
fail, since the signer and verifier will see a different component
value in their respective signature bases. For robustness, it is
RECOMMENDED that signed messages include only a single instance of
any field covered under the signature, particularly with the value
for any list-based fields serialized using the algorithm below. This
approach increases the chances of the field value remaining untouched
through intermediaries. Where that approach is not possible and
multiple instances of a field need to be sent separately, it is
RECOMMENDED that signers and verifiers process any list-based fields
taking all individual field values and combining them based on the
strict algorithm below, to counter possible intermediary behavior.
When the field in question is a Structured Field of type List or
Dictionary, this effect can be accomplished more directly by
requiring the strict Structured Field serialization of the field
value, as described in Section 2.1.1.

Note that some HTTP fields, such as Set-Cookie [COOKIE], do not
follow a syntax that allows for the combination of field values in
this manner (such that the combined output is unambiguous from
multiple inputs). Even though the component value is never parsed by
the message signature process and is used only as part of the
signature base (Section 2.5), caution needs to be taken when
including such fields in signatures, since the combined value could
be ambiguous. The bs parameter, as described in Section 2.1.3,
provides a method for wrapping such problematic fields. See
Section 7.5.6 for more discussion regarding this issue.

If the correctly combined value is not directly available for a given
field by an implementation, the following algorithm will produce
canonicalized results for list-based fields:

1. Create an ordered list of the field values of each instance of
the field in the message, in the order they occur (or will occur)
in the message.

2. Strip leading and trailing whitespace from each item in the list.
Note that since HTTP field values are not allowed to contain
leading and trailing whitespace, this would be a no-op in a
compliant implementation.

3. Remove any obsolete line folding within the line, and replace it
with a single space (" "), as discussed in Section 5.2 of
[HTTP/1.1]. Note that this behavior is specific to HTTP/1.1 and
does not apply to other versions of the HTTP specification, which
do not allow internal line folding.

4. Concatenate the list of values with a single comma (",") and a
single space (" ") between each item.

The resulting string is the component value for the field.

Note that some HTTP fields have values with multiple valid
serializations that have equivalent semantics, such as allowing case-
insensitive values that intermediaries could change. Applications
signing and processing such fields MUST consider how to handle the
values of such fields to ensure that the signer and verifier can
derive the same value, as discussed in Section 7.5.2.

The following are non-normative examples of component values for
header fields, given the following example HTTP message fragment:

Host: www.example.com
Date: Tue, 20 Apr 2021 02:07:56 GMT
X-OWS-Header: Leading and trailing whitespace.
X-Obs-Fold-Header: Obsolete
line folding.
Cache-Control: max-age=60
Cache-Control: must-revalidate
Example-Dict: a=1, b=2;x=1;y=2, c=(a b c)

The following example shows the component values for these example
header fields, presented using the signature base format defined in
Section 2.5:

"host": www.example.com
"date": Tue, 20 Apr 2021 02:07:56 GMT
"x-ows-header": Leading and trailing whitespace.
"x-obs-fold-header": Obsolete line folding.
"cache-control": max-age=60, must-revalidate
"example-dict": a=1, b=2;x=1;y=2, c=(a b c)

Empty HTTP fields can also be signed when present in a message. The
canonicalized value is the empty string. This means that the
following empty header, header field, with (SP) indicating a single trailing
space character before the empty field value:

X-Empty-Header:(SP)

is serialized by the signature base generation algorithm
(Section 2.5) with an empty string value following the colon and
space added after the content component identifier.

"x-empty-header":(SP)

Any HTTP field component identifiers MAY have the following
parameters in specific circumstances, each described in detail in
their own sections:

sf A boolean Boolean flag indicating that the component value is serialized
using strict encoding of the Structured Field value
(Section 2.1.1).

key A string String parameter used to select a single member value from a
Dictionary Structured Field (Section 2.1.2).

bs A boolean Boolean flag indicating that individual field values are
encoded using Byte Sequence data structures before being combined
into the component value (Section 2.1.3).

req A boolean Boolean flag for signed responses indicating that the
component value is derived from the request that triggered this
response message and not from the response message directly. Note
that this parameter can also be applied to any derived component
identifiers that target the request (Section 2.4).

tr A boolean Boolean flag indicating that the field value is taken from the
trailers of the message as defined in Section 6.5 of [HTTP]. If
this flag is absent, the field value is taken from the headers header
fields of the message as defined in Section 6.3 of [HTTP]
(Section 2.1.4).

Multiple parameters MAY be specified together, though some
combinations are redundant or incompatible. For example, the sf
parameter's functionality is already covered when the key parameter
is used on a Dictionary item, since key requires strict serialization
of the value. The bs parameter, which requires the raw bytes of the
field values from the message, is not compatible with the use of the
sf or key parameters, which require the parsed data structures of the
field values after combination.

Additional parameters can be defined in the "HTTP Signature Component
Parameters" registry established in Section 6.5.

2.1.1. Strict Serialization of HTTP Structured Fields

If the value of an HTTP field is known by the application to be a
Structured Field type (as defined in [STRUCTURED-FIELDS] or its
extensions or updates) and the expected type of the Structured Field
is known, the signer MAY include the sf parameter in the component
identifier. If this parameter is included with a component
identifier, the HTTP field value MUST be serialized using the formal
serialization rules specified in Section 4 of [STRUCTURED-FIELDS] (or
the applicable formal serialization section of its extensions or
updates) applicable to the type of the HTTP field. Note that this
process will replace any optional internal whitespace with a single
space character, among other potential transformations of the value.

If multiple field values occur within a message, these values MUST be
combined into a single List or Dictionary structure before
serialization.

If the application does not know the type of the field or does not
know how to serialize the type of the field, the use of this flag
will produce an error. As a consequence, the signer can only
reliably sign fields using this flag when the verifier's system knows
the type as well.

For example, the following Dictionary field is a valid serialization:

Example-Dict: a=1, b=2;x=1;y=2, c=(a b c)

If included in the signature base without parameters, its value would
be:

"example-dict": a=1, b=2;x=1;y=2, c=(a b c)

However, if the sf parameter is added, the value is reserialized as
follows:

"example-dict";sf: a=1, b=2;x=1;y=2, c=(a b c)

The resulting string is used as the component value; see Section 2.1.

2.1.2. Dictionary Structured Field Members

If a given field is known by the application to be a Dictionary
Structured Field, an individual member in the value of that
Dictionary is identified by using the parameter key and the
Dictionary member key as a String value.

If multiple field values occur within a message, these values MUST be
combined into a single Dictionary structure before serialization.

An individual member value of a Dictionary Structured Field is
canonicalized by applying the serialization algorithm described in
Section 4.1.2 of [STRUCTURED-FIELDS] on the member_value and its
parameters, not including the Dictionary key itself. Specifically,
the value is serialized as an Item or Inner List (the two possible
values of a Dictionary member), with all parameters and possible
subfields serialized using the strict serialization rules defined in
Section 4 of [STRUCTURED-FIELDS] (or the applicable section of its
extensions or updates).

Each parameterized key for a given field MUST NOT appear more than
once in the signature base. Parameterized keys MAY appear in any
order in the signature base, regardless of the order they occur in
the source Dictionary.

If a Dictionary key is named as a covered component but it does not
occur in the Dictionary, this MUST cause an error in the signature
base generation.

The following are non-normative examples of canonicalized values for
Dictionary Structured Field members, given the following example
header field, whose value is known by the application to be a
Dictionary:

Example-Dict: a=1, b=2;x=1;y=2, c=(a b c), d

The following example shows canonicalized values for different
component identifiers of this field, presented using the signature
base format discussed in Section 2.5:

"example-dict";key="a": 1
"example-dict";key="d": ?1
"example-dict";key="b": 2;x=1;y=2
"example-dict";key="c": (a b c)

Note that the value for key="c" has been reserialized according to
the strict member_value algorithm, and the value for key="d" has been
serialized as a Boolean value.

2.1.3. Binary-Wrapped HTTP Fields

If the value of the HTTP field in question is known by the
application to cause problems with serialization, particularly with
the combination of multiple values into a single line as discussed in
Section 7.5.6, the signer SHOULD include the bs parameter in a
component identifier to indicate that the values of the field need to
be wrapped as binary structures before being combined.

If this parameter is included with a component identifier, the
component value MUST be calculated using the following algorithm:

1. Let the input be the ordered set of values for a field, in the
order they appear in the message.

2. Create an empty List for accumulating processed field values.

3. For each field value in the set:

3.1. Strip leading and trailing whitespace from the field value.
Note that since HTTP field values are not allowed to
contain leading and trailing whitespace, this would be a
no-op in a compliant implementation.

3.2. Remove any obsolete line folding within the line, and
replace it with a single space (" "), as discussed in
Section 5.2 of [HTTP/1.1]. Note that this behavior is
specific to [HTTP/1.1] and does not apply to other versions
of the HTTP specification.

3.3. Encode the bytes of the resulting field value as a Byte
Sequence. Note that most fields are restricted to ASCII
characters, but other octets could be included in the value
in some implementations.

3.4. Add the Byte Sequence to the List accumulator.

4. The intermediate result is a List of Byte Sequence values.

5. Follow the strict serialization of a List as described in
Section 4.1.1 of [STRUCTURED-FIELDS], and return this output.

For example, the following field with internal commas prevents the
distinct field values from being safely combined:

Example-Header: value, with, lots
Example-Header: of, commas

In our example, the same field can be sent with a semantically
different single value:

Example-Header: value, with, lots, of, commas

Both of these versions are treated differently by the application.
However, if included in the signature base without parameters, the
component value would be the same in both cases:

"example-header": value, with, lots, of, commas

However, if the bs parameter is added, the two separate instances are
encoded and serialized as follows:

"example-header";bs: :dmFsdWUsIHdpdGgsIGxvdHM=:, :b2YsIGNvbW1hcw==:

For the single-instance field above, the encoding with the bs
parameter is:

"example-header";bs: :dmFsdWUsIHdpdGgsIGxvdHMsIG9mLCBjb21tYXM=:

This component value is distinct from the multiple-instance field
above, preventing a collision that could potentially be exploited.

2.1.4. Trailer Fields

If the signer wants to include a trailer field in the signature, the
signer MUST include the tr boolean Boolean parameter to indicate that the
value MUST be taken from the trailer fields and not from the header
fields.

For example, given the following message:

HTTP/1.1 200 OK
Content-Type: text/plain
Transfer-Encoding: chunked
Trailer: Expires

4
HTTP
7
Message
a
Signatures
0
Expires: Wed, 9 Nov 2022 07:28:00 GMT

The signer decides to add both the Trailer header field and the
Expires trailer field to the signature base, along with the status
code derived component:

"@status": 200
"trailer": Expires
"expires";tr: Wed, 9 Nov 2022 07:28:00 GMT

If a field is available as both a header and a trailer in a message,
both values MAY be signed, but the values MUST be signed separately.
The values of header fields and trailer fields of the same name MUST
NOT be combined for purposes of the signature.

Since trailer fields could be merged into the header fields or
dropped entirely by intermediaries as per Section 6.5.1 of [HTTP], it
is NOT RECOMMENDED to include trailers in the signature unless the
signer knows that the verifier will have access to the values of the
trailers as sent.

2.2. Derived Components

In addition to HTTP fields, there are a number of different
components that can be derived from the control data, signature
context, or other aspects of the HTTP message being signed. Such
derived components can be included in the signature base by defining
a component name, possible parameters, message targets, and the
derivation method for its component value.

Derived component names MUST start with the "at" (@) character. This
differentiates derived component names from HTTP field names, which
cannot contain the @ character as per Section 5.1 of [HTTP].
Processors of HTTP Message Signatures message signatures MUST treat derived component
names separately from field names, as discussed in Section 7.5.1.

This specification defines the following derived components:

@method The method used for a request (Section 2.2.1).

@target-uri The full target URI for a request (Section 2.2.2).

@authority The authority of the target URI for a request
(Section 2.2.3).

@scheme The scheme of the target URI for a request (Section 2.2.4).

@request-target The request target (Section 2.2.5).

@path The absolute path portion of the target URI for a request
(Section 2.2.6).

@query The query portion of the target URI for a request
(Section 2.2.7).

@query-param A parsed and encoded query parameter of the target URI
for a request (Section 2.2.8).

@status The status code for a response (Section 2.2.9).

Additional derived component names are defined in the "HTTP Signature
Derived Component Names" registry (Section 6.4).

Derived component values are taken from the context of the target
message for the signature. This context includes information about
the message itself, such as its control data, as well as any
additional state and context held by the signer or verifier. In
particular, when signing a response, the signer can include any
derived components from the originating request by using the req
parameter (Section 2.4).

request: Values derived from, and results applied to, an HTTP
request message as described in Section 3.4 of [HTTP]. If the
target message of the signature is a response, derived components
that target request messages can be included by using the req
parameter as defined in Section 2.4.

response: Values derived from, and results applied to, an HTTP
response message as described in Section 3.4 of [HTTP].

request, response: Values derived from, and results applied to,
either a request message or a response message.

A derived component definition MUST define all target message types
to which it can be applied.

Derived component values MUST be limited to printable characters and
spaces and MUST NOT contain any newline characters. Derived
component values MUST NOT start or end with whitespace characters.

2.2.1. Method

The @method derived component refers to the HTTP method of a request
message. The component value is canonicalized by taking the value of
the method as a string. Note that the method name is case sensitive
as per [HTTP], Section 9.1. While conventionally standardized method
names are uppercase [ASCII], no transformation to the input method
value's case is performed.

For example, the following request message:

POST /path?param=value HTTP/1.1
Host: www.example.com

would result in the following @method component value:

POST

and the following signature base line:

"@method": POST

2.2.2. Target URI

The @target-uri derived component refers to the target URI of a
request message. The component value is the target URI of the
request ([HTTP], Section 7.1), assembled from all available URI
components, including the authority.

For example, the following message sent over HTTPS:

POST /path?param=value HTTP/1.1
Host: www.example.com

would result in the following @target-uri component value:

https://www.example.com/path?param=value

and the following signature base line:

"@target-uri": https://www.example.com/path?param=value

2.2.3. Authority

The @authority derived component refers to the authority component of
the target URI of the HTTP request message, as defined in [HTTP],
Section 7.2. In HTTP/1.1, this is usually conveyed using the Host
header,
header field, while in HTTP/2 and HTTP/3 it is conveyed using the
:authority pseudo-header. The value is the fully qualified authority
component of the request, comprised of the host and, optionally, port
of the request target, as a string. The component value MUST be
normalized according to the rules provided in [HTTP], Section 4.2.3.
Namely, the hostname is normalized to lowercase, and the default port
is omitted.

For example, the following request message:

POST /path?param=value HTTP/1.1
Host: www.example.com

would result in the following @authority component value:

www.example.com

and the following signature base line:

"@authority": www.example.com

The @authority derived component SHOULD be used instead of signing
the Host header field directly. See Section 7.2.4.

2.2.4. Scheme

The @scheme derived component refers to the scheme of the target URL
of the HTTP request message. The component value is the scheme as a
lowercase string as defined in [HTTP], Section 4.2. While the scheme
itself is case insensitive, it MUST be normalized to lowercase for
inclusion in the signature base.

For example, the following request message sent over plain HTTP:

POST /path?param=value HTTP/1.1
Host: www.example.com

would result in the following @scheme component value:

http

and the following signature base line:

"@scheme": http

2.2.5. Request Target

The @request-target derived component refers to the full request
target of the HTTP request message, as defined in [HTTP],
Section 7.1. The component value of the request target can take
different forms, depending on the type of request, as described
below.

For HTTP/1.1, the component value is equivalent to the request target
portion of the request line. However, this value is more difficult
to reliably construct in other versions of HTTP. Therefore, it is
NOT RECOMMENDED that this component be used when versions of HTTP
other than 1.1 might be in use.

The origin form value is a combination of the absolute path and query
components of the request URL.

For example, the following request message:

POST /path?param=value HTTP/1.1
Host: www.example.com

would result in the following @request-target component value:

/path?param=value

and the following signature base line:

"@request-target": /path?param=value

The following request to an HTTP proxy with the absolute-form value,
containing the fully qualified target URI:

GET https://www.example.com/path?param=value HTTP/1.1

would result in the following @request-target component value:

https://www.example.com/path?param=value

and the following signature base line:

"@request-target": https://www.example.com/path?param=value

The following CONNECT request with an authority-form value,
containing the host and port of the target:

CONNECT www.example.com:80 HTTP/1.1
Host: www.example.com

would result in the following @request-target component value:

www.example.com:80

and the following signature base line:

"@request-target": www.example.com:80

The following OPTIONS request message with the asterisk-form value,
containing a single asterisk (*) character:

OPTIONS * HTTP/1.1
Host: www.example.com

would result in the following @request-target component value:

and the following signature base line:

"@request-target": *

2.2.6. Path

The @path derived component refers to the target path of the HTTP
request message. The component value is the absolute path of the
request target defined by [URI], with no query component and no
trailing question mark (?) character. The value is normalized
according to the rules provided in [HTTP], Section 4.2.3. Namely, an
empty path string is normalized as a single slash (/) character.
Path components are represented by their values before decoding any
percent-encoded octets, as described in the simple string comparison
rules provided in Section 6.2.1 of [URI].

For example, the following request message:

GET /path?param=value HTTP/1.1
Host: www.example.com

would result in the following @path component value:

/path

and the following signature base line:

"@path": /path

2.2.7. Query

The @query derived component refers to the query component of the
HTTP request message. The component value is the entire normalized
query string defined by [URI], including the leading ? character.
The value is read using the simple string comparison rules provided
in Section 6.2.1 of [URI]. Namely, percent-encoded octets are not
decoded.

For example, the following request message:

GET /path?param=value&foo=bar&baz=bat%2Dman HTTP/1.1
Host: www.example.com

would result in the following @query component value:

?param=value&foo=bar&baz=bat%2Dman

and the following signature base line:

"@query": ?param=value&foo=bar&baz=bat%2Dman

The following request message:

POST /path?queryString HTTP/1.1
Host: www.example.com

would result in the following @query component value:

?queryString

and the following signature base line:

"@query": ?queryString

Just like including an empty path component, the signer can include
an empty query component to indicate that this component is not used
in the message. If the query string is absent from the request
message, the component value is the leading ? character alone:

resulting in the following signature base line:

"@query": ?

2.2.8. Query Parameters

If the query portion of a request target URI uses HTML form
parameters in the format defined in Section 5 ("application/
x-www-form-urlencoded") of [HTMLURL], the @query-param derived
component allows addressing of these individual query parameters.
The query parameters MUST be parsed according to Section 5.1
("application/x-www-form-urlencoded parsing") of [HTMLURL], resulting
in a list of (nameString, valueString) tuples. The REQUIRED name
parameter of each component identifier contains the encoded
nameString of a single query parameter as a String value. The
component value of a single named parameter is the encoded
valueString of that single query parameter. Several different named
query parameters MAY be included in the covered components. Single
named parameters MAY occur in any order in the covered components,
regardless of the order they occur in the query string.

The value of the name parameter and the component value of a single
named parameter are calculated via the following process:

1. Parse the nameString or valueString of the named query parameter
defined by Section 5.1 ("application/x-www-form-urlencoded
parsing") of [HTMLURL]; this is the value after percent-encoded
octets are decoded.

2. Encode the nameString or valueString using the "percent-encode
after encoding" process defined by Section 5.2 ("application/
x-www-form-urlencoded serializing") of [HTMLURL]; this results in
an ASCII string [ASCII].

3. Output the ASCII string.

Note that the component value does not include any leading ? question
mark (?) characters, equals sign (=) characters, or separating
ampersand (&) characters. Named query parameters with an empty
valueString have an empty string as the component value. Note that
due to inconsistencies in implementations, some query parameter
parsing libraries drop such empty values.

If a query parameter is named as a covered component but it does not
occur in the query parameters, this MUST cause an error in the
signature base generation.

For example, for the following request:

GET /path?param=value&foo=bar&baz=batman&qux= HTTP/1.1
Host: www.example.com

Indicating the baz, qux, and param named query parameters would
result in the following @query-param component values:

_baz_: batman

_qux_: an empty string

_param_: value

and the following signature base lines, with (SP) indicating a single
trailing space character before the empty component value:

"@query-param";name="baz": batman
"@query-param";name="qux":(SP)
"@query-param";name="param": value

This derived component has some limitations. Specifically, the
algorithms provided in Section 5 ("application/
x-www-form-urlencoded") of [HTMLURL] only support query parameters
using percent-escaped UTF-8 encoding. Other encodings are not
supported. Additionally, multiple instances of a named parameter are
not reliably supported in the wild. If a parameter name occurs
multiple times in a request, the named query parameter MUST NOT be
included. If multiple parameters are common within an application,
it is RECOMMENDED to sign the entire query string using the @query
component identifier defined in Section 2.2.7.

The encoding process allows query parameters that include newlines or
other problematic characters in their values, or with alternative
encodings such as using the plus (+) character to represent spaces.
For the query parameters in this message:

NOTE: '\' line wrapping per RFC 8792

GET /parameters?var=this%20is%20a%20big%0Amultiline%20value&\
bar=with+plus+whitespace&fa%C3%A7ade%22%3A%20=something HTTP/1.1
Host: www.example.com
Date: Tue, 20 Apr 2021 02:07:56 GMT

The resulting values are encoded as follows:

"@query-param";name="var": this%20is%20a%20big%0Amultiline%20value
"@query-param";name="bar": with%20plus%20whitespace
"@query-param";name="fa%C3%A7ade%22%3A%20": something

If the encoding were not applied, the resultant values would be:

"@query-param";name="var": this is a big
multiline value
"@query-param";name="bar": with plus whitespace
"@query-param";name="façade\": ": something

This base string contains characters that violate the constraints on
component names and values and is therefore invalid.

2.2.9. Status Code

The @status derived component refers to the three-digit numeric HTTP
status code of a response message as defined in [HTTP], Section 15.
The component value is the serialized three-digit integer of the HTTP
status code, with no descriptive text.

For example, the following response message:

HTTP/1.1 200 OK
Date: Fri, 26 Mar 2010 00:05:00 GMT

would result in the following @status component value:

200

and the following signature base line:

"@status": 200

The @status component identifier MUST NOT be used in a request
message.

2.3. Signature Parameters

HTTP Message Signatures message signatures have metadata properties that provide
information regarding the signature's generation and verification,
consisting of the ordered set of covered components and the ordered
set of parameters, where the parameters include a timestamp of
signature creation, identifiers for verification key material, and
other utilities. This metadata is represented by a special message
component in the signature base for signature parameters; this
special message component is treated slightly differently from other
message components. Specifically, the signature parameters message
component is REQUIRED as the last line of the signature base
(Section 2.5), and the component identifier MUST NOT be enumerated
within the set of covered components for any signature, including
itself.

The signature parameters component name is @signature-params.

The signature parameters component value is the serialization of the
signature parameters for this signature, including the covered
components ordered set with all associated parameters. These
parameters include any of the following:

created: Creation time as a UNIX timestamp value of type Integer.
Sub-second precision is not supported. The inclusion of this
parameter is RECOMMENDED.

expires: Expiration time as a UNIX timestamp value of type Integer.
Sub-second precision is not supported.

nonce: A random unique value generated for this signature as a
String value.

alg: The HTTP message signature algorithm from the "HTTP Signature
Algorithms" registry, as a String value.

keyid: The identifier for the key material as a String value.

tag: An application-specific tag for the signature as a String
value. This value is used by applications to help identify
signatures relevant for specific applications or protocols.

Additional parameters can be defined in the "HTTP Signature Metadata
Parameters" registry (Section 6.3). Note that the parameters are not
in any general order, but once an ordering is chosen for a given set
of parameters, it cannot be changed without altering the signature
parameters value.

The signature parameters component value is serialized as a
parameterized Inner List using the rules provided in Section 4 of
[STRUCTURED-FIELDS] as follows:

1. Let the output be an empty string.

2. Determine an order for the component identifiers of the covered
components, not including the @signature-params component
identifier itself. Once this order is chosen, it cannot be
changed. This order MUST be the same order as that used in
creating the signature base (Section 2.5).

3. Serialize the component identifiers of the covered components,
including all parameters, as an ordered Inner List of String
values according to Section 4.1.1.1 of [STRUCTURED-FIELDS]; then,
append this to the output. Note that the component identifiers
can include their own parameters, and these parameters are
ordered sets. Once an order is chosen for a component's
parameters, the order cannot be changed.

4. Determine an order for any signature parameters. Once this order
is chosen, it cannot be changed.

5. Append the parameters to the Inner List in order according to
Section 4.1.1.2 of [STRUCTURED-FIELDS], skipping parameters that
are not available or not used for this message signature.

6. The output contains the signature parameters component value.

Note that the Inner List serialization from Section 4.1.1.1 of
[STRUCTURED-FIELDS] is used for the covered component value instead
of the List serialization from Section 4.1.1 of [STRUCTURED-FIELDS]
in order to facilitate parallelism with this value's inclusion in the
Signature-Input field, as discussed in Section 4.1.

This example shows the serialized component value for the parameters
of an example message signature:

NOTE: '\' line wrapping per RFC 8792

("@target-uri" "@authority" "date" "cache-control")\
;keyid="test-key-rsa-pss";alg="rsa-pss-sha512";\
created=1618884475;expires=1618884775

Note that an HTTP message could contain multiple signatures
(Section 4.3), but only the signature parameters used for a single
signature are included in a given signature parameters entry.

2.4. Signing Request Components in a Response Message

When a request message results in a signed response message, the
signer can include portions of the request message in the signature
base by adding the req parameter to the component identifier.

req A boolean Boolean flag indicating that the component value is derived
from the request that triggered this response message and not from
the response message directly.

This parameter can be applied to both HTTP fields and derived
components that target the request, with the same semantics. The
component value for a message component using this parameter is
calculated in the same manner as it is normally, but data is pulled
from the request message instead of the target response message to
which the signature is applied.

Note that the same component name MAY be included with and without
the req parameter in a single signature base, indicating the same
named component from both the request message and the response
message.

The req parameter MAY be combined with other parameters as
appropriate for the component identifier, such as the key parameter
for a Dictionary field.

For example, when serving a response for this request:

NOTE: '\' line wrapping per RFC 8792

{"hello": "world"}

This would result in the following unsigned response message:

NOTE: '\' line wrapping per RFC 8792

{"busy": true, "message": "Your call is very important to us"}

The server signs the response with its own key, including the @status
code and several header fields in the covered components. While this
covers a reasonable amount of the response for this application, the
server additionally includes several components derived from the
original request message that triggered this response. In this
example, the server includes the method, authority, path, and content
digest from the request in the covered components of the response.
The Content-Digest for both the request and the response is included
under the response signature. For the application in this example,
the query is deemed not to be relevant to the response and is
therefore not covered. Other applications would make different
decisions based on application needs, as discussed in Section 1.4.

The signature base for this example is:

NOTE: '\' line wrapping per RFC 8792

"@status": 503
"content-digest": sha-512=:0Y6iCBzGg5rZtoXS95Ijz03mslf6KAMCloESHObf\
wnHJDbkkWWQz6PhhU9kxsTbARtY2PTBOzq24uJFpHsMuAg==:
"content-type": application/json
"@authority";req: example.com
"@method";req: POST
"@path";req: /foo
"content-digest";req: sha-512=:WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A\
2svX+TaPm+AbwAgBWnrIiYllu7BNNyealdVLvRwEmTHWXvJwew==:
"@signature-params": ("@status" "content-digest" "content-type" \
"@authority";req "@method";req "@path";req "content-digest";req)\
;created=1618884479;keyid="test-key-ecc-p256"

The signed response message is:

NOTE: '\' line wrapping per RFC 8792

HTTP/1.1 503 Service Unavailable
Date: Tue, 20 Apr 2021 02:07:56 GMT
Content-Type: application/json
Content-Length: 62
Content-Digest: sha-512=:0Y6iCBzGg5rZtoXS95Ijz03mslf6KAMCloESHObfwn\
HJDbkkWWQz6PhhU9kxsTbARtY2PTBOzq24uJFpHsMuAg==:
Signature-Input: reqres=("@status" "content-digest" "content-type" \
"@authority";req "@method";req "@path";req "content-digest";req)\
;created=1618884479;keyid="test-key-ecc-p256"
Signature: reqres=:dMT/A/76ehrdBTD/2Xx8QuKV6FoyzEP/I9hdzKN8LQJLNgzU\
4W767HK05rx1i8meNQQgQPgQp8wq2ive3tV5Ag==:

{"busy": true, "message": "Your call is very important to us"}

Note that the ECDSA signature algorithm in use here is non-
deterministic, meaning that a different signature value will be
created every time the algorithm is run. The signature value
provided here can be validated against the given keys, but newly
generated signature values are not expected to match the example.
See Section 7.3.5.

Since the component values from the request are not repeated in the
response message, the requester MUST keep the original message
component values around long enough to validate the signature of the
response that uses this component identifier parameter. In most
cases, this means the requester needs to keep the original request
message around, since the signer could choose to include any portions
of the request in its response, according to the needs of the
application. Since it is possible for an intermediary to alter a
request message before it is processed by the server, applications
need to take care not to sign such altered values, as the client
would not be able to validate the resulting signature.

It is also possible for a server to create a signed response in
response to a signed request. For this example of a signed request:

NOTE: '\' line wrapping per RFC 8792

POST /foo?param=Value&Pet=dog HTTP/1.1
Host: example.com
Date: Tue, 20 Apr 2021 02:07:55 GMT
Content-Digest: sha-512=:WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+T\
aPm+AbwAgBWnrIiYllu7BNNyealdVLvRwEmTHWXvJwew==:
Content-Type: application/json
Content-Length: 18
Signature-Input: sig1=("@method" "@authority" "@path" "@query" \
"content-digest" "content-type" "content-length")\
;created=1618884475;keyid="test-key-rsa-pss"
Signature: sig1=:e8UJ5wMiRaonlth5ERtE8GIiEH7Akcr493nQ07VPNo6y3qvjdK\
t0fo8VHO8xXDjmtYoatGYBGJVlMfIp06eVMEyNW2I4vN7XDAz7m5v1108vGzaDljr\
d0H8+SJ28g7bzn6h2xeL/8q+qUwahWA/JmC8aOC9iVnwbOKCc0WSrLgWQwTY6VLp4\
2Qt7jjhYT5W7/wCvfK9A1VmHH1lJXsV873Z6hpxesd50PSmO+xaNeYvDLvVdZlhtw\
5PCtUYzKjHqwmaQ6DEuM8udRjYsoNqp2xZKcuCO1nKc0V3RjpqMZLuuyVbHDAbCzr\
0pg2d2VM/OC33JAU7meEjjaNz+d7LWPg==:

{"hello": "world"}

The server could choose to sign portions of this response, including
several portions of the request, resulting in this signature base:

NOTE: '\' line wrapping per RFC 8792

"@status": 503
"content-digest": sha-512=:0Y6iCBzGg5rZtoXS95Ijz03mslf6KAMCloESHObf\
wnHJDbkkWWQz6PhhU9kxsTbARtY2PTBOzq24uJFpHsMuAg==:
"content-type": application/json
"@authority";req: example.com
"@method";req: POST
"@path";req: /foo
"@query";req: ?param=Value&Pet=dog
"content-digest";req: sha-512=:WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A\
2svX+TaPm+AbwAgBWnrIiYllu7BNNyealdVLvRwEmTHWXvJwew==:
"content-type";req: application/json
"content-length";req: 18
"@signature-params": ("@status" "content-digest" "content-type" \
"@authority";req "@method";req "@path";req "@query";req \
"content-digest";req "content-type";req "content-length";req)\
;created=1618884479;keyid="test-key-ecc-p256"

and the following signed response:

NOTE: '\' line wrapping per RFC 8792

HTTP/1.1 503 Service Unavailable
Date: Tue, 20 Apr 2021 02:07:56 GMT
Content-Type: application/json
Content-Length: 62
Content-Digest: sha-512=:0Y6iCBzGg5rZtoXS95Ijz03mslf6KAMCloESHObfwn\
HJDbkkWWQz6PhhU9kxsTbARtY2PTBOzq24uJFpHsMuAg==:
Signature-Input: reqres=("@status" "content-digest" "content-type" \
"@authority";req "@method";req "@path";req "@query";req \
"content-digest";req "content-type";req "content-length";req)\
;created=1618884479;keyid="test-key-ecc-p256"
Signature: reqres=:C73J41GVKc+TYXbSobvZf0CmNcptRiWN+NY1Or0A36ISg6ym\
dRN6ZgR2QfrtopFNzqAyv+CeWrMsNbcV2Ojsgg==:

{"busy": true, "message": "Your call is very important to us"}

Applications signing a response to a signed request SHOULD sign all
of the components of the request signature value to provide
sufficient coverage and protection against a class of collision
attacks, as discussed in Section 7.3.7. The server in this example
has included all components listed in the Signature-Input field of
the client's signature on the request in the response signature, in
addition to components of the response.

While it is syntactically possible to include the Signature and
Signature-Input fields of the request message in the signature
components of a response to a message using this mechanism, this
practice is NOT RECOMMENDED. This is because signatures of
signatures do not provide transitive coverage of covered components
as one might expect, and the practice is susceptible to several
attacks as discussed in Section 7.3.7. An application that needs to
signal successful processing or receipt of a signature would need to
carefully specify alternative mechanisms for sending such a signal
securely.

The response signature can only ever cover what is included in the
request message when using this flag. Consequently, if an
application needs to include the message content of the request under
the signature of its response, the client needs to include a means
for covering that content, such as a Content-Digest field. See the
discussion in Section 7.2.8 for more information.

The req parameter MUST NOT be used for any component in a signature
that targets a request message.

2.5. Creating the Signature Base

The signature base is an ASCII string [ASCII] containing the
canonicalized HTTP message components covered by the signature. The
input to the signature base creation algorithm is the ordered set of
covered component identifiers and their associated values, along with
any additional signature parameters discussed in Section 2.3.

Component identifiers are serialized using the strict serialization
rules defined by [STRUCTURED-FIELDS], Section 4. The component
identifier has a component name, which is a String Item value
serialized using the sf-string ABNF rule. The component identifier
MAY also include defined parameters that are serialized using the
parameters ABNF rule. The signature parameters line defined in
Section 2.3 follows this same pattern, but the component identifier
is a String Item with a fixed value and no parameters, and the
component value is always an Inner List with optional parameters.

Note that this means the serialization of the component name itself
is encased in double quotes, with parameters following as a
semicolon-separated list, such as "cache-control", "@authority",
"@signature-params", or "example-dictionary";key="foo".

The output is the ordered set of bytes that form the signature base,
which conforms to the following ABNF:

signature-base = *( signature-base-line LF ) signature-params-line
signature-base-line = component-identifier ":" SP
( derived-component-value / *field-content )
; no obs-fold nor obs-text
component-identifier = component-name parameters
component-name = sf-string
derived-component-value = *( VCHAR / SP )
signature-params-line = DQUOTE "@signature-params" DQUOTE
":" SP inner-list

To create the signature base, the signer or verifier concatenates
entries for each component identifier in the signature's covered
components (including their parameters) using the following
algorithm. All errors produced as described MUST fail the algorithm
immediately, without outputting a signature base.

1. Let the output be an empty string.

2. For each message component item in the covered components set (in
order):

2.1. If the component identifier (including its parameters) has
already been added to the signature base, produce an error.

2.2. Append the component identifier for the covered component
serialized according to the component-identifier ABNF rule.
Note that this serialization places the component name in
double quotes and appends any parameters outside of the
quotes.

2.3. Append a single colon (:).

2.4. Append a single space (" ").

2.5. Determine the component value for the component identifier.

* If the component identifier has a parameter that is not
understood, produce an error.

* If the component identifier has parameters that are
mutually incompatible with one another, such as bs and
sf, produce an error.

* If the component identifier contains the req parameter
and the target message is a request, produce an error.

* If the component identifier contains the req parameter
and the target message is a response, the context for
the component value is the related request message of
the target response message. Otherwise, the context for
the component value is the target message.

* If the component name starts with an "at" (@) character,
derive the component's value from the message according
to the specific rules defined for the derived component,
as provided in Section 2.2, including processing of any
known valid parameters. If the derived component name
is unknown or the value cannot be derived, produce an
error.

* If the component name does not start with an "at" (@)
character, canonicalize the HTTP field value as
described in Section 2.1, including processing of any
known valid parameters. If the field cannot be found in
the message or the value cannot be obtained in the
context, produce an error.

2.6. Append the covered component's canonicalized component
value.

2.7. Append a single newline (\n).

3. Append the signature parameters component (Section 2.3) according
to the signature-params-line rule as follows:

3.1. Append the component identifier for the signature
parameters serialized according to the component-identifier
rule, i.e., the exact value "@signature-params" (including
double quotes).

3.2. Append a single colon (:).

3.3. Append a single space (" ").

3.4. Append the signature parameters' canonicalized component
values as defined in Section 2.3, i.e., Inner List
Structured Field values with parameters.

4. Produce an error if the output string contains any non-ASCII
characters [ASCII].

5. Return the output string.

If covered components reference a component identifier that cannot be
resolved to a component value in the message, the implementation MUST
produce an error and not create a signature base. Such situations
include, but are not limited to, the following:

* The signer or verifier does not understand the derived component
name.

* The component name identifies a field that is not present in the
message or whose value is malformed.

* The component identifier includes a parameter that is unknown or
does not apply to the component identifier to which it is
attached.

* The component identifier indicates that a Structured Field
serialization is used (via the sf parameter), but the field in
question is known to not be a Structured Field or the type of
Structured Field is not known to the implementation.

* The component identifier is a Dictionary member identifier that
references a field that is not present in the message, that is not
a Dictionary Structured Field, or whose value is malformed.

* The component identifier is a Dictionary member identifier or a
named query parameter identifier that references a member that is
not present in the component value or whose value is malformed.
For example, the identifier is "example-dict";key="c", and the
value of the Example-Dict header field is a=1, b=2, which does not
have the c value.

In the following non-normative example, the HTTP message being signed
is the following request:

NOTE: '\' line wrapping per RFC 8792

{"hello": "world"}

The covered components consist of the @method, @authority, and @path
derived components followed by the Content-Digest, Content-Length,
and Content-Type HTTP header fields, in order. The signature
parameters consist of a creation timestamp of 1618884473 and a key
identifier of test-key-rsa-pss. Note that no explicit alg parameter
is given here, since the verifier is known by the application to use
the RSA-PSS algorithm based on the identified key. The signature
base for this message with these parameters is:

NOTE: '\' line wrapping per RFC 8792

"@method": POST
"@authority": example.com
"@path": /foo
"content-digest": sha-512=:WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX\
+TaPm+AbwAgBWnrIiYllu7BNNyealdVLvRwEmTHWXvJwew==:
"content-length": 18
"content-type": application/json
"@signature-params": ("@method" "@authority" "@path" \
"content-digest" "content-length" "content-type")\
;created=1618884473;keyid="test-key-rsa-pss"

Figure 1: Non-normative Example Signature Base

Note that the example signature base above does not include the final
newline that ends the displayed example, nor do other example
signature bases displayed elsewhere in this specification.

3. HTTP Message Signatures

An HTTP Message Signature message signature is a signature over a string generated from
a subset of the components of an HTTP message in addition to metadata
about the signature itself. When successfully verified against an
HTTP message, an HTTP Message Signature message signature provides cryptographic proof
that the message is semantically equivalent to the message for which
the signature was generated, with respect to the subset of message
components that was signed.

3.1. Creating a Signature

Creation of an HTTP message signature is a process that takes as its
input the signature context (including the target message) and the
requirements for the application. The output is a signature value
and set of signature parameters that can be communicated to the
verifier by adding them to the message.

In order to create a signature, a signer MUST apply the following
algorithm:

1. The signer chooses an HTTP signature algorithm and key material
for signing from the set of potential signing algorithms. The
set of potential algorithms is determined by the application and
is out of scope for this document. The signer MUST choose key
material that is appropriate for the signature's algorithm and
that conforms to any requirements defined by the algorithm, such
as key size or format. The mechanism by which the signer chooses
the algorithm and key material is out of scope for this document.

2. The signer sets the signature's creation time to the current
time.

3. If applicable, the signer sets the signature's expiration time
property to the time at which the signature is to expire. The
expiration is a hint to the verifier, expressing the time at
which the signer is no longer willing to vouch for the signature.
An appropriate expiration length, and the processing requirements
of this parameter, are application specific.

4. The signer creates an ordered set of component identifiers
representing the message components to be covered by the
signature and attaches signature metadata parameters to this set.
The serialized value of this set is later used as the value of
the Signature-Input field as described in Section 4.1.

* Once an order of covered components is chosen, the order MUST
NOT change for the life of the signature.

* Each covered component identifier MUST be either (1) an HTTP
field (Section 2.1) in the signature context or (2) a derived
component listed in Section 2.2 or in the "HTTP Signature
Derived Component Names" registry.

* Signers of a request SHOULD include some or all of the message
control data in the covered components, such as the @method,
@authority, @target-uri, or some combination thereof.

* Signers SHOULD include the created signature metadata
parameter to indicate when the signature was created.

* The @signature-params derived component identifier MUST NOT be
present in the list of covered component identifiers. The
derived component is required to always be the last line in
the signature base, ensuring that a signature always covers
its own metadata and the metadata cannot be substituted.

* Further guidance on what to include in this set and in what
order is out of scope for this document.

5. The signer creates the signature base using these parameters and
the signature base creation algorithm (Section 2.5).

6. The signer uses the HTTP_SIGN primitive function to sign the
signature base with the chosen signing algorithm using the key
material chosen by the signer. The HTTP_SIGN primitive and
several concrete applications of signing algorithms are defined
in Section 3.3.

7. The byte array output of the signature function is the HTTP
message signature output value to be included in the Signature
field as defined in Section 4.2.

For example, given the HTTP message and signature parameters in the
example in Section 2.5, the example signature base is signed with the
test-key-rsa-pss key (see Appendix B.1.2) and the RSASSA-PSS
algorithm described in Section 3.3.1, giving the following message
signature output value, encoded in Base64:

NOTE: '\' line wrapping per RFC 8792

HIbjHC5rS0BYaa9v4QfD4193TORw7u9edguPh0AW3dMq9WImrlFrCGUDih47vAxi4L2\
YRZ3XMJc1uOKk/J0ZmZ+wcta4nKIgBkKq0rM9hs3CQyxXGxHLMCy8uqK488o+9jrptQ\
+xFPHK7a9sRL1IXNaagCNN3ZxJsYapFj+JXbmaI5rtAdSfSvzPuBCh+ARHBmWuNo1Uz\
VVdHXrl8ePL4cccqlazIJdC4QEjrF+Sn4IxBQzTZsL9y9TP5FsZYzHvDqbInkTNigBc\
E9cKOYNFCn4D/WM7F6TNuZO9EgtzepLWcjTymlHzK7aXq6Am6sfOrpIC49yXjj3ae6H\
RalVc/g==

Figure 2: Non-normative Example Signature Value

Note that the RSA-PSS algorithm in use here is non-deterministic,
meaning that a different signature value will be created every time
the algorithm is run. The signature value provided here can be
validated against the given keys, but newly generated signature
values are not expected to match the example. See Section 7.3.5.

3.2. Verifying a Signature

Verification of an HTTP message signature is a process that takes as
its input the signature context (including the target message,
particularly its Signature and Signature-Input fields) and the
requirements for the application. The output of the verification is
either a positive verification or an error.

In order to verify a signature, a verifier MUST apply the following
algorithm:

1. Parse the Signature and Signature-Input fields as described in
Sections 4.1 and 4.2, and extract the signatures to be verified. verified
and their labels.

1.1. If there is more than one signature value present,
determine which signature should be processed for this
message based on the policy and configuration of the
verifier. If an applicable signature is not found, produce
an error.

1.2. If the chosen Signature field value does not have a
corresponding Signature-Input value, field value (i.e., one with
the same label), produce an error.

2. Parse the values of the chosen Signature-Input field as a
parameterized Inner List to get the ordered list of covered
components and the signature parameters for the signature to be
verified.

3. Parse the value of the corresponding Signature field to get the
byte array value of the signature to be verified.

4. Examine the signature parameters to confirm that the signature
meets the requirements described in this document, as well as any
additional requirements defined by the application such as which
message components are required to be covered by the signature
(Section 3.2.1).

5. Determine the verification key material for this signature. If
the key material is known through external means such as static
configuration or external protocol negotiation, the verifier will
use the applicable technique to obtain the key material from this
external knowledge. If the key is identified in the signature
parameters, the verifier will dereference the key identifier to
appropriate key material to use with the signature. The verifier
has to determine the trustworthiness of the key material for the
context in which the signature is presented. If a key is
identified that the verifier does not know or trust for this
request or that does not match something preconfigured, the
verification MUST fail.

6. Determine the algorithm to apply for verification:

6.1. Start with the set of allowable algorithms known to the
application. If any of the following steps select an
algorithm that is not in this set, the signature validation
fails.

6.2. If the algorithm is known through external means such as
static configuration or external protocol negotiation, the
verifier will use that algorithm.

6.3. If the algorithm can be determined from the keying
material, such as through an algorithm field on the key
value itself, the verifier will use that algorithm.

6.4. If the algorithm is explicitly stated in the signature
parameters using a value from the "HTTP Signature
Algorithms" registry, the verifier will use the referenced
algorithm.

6.5. If the algorithm is specified in more than one location
(e.g., a combination of static configuration, the algorithm
signature parameter, and the key material itself), the
resolved algorithms MUST be the same. If the algorithms
are not the same, the verifier MUST fail the verification.

7. Use the received HTTP message and the parsed signature parameters
to recreate the signature base, using the algorithm defined in
Section 2.5. The value of the @signature-params input is the
value of the Signature-Input field for this signature serialized
according to the rules described in Section 2.3. Note that this
does not include the signature's label from the Signature-Input
field.

8. If the key material is appropriate for the algorithm, apply the
appropriate HTTP_VERIFY cryptographic verification algorithm to
the signature, recalculated signature base, key material, and
signature value. The HTTP_VERIFY primitive and several concrete
algorithms are defined in Section 3.3.

9. The results of the verification algorithm function are the final
results of the cryptographic verification function.

If any of the above steps fail or produce an error, the signature
validation fails.

For example, verifying the signature with the key label sig1 of the
following message with the test-key-rsa-pss key (see Appendix B.1.2)
and the RSASSA-PSS algorithm described in Section 3.3.1:

NOTE: '\' line wrapping per RFC 8792

POST /foo?param=Value&Pet=dog HTTP/1.1
Host: example.com
Date: Tue, 20 Apr 2021 02:07:55 GMT
Content-Type: application/json
Content-Digest: sha-512=:WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+T\
aPm+AbwAgBWnrIiYllu7BNNyealdVLvRwEmTHWXvJwew==:
Content-Length: 18
Signature-Input: sig1=("@method" "@authority" "@path" \
"content-digest" "content-length" "content-type")\
;created=1618884473;keyid="test-key-rsa-pss"
Signature: sig1=:HIbjHC5rS0BYaa9v4QfD4193TORw7u9edguPh0AW3dMq9WImrl\
FrCGUDih47vAxi4L2YRZ3XMJc1uOKk/J0ZmZ+wcta4nKIgBkKq0rM9hs3CQyxXGxH\
LMCy8uqK488o+9jrptQ+xFPHK7a9sRL1IXNaagCNN3ZxJsYapFj+JXbmaI5rtAdSf\
SvzPuBCh+ARHBmWuNo1UzVVdHXrl8ePL4cccqlazIJdC4QEjrF+Sn4IxBQzTZsL9y\
9TP5FsZYzHvDqbInkTNigBcE9cKOYNFCn4D/WM7F6TNuZO9EgtzepLWcjTymlHzK7\
aXq6Am6sfOrpIC49yXjj3ae6HRalVc/g==:

{"hello": "world"}

With the additional requirements that at least the method, authority,
path, content-digest, content-length, and content-type entries be
signed, and that the signature creation timestamp be recent enough at
the time of verification, the verification passes.

3.2.1. Enforcing Application Requirements

The verification requirements specified in this document are intended
as a baseline set of restrictions that are generally applicable to
all use cases. Applications using HTTP Message Signatures message signatures MAY impose
requirements above and beyond those specified by this document, as
appropriate for their use case.

Some non-normative examples of additional requirements an application
might define are:

* Requiring a specific set of header fields to be signed (e.g.,
Authorization, Digest). Content-Digest).

* Enforcing a maximum signature age from the time of the created
timestamp.

* Rejecting signatures past the expiration time in the expires
timestamp. Note that the expiration time is a hint from the
signer and that a verifier can always reject a signature ahead of
its expiration time.

* Prohibiting certain signature metadata parameters, such as runtime
algorithm signaling with the alg parameter when the algorithm is
determined from the key information.

* Ensuring successful dereferencing of the keyid parameter to valid
and appropriate key material.

* Prohibiting the use of certain algorithms or mandating the use of
a specific algorithm.

* Requiring keys to be of a certain size (e.g., 2048 bits vs. 1024
bits).

* Enforcing uniqueness of the nonce parameter.

* Requiring an application-specific value for the tag parameter.

Application-specific requirements are expected and encouraged. When
an application defines additional requirements, it MUST enforce them
during the signature verification process, and signature verification
MUST fail if the signature does not conform to the application's
requirements.

Applications MUST enforce the requirements defined in this document.
Regardless of use case, applications MUST NOT accept signatures that
do not conform to these requirements.

3.3. Signature Algorithms

An HTTP Message message signature MUST use a cryptographic digital signature
or MAC method that is appropriate for the key material, environment,
and needs of the signer and verifier. This specification does not
strictly limit the available signature algorithms, and any signature
algorithm that meets these basic requirements MAY be used by an
application of HTTP message signatures.

For each signing method, HTTP_SIGN takes as its input the signature
base defined in Section 2.5 as a byte array (M) and the signing key
material (Ks), and outputs the resultant signature as a byte array
(S):

HTTP_SIGN (M, Ks) -> S

For each verification method, HTTP_VERIFY takes as its input the
regenerated signature base defined in Section 2.5 as a byte array
(M), the verification key material (Kv), and the presented signature
to be verified as a byte array (S), and outputs the verification
result (V) as a boolean: Boolean:

HTTP_VERIFY (M, Kv, S) -> V

The following sections contain several common signature algorithms
and demonstrate how these cryptographic primitives map to the
HTTP_SIGN and HTTP_VERIFY definitions above. Which method to use can
be communicated through the explicit algorithm (alg) signature
parameter (Section 2.3), by reference to the key material, or through
mutual agreement between the signer and verifier. Signature
algorithms selected using the alg parameter MUST use values from the
"HTTP Signature Algorithms" registry (Section 6.2).

3.3.1. RSASSA-PSS Using SHA-512

To sign using this algorithm, the signer applies the RSASSA-PSS-SIGN
(K, M) function defined in [RFC8017] with the signer's private
signing key (K) and the signature base (M) (Section 2.5). The mask
generation function is MGF1 as specified in [RFC8017] with a hash
function of SHA-512 [RFC6234]. The salt length (sLen) is 64 bytes.
The hash function (Hash) SHA-512 [RFC6234] is applied to the
signature base to create the digest content to which the digital
signature is applied. The resulting signed content byte array (S) is
the HTTP message signature output used in Section 3.1.

To verify using this algorithm, the verifier applies the RSASSA-PSS-
VERIFY ((n, e), M, S) function [RFC8017] using the public key portion
of the verification key material (n, e) and the signature base (M)
recreated as described in Section 3.2. The mask generation function
is MGF1 as specified in [RFC8017] with a hash function of SHA-512
[RFC6234]. The salt length (sLen) is 64 bytes. The hash function
(Hash) SHA-512 [RFC6234] is applied to the signature base to create
the digest content to which the verification function is applied.
The verifier extracts the HTTP message signature to be verified (S)
as described in Section 3.2. The results of the verification
function indicate whether the signature presented is valid.

Note that the output of the RSASSA-PSS algorithm is non-
deterministic; therefore, it is not correct to recalculate a new
signature on the signature base and compare the results to an
existing signature. Instead, the verification algorithm defined here
needs to be used. See Section 7.3.5.

The use of this algorithm can be indicated at runtime using the rsa-
pss-sha512 value for the alg signature parameter.

3.3.2. RSASSA-PKCS1-v1_5 Using SHA-256

To sign using this algorithm, the signer applies the RSASSA-
PKCS1-V1_5-SIGN (K, M) function defined in [RFC8017] with the
signer's private signing key (K) and the signature base (M)
(Section 2.5). The hash SHA-256 [RFC6234] is applied to the
signature base to create the digest content to which the digital
signature is applied. The resulting signed content byte array (S) is
the HTTP message signature output used in Section 3.1.

To verify using this algorithm, the verifier applies the RSASSA-
PKCS1-V1_5-VERIFY ((n, e), M, S) function [RFC8017] using the public
key portion of the verification key material (n, e) and the signature
base (M) recreated as described in Section 3.2. The hash function
SHA-256 [RFC6234] is applied to the signature base to create the
digest content to which the verification function is applied. The
verifier extracts the HTTP message signature to be verified (S) as
described in Section 3.2. The results of the verification function
indicate whether the signature presented is valid.

The use of this algorithm can be indicated at runtime using the rsa-
v1_5-sha256 value for the alg signature parameter.

3.3.3. HMAC Using SHA-256

To sign and verify using this algorithm, the signer applies the HMAC
function [RFC2104] with the shared signing key (K) and the signature
base (text) (Section 2.5). The hash function SHA-256 [RFC6234] is
applied to the signature base to create the digest content to which
the HMAC is applied, giving the signature result.

For signing, the resulting value is the HTTP message signature output
used in Section 3.1.

For verification, the verifier extracts the HTTP message signature to
be verified (S) as described in Section 3.2. The output of the HMAC
function is compared bytewise to the value of the HTTP message
signature, and the results of the comparison determine the validity
of the signature presented.

The use of this algorithm can be indicated at runtime using the hmac-
sha256 value for the alg signature parameter.

3.3.4. ECDSA Using Curve P-256 DSS and SHA-256

To sign using this algorithm, the signer applies the ECDSA signature
algorithm defined in [FIPS186-5] using curve P-256 with the signer's
private signing key and the signature base (Section 2.5). The hash
SHA-256 [RFC6234] is applied to the signature base to create the
digest content to which the digital signature is applied (M). The
signature algorithm returns two integer values: r and s. These are
both encoded as big-endian unsigned integers, zero-padded to 32
octets each. These encoded values are concatenated into a single
64-octet array consisting of the encoded value of r followed by the
encoded value of s. The resulting concatenation of (r, s) is a byte
array of the HTTP message signature output used in Section 3.1.

To verify using this algorithm, the verifier applies the ECDSA
signature algorithm defined in [FIPS186-5] using the public key
portion of the verification key material and the signature base
recreated as described in Section 3.2. The hash function SHA-256
[RFC6234] is applied to the signature base to create the digest
content to which the signature verification function is applied (M).
The verifier extracts the HTTP message signature to be verified (S)
as described in Section 3.2. This value is a 64-octet array
consisting of the encoded values of r and s concatenated in order.
These are both encoded as big-endian unsigned integers, zero-padded
to 32 octets each. The resulting signature value (r, s) is used as
input to the signature verification function. The results of the
verification function indicate whether the signature presented is
valid.

Note that the output of ECDSA signature algorithms is non-
deterministic; therefore, it is not correct to recalculate a new
signature on the signature base and compare the results to an
existing signature. Instead, the verification algorithm defined here
needs to be used. See Section 7.3.5.

The use of this algorithm can be indicated at runtime using the
ecdsa-p256-sha256 value for the alg signature parameter.

3.3.5. ECDSA Using Curve P-384 DSS and SHA-384

To sign using this algorithm, the signer applies the ECDSA signature
algorithm defined in [FIPS186-5] using curve P-384 with the signer's
private signing key and the signature base (Section 2.5). The hash
SHA-384 [RFC6234] is applied to the signature base to create the
digest content to which the digital signature is applied (M). The
signature algorithm returns two integer values: r and s. These are
both encoded as big-endian unsigned integers, zero-padded to 48
octets each. These encoded values are concatenated into a single
96-octet array consisting of the encoded value of r followed by the
encoded value of s. The resulting concatenation of (r, s) is a byte
array of the HTTP message signature output used in Section 3.1.

To verify using this algorithm, the verifier applies the ECDSA
signature algorithm defined in [FIPS186-5] using the public key
portion of the verification key material and the signature base
recreated as described in Section 3.2. The hash function SHA-384
[RFC6234] is applied to the signature base to create the digest
content to which the signature verification function is applied (M).
The verifier extracts the HTTP message signature to be verified (S)
as described in Section 3.2. This value is a 96-octet array
consisting of the encoded values of r and s concatenated in order.
These are both encoded as big-endian unsigned integers, zero-padded
to 48 octets each. The resulting signature value (r, s) is used as
input to the signature verification function. The results of the
verification function indicate whether the signature presented is
valid.

The use of this algorithm can be indicated at runtime using the
ecdsa-p384-sha384 value for the alg signature parameter.

3.3.6. EdDSA Using Curve edwards25519

To sign using this algorithm, the signer applies the Ed25519
algorithm defined in Section 5.1.6 of [RFC8032] with the signer's
private signing key and the signature base (Section 2.5). The
signature base is taken as the input message (M) with no prehash
function. The signature is a 64-octet concatenation of R and S as
specified in Section 5.1.6 of [RFC8032], and this is taken as a byte
array for the HTTP message signature output used in Section 3.1.

To verify using this algorithm, the signer applies the Ed25519
algorithm defined in Section 5.1.7 of [RFC8032] using the public key
portion of the verification key material (A) and the signature base
recreated as described in Section 3.2. The signature base is taken
as the input message (M) with no prehash function. The signature to
be verified is processed as the 64-octet concatenation of R and S as
specified in Section 5.1.7 of [RFC8032]. The results of the
verification function indicate whether the signature presented is
valid.

The use of this algorithm can be indicated at runtime using the
ed25519 value for the alg signature parameter.

3.3.7. JSON Web Signature (JWS) Algorithms

If the signing algorithm is a JSON Object Signing and Encryption
(JOSE) signing algorithm from the "JSON Web Signature and Encryption
Algorithms" registry established by [RFC7518], the JWS algorithm
definition determines the signature and hashing algorithms to apply
for both signing and verification.

For both signing and verification, the HTTP message's signature base
(Section 2.5) is used as the entire "JWS Signing Input". The JOSE
Header [JWS] [RFC7517] is not used, and the signature base is not
first encoded in Base64 before applying the algorithm. The output of
the JWS Signature is taken as a byte array prior to the Base64url
encoding used in JOSE.

The JWS algorithm MUST NOT be "none" and MUST NOT be any algorithm
with a JOSE Implementation Requirement of "Prohibited".

JSON Web Algorithm (JWA) values from the "JSON Web Signature and
Encryption Algorithms" registry are not included as signature
parameters. Typically, the JWS algorithm can be signaled using JSON
Web Keys (JWKs) or other mechanisms common to JOSE implementations.
In fact, JWA values are not registered in the "HTTP Signature
Algorithms" registry (Section 6.2), and so the explicit alg signature
parameter is not used at all when using JOSE signing algorithms.

4. Including a Message Signature in a Message

HTTP message signatures can be included within an HTTP message via
the Signature-Input and Signature fields, both defined within this
specification.

The Signature-Input field identifies the covered components and
parameters that describe how the signature was generated, while the
Signature field contains the signature value. Each field MAY contain
multiple labeled values.

An HTTP message signature is identified by a label within an HTTP
message. This label MUST be unique within a given HTTP message and
MUST be used in both the Signature-Input field and the Signature
field. The label is chosen by the signer, except where a specific
label is dictated by protocol negotiations such as those described in
Section 5.

An HTTP message signature MUST use both the Signature-Input field and
the Signature field, and each field MUST contain the same labels.
The presence of a label in one field but not the other is an error.

4.1. The Signature-Input HTTP Field

The Signature-Input field is a Dictionary Structured Field (defined
in Section 3.2 of [STRUCTURED-FIELDS]) containing the metadata for
one or more message signatures generated from components within the
HTTP message. Each member describes a single message signature. The
member's key is the label that uniquely identifies the message
signature within the HTTP message. The member's value is the covered
components ordered set serialized as an Inner List, including all
signature metadata parameters identified by the label:

NOTE: '\' line wrapping per RFC 8792

Signature-Input: sig1=("@method" "@target-uri" "@authority" \
"content-digest" "cache-control");\
created=1618884475;keyid="test-key-rsa-pss"

To facilitate signature validation, the Signature-Input field value
MUST contain the same serialized value used in generating the
signature base's @signature-params value defined in Section 2.3.
Note that in a Structured Field value, list order and parameter order
have to be preserved.

The signer MAY include the Signature-Input field as a trailer to
facilitate signing a message after its content has been processed by
the signer. However, since intermediaries are allowed to drop
trailers as per [HTTP], it is RECOMMENDED that the Signature-Input
field be included only as a header field to avoid signatures being
inadvertently stripped from a message.

Multiple Signature-Input fields MAY be included in a single HTTP
message. The signature labels MUST be unique across all field
values.

4.2. The Signature HTTP Field

The Signature field is a Dictionary Structured Field (defined in
Section 3.2 of [STRUCTURED-FIELDS]) containing one or more message
signatures generated from the signature context of the target
message. The member's key is the label that uniquely identifies the
message signature within the HTTP message. The member's value is a
Byte Sequence containing the signature value for the message
signature identified by the label:

NOTE: '\' line wrapping per RFC 8792

Signature: sig1=:P0wLUszWQjoi54udOtydf9IWTfNhy+r53jGFj9XZuP4uKwxyJo\
1RSHi+oEF1FuX6O29d+lbxwwBao1BAgadijW+7O/PyezlTnqAOVPWx9GlyntiCiHz\
C87qmSQjvu1CFyFuWSjdGa3qLYYlNm7pVaJFalQiKWnUaqfT4LyttaXyoyZW84jS8\
gyarxAiWI97mPXU+OVM64+HVBHmnEsS+lTeIsEQo36T3NFf2CujWARPQg53r58Rmp\
Z+J9eKR2CD6IJQvacn5A4Ix5BUAVGqlyp8JYm+S/CWJi31PNUjRRCusCVRj05NrxA\
BNFv3r5S9IXf2fYJK+eyW4AiGVMvMcOg==:

The signer MAY include the Signature field as a trailer to facilitate
signing a message after its content has been processed by the signer.
However, since intermediaries are allowed to drop trailers as per
[HTTP], it is RECOMMENDED that the Signature field be included only
as a header field to avoid signatures being inadvertently stripped
from a message.

Multiple Signature fields MAY be included in a single HTTP message.
The signature labels MUST be unique across all field values.

4.3. Multiple Signatures

Multiple distinct signatures MAY be included in a single message.
Each distinct signature MUST have a unique label. These multiple
signatures could all be added by the same signer, or they could come
from several different signers. For example, a signer may include
multiple signatures signing the same message components with
different keys or algorithms to support verifiers with different
capabilities, or a reverse proxy may include information about the
client in fields when forwarding the request to a service host,
including a signature over the client's original signature values.

The following non-normative example starts with a signed request from
the client. A reverse proxy takes this request and validates the
client's signature:

NOTE: '\' line wrapping per RFC 8792

POST /foo?param=Value&Pet=dog HTTP/1.1
Host: example.com
Date: Tue, 20 Apr 2021 02:07:55 GMT
Content-Type: application/json
Content-Length: 18
Content-Digest: sha-512=:WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+T\
aPm+AbwAgBWnrIiYllu7BNNyealdVLvRwEmTHWXvJwew==:
Signature-Input: sig1=("@method" "@authority" "@path" \
"content-digest" "content-type" "content-length")\
;created=1618884475;keyid="test-key-ecc-p256"
Signature: sig1=:X5spyd6CFnAG5QnDyHfqoSNICd+BUP4LYMz2Q0JXlb//4Ijpzp\
+kve2w4NIyqeAuM7jTDX+sNalzA8ESSaHD3A==:

{"hello": "world"}

The proxy then alters the message before forwarding it on to the
origin server, changing the target host and adding the Forwarded
header field defined in [RFC7239]:

NOTE: '\' line wrapping per RFC 8792

{"hello": "world"}

The proxy is in a position to validate the incoming client's
signature and make its own statement to the origin server about the
nature of the request that it is forwarding by adding its own
signature over the new message before passing it along to the origin
server. The proxy also includes all the elements from the original
message that are relevant to the origin server's processing. In many
cases, the proxy will want to cover all the same components that were
covered by the client's signature, which is the case in the following
example. Note that in this example, the proxy is signing over the
new authority value, which it has changed. The proxy also adds the
Forwarded header field to its own signature value. The proxy
identifies its own key and algorithm and, in this example, includes
an expiration for the signature to indicate to downstream systems
that the proxy will not vouch for this signed message past this short
time window. This results in a signature base of:

NOTE: '\' line wrapping per RFC 8792

"@method": POST
"@authority": origin.host.internal.example
"@path": /foo
"content-digest": sha-512=:WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX\
+TaPm+AbwAgBWnrIiYllu7BNNyealdVLvRwEmTHWXvJwew==:
"content-type": application/json
"content-length": 18
"forwarded": for=192.0.2.123;host=example.com;proto=https
"@signature-params": ("@method" "@authority" "@path" \
"content-digest" "content-type" "content-length" "forwarded")\
;created=1618884480;keyid="test-key-rsa";alg="rsa-v1_5-sha256"\
;expires=1618884540

and a signature output value of:

NOTE: '\' line wrapping per RFC 8792

S6ZzPXSdAMOPjN/6KXfXWNO/f7V6cHm7BXYUh3YD/fRad4BCaRZxP+JH+8XY1I6+8Cy\
+CM5g92iHgxtRPz+MjniOaYmdkDcnL9cCpXJleXsOckpURl49GwiyUpZ10KHgOEe11s\
x3G2gxI8S0jnxQB+Pu68U9vVcasqOWAEObtNKKZd8tSFu7LB5YAv0RAGhB8tmpv7sFn\
Im9y+7X5kXQfi8NMaZaA8i2ZHwpBdg7a6CMfwnnrtflzvZdXAsD3LH2TwevU+/PBPv0\
B6NMNk93wUs/vfJvye+YuI87HU38lZHowtznbLVdp770I6VHR6WfgS9ddzirrswsE1w\
5o0LV/g==

These values are added to the HTTP request message by the proxy. The
original signature is included under the identifier label sig1, and the reverse
proxy's signature is included under the label proxy_sig. The proxy
uses the key test-key-rsa to create its signature using the
rsa-v1_5-sha256 rsa-
v1_5-sha256 signature algorithm, while the client's original
signature was made using the keyid of key test-key-rsa-pss and an RSA-PSS
signature algorithm:

NOTE: '\' line wrapping per RFC 8792

POST /foo?param=Value&Pet=dog HTTP/1.1
Host: origin.host.internal.example
Date: Tue, 20 Apr 2021 02:07:56 GMT
Content-Type: application/json
Content-Length: 18
Forwarded: for=192.0.2.123;host=example.com;proto=https
Content-Digest: sha-512=:WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+T\
aPm+AbwAgBWnrIiYllu7BNNyealdVLvRwEmTHWXvJwew==:
Signature-Input: sig1=("@method" "@authority" "@path" \
"content-digest" "content-type" "content-length")\
;created=1618884475;keyid="test-key-ecc-p256", \
proxy_sig=("@method" "@authority" "@path" "content-digest" \
"content-type" "content-length" "forwarded")\
;created=1618884480;keyid="test-key-rsa";alg="rsa-v1_5-sha256"\
;expires=1618884540
Signature: sig1=:X5spyd6CFnAG5QnDyHfqoSNICd+BUP4LYMz2Q0JXlb//4Ijpzp\
+kve2w4NIyqeAuM7jTDX+sNalzA8ESSaHD3A==:, \
proxy_sig=:S6ZzPXSdAMOPjN/6KXfXWNO/f7V6cHm7BXYUh3YD/fRad4BCaRZxP+\
JH+8XY1I6+8Cy+CM5g92iHgxtRPz+MjniOaYmdkDcnL9cCpXJleXsOckpURl49G\
wiyUpZ10KHgOEe11sx3G2gxI8S0jnxQB+Pu68U9vVcasqOWAEObtNKKZd8tSFu7\
LB5YAv0RAGhB8tmpv7sFnIm9y+7X5kXQfi8NMaZaA8i2ZHwpBdg7a6CMfwnnrtf\
lzvZdXAsD3LH2TwevU+/PBPv0B6NMNk93wUs/vfJvye+YuI87HU38lZHowtznbL\
Vdp770I6VHR6WfgS9ddzirrswsE1w5o0LV/g==:

{"hello": "world"}

While the proxy could additionally include the client's Signature
field value and Signature-Input fields from the original message in
the new signature's covered components, this practice is NOT
RECOMMENDED due to known weaknesses in signing signature values as
discussed in Section 7.3.7. The proxy is in a position to validate
the client's signature; the changes the proxy makes to the message
will invalidate the existing signature when the message is seen by
the origin server. In this example, it is possible for the origin
server to have additional information in its signature context to
account for the change in authority, though this practice requires
additional configuration and extra care as discussed in
Section 7.4.4. In other applications, the origin server will not be
able to verify the original signature itself but will still want to
verify that the proxy has done the appropriate validation of the
client's signature. An application that needs to signal successful
processing or receipt of a signature would need to carefully specify
alternative mechanisms for sending such a signal securely.

5. Requesting Signatures

While a signer is free to attach a signature to a request or response
without prompting, it is often desirable for a potential verifier to
signal that it expects a signature from a potential signer using the
Accept-Signature field.

When the Accept-Signature field is sent in an HTTP request message,
the field indicates that the client desires the server to sign the
response using the identified parameters, and the target message is
the response to this request. All responses from resources that
support such signature negotiation SHOULD either be uncacheable or
contain a Vary header field that lists Accept-Signature, in order to
prevent a cache from returning a response with a signature intended
for a different request.

When the Accept-Signature field is used in an HTTP response message,
the field indicates that the server desires the client to sign its
next request to the server with the identified parameters, and the
target message is the client's next request. The client can choose
to also continue signing future requests to the same server in the
same way.

The target message of an Accept-Signature field MUST include all
labeled signatures indicated in the Accept-Signature field, each
covering the same identified components of the Accept-Signature
field.

The sender of an Accept-Signature field MUST include only identifiers
that are appropriate for the type of the target message. For
example, if the target message is a request, the covered components
cannot include the @status component identifier.

5.1. The Accept-Signature Field

The Accept-Signature field is a Dictionary Structured Field (defined
in Section 3.2 of [STRUCTURED-FIELDS]) containing the metadata for
one or more requested message signatures to be generated from message
components of the target HTTP message. Each member describes a
single message signature. The member's key is the label that
uniquely identifies the requested message signature within the
context of the target HTTP message.

The member's value is the serialization of the desired covered
components of the target message, including any allowed component
metadata parameters, using the serialization process defined in
Section 2.3:

NOTE: '\' line wrapping per RFC 8792

Accept-Signature: sig1=("@method" "@target-uri" "@authority" \
"content-digest" "cache-control");\
keyid="test-key-rsa-pss";created;tag="app-123"

The list of component identifiers indicates the exact set of
component identifiers to be included in the requested signature,
including all applicable component parameters.

The signature request MAY include signature metadata parameters that
indicate desired behavior for the signer. The following behavior is
defined by this specification:

created: The signer is requested to generate and include a creation
time. This parameter has no associated value when sent as a
signature request.

expires: The signer is requested to generate and include an
expiration time. This parameter has no associated value when sent
as a signature request.

nonce: The signer is requested to include the value of this
parameter as the signature nonce in the target signature.

alg: The signer is requested to use the indicated signature
algorithm from the "HTTP Signature Algorithms" registry to create
the target signature.

keyid: The signer is requested to use the indicated key material to
create the target signature.

tag: The signer is requested to include the value of this parameter
as the signature tag in the target signature.

5.2. Processing an Accept-Signature

The receiver of an Accept-Signature field fulfills that header as
follows:

1. Parse the field value as a Dictionary.

2. For each member of the Dictionary:

2.1. The key is taken as the label of the output signature as
specified in Section 4.1.

2.2. Parse the value of the member to obtain the set of covered
component identifiers.

2.3. Determine that the covered components are applicable to the
target message. If not, the process fails and returns an
error.

2.4. Process the requested parameters, such as the signing
algorithm and key material. If any requested parameters
cannot be fulfilled or if the requested parameters conflict
with those deemed appropriate to the target message, the
process fails and returns an error.

2.5. Select and generate any additional parameters necessary for
completing the signature.

2.6. Create the HTTP message signature over the target message.

2.7. Create the Signature-Input and Signature field values, and
associate them with the label.

3. Optionally create any additional Signature-Input and Signature
field values, with unique labels not found in the Accept-Signature Accept-
Signature field.

4. Combine all labeled Signature-Input and Signature field values,
and attach both fields to the target message.

By this process, a signature applied to a target message MUST have
the same label, MUST include the same set of covered components, MUST
process all requested parameters, and MAY have additional parameters.

The receiver of an Accept-Signature field MAY ignore any signature
request that does not fit application parameters.

The target message MAY include additional signatures not specified by
the Accept-Signature field. For example, to cover additional message
components, the signer can create a second signature that includes
the additional components as well as the signature output of the
requested signature.

6. IANA Considerations

IANA has updated one registry and created four new registries,
according to the following sections.

6.1. HTTP Field Name Registration

IANA has updated the entries in the "Hypertext Transfer Protocol
(HTTP) Field Name Registry" as follows:

Table 1: Updates to the Hypertext Transfer Protocol
(HTTP) Field Name Registry

6.2. HTTP Signature Algorithms Registry

This document defines HTTP signature algorithms, for which IANA has
created and now maintains a new registry titled "HTTP Signature
Algorithms". Initial values for this registry are given in
Section 6.2.2. Future assignments and modifications to existing
assignments are to be made through the Specification Required
registration policy [RFC8126].

The algorithms listed in this registry identify some possible
cryptographic algorithms for applications to use with this
specification, but the entries neither represent an exhaustive list
of possible algorithms nor indicate fitness for purpose with any
particular application of this specification. An application is free
to implement any algorithm that suits its needs, provided the signer
and verifier can agree to the parameters of that algorithm in a
secure and deterministic fashion. When an application needs to
signal the use of a particular algorithm at runtime using the alg
signature parameter, this registry provides a mapping between the
value of that parameter and a particular algorithm. However, the use
of the alg parameter needs to be treated with caution to avoid
various forms of algorithm confusion and substitution attacks, as
discussed in Section 7.

The Status value should reflect standardization status and the broad
opinion of relevant interest groups such as the IETF or security-
related Standards Development Organizations (SDOs). When an
algorithm is first registered, the designated expert (DE) should set
the Status field to "Active" if there is consensus for the algorithm
to be generally recommended as secure or "Provisional" if the
algorithm has not reached that consensus, e.g., for an experimental
algorithm. A status of "Provisional" does not mean that the
algorithm is known to be insecure but instead indicates that the
algorithm has not reached consensus regarding its properties. If at
a future time the algorithm as registered is found to have flaws, the
registry entry can be updated and the algorithm can be marked as
"Deprecated" to indicate that the algorithm has been found to have
problems. This status does not preclude an application from using a
particular algorithm; rather, it serves to provide a warning
regarding possible known issues with an algorithm that need to be
considered by the application. The DE can further ensure that the
registration includes an explanation and reference for the Status
value; this is particularly important for deprecated algorithms.

The DE is expected to do the following:

* Ensure that the algorithms referenced by a registered algorithm
identifier are fully defined with all parameters (e.g., salt,
hash, required key length) fixed by the defining text.

* Ensure that the algorithm definition fully specifies the HTTP_SIGN
and HTTP_VERIFY primitive functions, including how all defined
inputs and outputs map to the underlying cryptographic algorithm.

* Reject any registrations that are aliases of existing
registrations.

* Ensure that all registrations follow the template presented in
Section 6.2.1; this includes ensuring that the length of the name
is not excessive while still being unique and recognizable.

This specification creates algorithm identifiers by including major
parameters in the identifier string String in order to make the algorithm
name unique and recognizable by developers. However, algorithm
identifiers in this registry are to be interpreted as whole string String
values and not as a combination of parts. That is to say, it is
expected that implementors understand rsa-pss-sha512 as referring to
one specific algorithm with its hash, mask, and salt values set as
defined in the defining text that establishes the identifier in
question. Implementors do not parse out the rsa, pss, and sha512
portions of the identifier to determine parameters of the signing
algorithm from the string, String, and the registration of one combination of
parameters does not imply the registration of other combinations.

6.2.1. Registration Template

Algorithm Name:
An identifier for the HTTP signature algorithm. The name MUST be
an ASCII string that conforms to the sf-string ABNF rule in
Section 3.3.3 of [STRUCTURED-FIELDS] and SHOULD NOT exceed 20
characters in length. The identifier MUST be unique within the
context of the registry.

Description:
A brief description of the algorithm used to sign the signature
base.

Status:
The status of the algorithm. MUST start with one of the following
values and MAY contain additional explanatory text. The options
are:

"Active": For algorithms without known problems. The signature
algorithm is fully specified, and its security properties are
understood.

"Provisional": For unproven algorithms. The signature algorithm
is fully specified, but its security properties are not known
or proven.

"Deprecated": For algorithms with known security issues. The
signature algorithm is no longer recommended for general use
and might be insecure or unsafe in some known circumstances.

Specification document(s):

Reference:
Reference to the document or documents that specify the algorithm,
preferably including a URI that can be used to retrieve a copy of
the document(s). An indication of the relevant sections may also
be included but is not required.

6.2.2. Initial Contents

The table below contains the initial contents of the "HTTP Signature
Algorithms" registry.

Table 2: Initial Contents of the HTTP Signature Algorithms
Registry

6.3. HTTP Signature Metadata Parameters Registry

This document defines the signature parameters structure
(Section 2.3), which may have parameters containing metadata about a
message signature. IANA has created and now maintains a new registry
titled "HTTP Signature Metadata Parameters" to record and maintain
the set of parameters defined for use with member values in the
signature parameters structure. Initial values for this registry are
given in Section 6.3.2. Future assignments and modifications to
existing assignments are to be made through the Expert Review
registration policy [RFC8126].

The DE is expected to do the following:

* Ensure that the name follows the template presented in
Section 6.3.1; this includes ensuring that the length of the name
is not excessive while still being unique and recognizable for its
defined function.

* Ensure that the defined functionality is clear and does not
conflict with other registered parameters.

* Ensure that the definition of the metadata parameter includes its
behavior when used as part of the normal signature process as well
as when used in an Accept-Signature field.

6.3.1. Registration Template

Name:
An identifier for the HTTP signature metadata parameter. The name
MUST be an ASCII string that conforms to the key ABNF rule defined
in Section 3.1.2 of [STRUCTURED-FIELDS] and SHOULD NOT exceed 20
characters in length. The identifier MUST be unique within the
context of the registry.

Description:
A brief description of the metadata parameter and what it
represents.

Specification document(s):

Reference:
Reference to the document or documents that specify the parameter,
preferably including a URI that can be used to retrieve a copy of
the document(s). An indication of the relevant sections may also
be included but is not required.

6.3.2. Initial Contents

The table below contains the initial contents of the "HTTP Signature
Metadata Parameters" registry. Each row in the table represents a
distinct entry in the registry.

+=========+===============================+===============+

Table 3: Initial Contents of the HTTP Signature
Metadata Parameters Registry

6.4. HTTP Signature Derived Component Names Registry

This document defines a method for canonicalizing HTTP message
components, including components that can be derived from the context
of the target message outside of the HTTP fields. These derived
components are identified by a unique string, String, known as the component
name. Component names for derived components always start with the
"at" (@) symbol to distinguish them from HTTP field names. IANA has
created and now maintains a new registry titled "HTTP Signature
Derived Component Names" to record and maintain the set of non-field
component names and the methods used to produce their associated
component values. Initial values for this registry are given in
Section 6.4.2. Future assignments and modifications to existing
assignments are to be made through the Expert Review registration
policy [RFC8126].

The DE is expected to do the following:

* Ensure that the name follows the template presented in
Section 6.4.1; this includes ensuring that the length of the name
is not excessive while still being unique and recognizable for its
defined function.

* Ensure that the component value represented by the registration
request can be deterministically derived from the target HTTP
message.

* Ensure that any parameters defined for the registration request
are clearly documented, along with their effects on the component
value.

The DE should ensure that a registration is sufficiently distinct
from existing derived component definitions to warrant its
registration.

When setting a registered item's status to "Deprecated", the DE
should ensure that a reason for the deprecation is documented, along
with instructions for moving away from the deprecated functionality.

6.4.1. Registration Template

Name:
A name for the HTTP derived component. The name MUST begin with
the "at" (@) character followed by an ASCII string consisting only
of lowercase characters ("a"-"z"), digits ("0"-"9"), and hyphens
("-"), and SHOULD NOT exceed 20 characters in length. The name
MUST be unique within the context of the registry.

Description:
A description of the derived component.

Status:
A brief text description of the status of the algorithm. The
description MUST begin with one of "Active" or "Deprecated" and
MAY provide further context or explanation as to the reason for
the status. A value of "Deprecated" indicates that the derived
component name is no longer recommended for use.

Target:
The valid message targets for the derived parameter. MUST be one
of the values "Request", "Response", or "Request, Response". The
semantics of these entries are defined in Section 2.2.

Specification document(s):

Reference:
Reference to the document or documents that specify the derived
component, preferably including a URI that can be used to retrieve
a copy of the document(s). An indication of the relevant sections
may also be included but is not required.

6.4.2. Initial Contents

The table below contains the initial contents of the "HTTP Signature
Derived Component Names" registry.

+===================+==============+======+========+===============+

Table 4: Initial Contents of the HTTP Signature Derived
Component Names Registry

6.5. HTTP Signature Component Parameters Registry

This document defines several kinds of component identifiers, some of
which can be parameterized in specific circumstances to provide
unique modified behavior. IANA has created and now maintains a new
registry titled "HTTP Signature Component Parameters" to record and
maintain the set of parameter names, the component identifiers they
are associated with, and the modifications these parameters make to
the component value. Definitions of parameters MUST define the
targets to which they apply (such as specific field types, derived
components, or contexts). Initial values for this registry are given
in Section 6.5.2. Future assignments and modifications to existing
assignments are to be made through the Expert Review registration
policy [RFC8126].

The DE is expected to do the following:

* Ensure that the name follows the template presented in
Section 6.5.1; this includes ensuring that the length of the name
is not excessive while still being unique and recognizable for its
defined function.

* Ensure that the definition of the field sufficiently defines any
interactions or incompatibilities with other existing parameters
known at the time of the registration request.

* Ensure that the component value defined by the component
identifier with the parameter applied can be deterministically
derived from the target HTTP message in cases where the parameter
changes the component value.

6.5.1. Registration Template

Name:
A name for the parameter. The name MUST be an ASCII string that
conforms to the key ABNF rule defined in Section 3.1.2 of
[STRUCTURED-FIELDS] and SHOULD NOT exceed 20 characters in length.
The name MUST be unique within the context of the registry.

Description:
A description of the parameter's function.

Specification document(s):

6.5.2. Initial Contents

The table below contains the initial contents of the "HTTP Signature
Component Parameters" registry.

+======+==============================+===========================+

Table 5: Initial Contents of the HTTP Signature Component
Parameters Registry

7. Security Considerations

In order for an HTTP message to be considered _covered_ by a
signature, all of the following conditions have to be true:

* A signature is expected or allowed on the message by the verifier.

* The signature exists on the message.

* The signature is verified against the identified key material and
algorithm.

* The key material and algorithm are appropriate for the context of
the message.

* The signature is within expected time boundaries.

* The signature covers the expected content, including any critical
components.

* The list of covered components is applicable to the context of the
message.

In addition to the application requirement definitions listed in
Section 1.4, the following security considerations provide discussion
and context regarding the requirements of creating and verifying
signatures on HTTP messages.

7.1. General Considerations

7.1.1. Skipping Signature Verification

HTTP Message Signatures message signatures only provide security if the signature is
verified by the verifier. Since the message to which the signature
is attached remains a valid HTTP message without the Signature or
Signature-Input fields, it is possible for a verifier to ignore the
output of the verification function and still process the message.
Common reasons for this could be relaxed requirements in a
development environment or a temporary suspension of enforcing
verification while debugging an overall system. Such temporary
suspensions are difficult to detect under positive-example testing,
since a good signature will always trigger a valid response whether
or not it has been checked.

To detect this, verifiers should be tested using both valid and
invalid signatures, ensuring that an invalid signature fails as
expected.

7.1.2. Use of TLS

The use of HTTP Message Signatures message signatures does not negate the need for TLS
or its equivalent to protect information in transit. Message
signatures provide message integrity over the covered message
components but do not provide any confidentiality for communication
between parties.

TLS provides such confidentiality between the TLS endpoints. As part
of this, TLS also protects the signature data itself from being
captured by an attacker. This is an important step in preventing
signature replay (Section 7.2.2).

When TLS is used, it needs to be deployed according to the
recommendations provided in [BCP195].

7.2. Message Processing and Selection

7.2.1. Insufficient Coverage

Any portions of the message not covered by the signature are
susceptible to modification by an attacker without affecting the
signature. An attacker can take advantage of this by introducing or
modifying a header field or other message component that will change
the processing of the message but will not be covered by the
signature. Such an altered message would still pass signature
verification, but when the verifier processes the message as a whole,
the unsigned content injected by the attacker would subvert the trust
conveyed by the valid signature and change the outcome of processing
the message.

To combat this, an application of this specification should require
as much of the message as possible to be signed, within the limits of
the application and deployment. The verifier should only trust
message components that have been signed. Verifiers could also strip
out any sensitive unsigned portions of the message before processing
of the message continues.

7.2.2. Signature Replay

Since HTTP Message Signatures message signatures allow sub-portions of the HTTP message
to be signed, it is possible for two different HTTP messages to
validate against the same signature. The most extreme form of this
would be a signature over no message components. If such a signature
were intercepted, it could be replayed at will by an attacker,
attached to any HTTP message. Even with sufficient component
coverage, a given signature could be applied to two similar HTTP
messages, allowing a message to be replayed by an attacker with the
signature intact.

To counteract these kinds of attacks, it's first important for the
signer to cover sufficient portions of the message to differentiate
it from other messages. In addition, the signature can use the nonce
signature parameter to provide a per-message unique value to allow
the verifier to detect replay of the signature itself if a nonce
value is repeated. Furthermore, the signer can provide a timestamp
for when the signature was created and a time at which the signer
considers the signature to be expired, limiting the utility of a
captured signature value.

If a verifier wants to trigger a new signature from a signer, it can
send the Accept-Signature header field with a new nonce parameter.
An attacker that is simply replaying a signature would not be able to
generate a new signature with the chosen nonce value.

7.2.3. Choosing Message Components

Applications of HTTP Message Signatures message signatures need to decide which message
components will be covered by the signature. Depending on the
application, some components could be expected to be changed by
intermediaries prior to the signature's verification. If these
components are covered, such changes would, by design, break the
signature.

However, this document allows for flexibility in determining which
components are signed precisely so that a given application can
choose the appropriate portions of the message that need to be
signed, avoiding problematic components. For example, a web
application framework that relies on rewriting query parameters might
avoid using the @query derived component in favor of sub-indexing the
query value using @query-param derived components instead.

Some components are expected to be changed by intermediaries and
ought not to be signed under most circumstances. The Via and
Forwarded header fields, for example, are expected to be manipulated
by proxies and other middleboxes, including replacing or entirely
dropping existing values. These fields should not be covered by the
signature, except in very limited and tightly coupled scenarios.

Additional considerations for choosing signature aspects are
discussed in Section 1.4.

7.2.4. Choosing Signature Parameters and Derived Components over HTTP
Fields

Some HTTP fields have values and interpretations that are similar to
HTTP signature parameters or derived components. In most cases, it
is more desirable to sign the non-field alternative. In particular,
the following fields should usually not be included in the signature
unless the application specifically requires it:

"date" The "date" Date header field value represents the timestamp of the
HTTP message. However, the creation time of the signature itself
is encoded in the created signature parameter. These two values
can be different, depending on how the signature and the HTTP
message are created and serialized. Applications processing
signatures for valid time windows should use the created signature
parameter for such calculations. An application could also put
limits on how much skew there is between the "date" Date field and the
created signature parameter, in order to limit the application of
a generated signature to different HTTP messages. See also
Sections 7.2.2 and 7.2.1.

"host" The "host" Host header field is specific to HTTP/1.1, and its
functionality is subsumed by the @authority derived component,
defined in Section 2.2.3. In order to preserve the value across
different HTTP versions, applications should always use the
@authority derived component. See also Section 7.5.4.

7.2.5. Signature Labels

HTTP Message Signature message signature values are identified in the Signature and
Signature-Input field values by unique labels. These labels are
chosen only when attaching the signature values to the message and
are not accounted for during the signing process. An intermediary is
allowed to relabel an existing signature when processing the message.

Therefore, applications should not rely on specific labels being
present, and applications should not put semantic meaning on the
labels themselves. Instead, additional signature parameters can be
used to convey whatever additional meaning is required to be attached
to, and covered by, the signature. In particular, the tag parameter
can be used to define an application-specific value as described in
Section 7.2.7.

7.2.6. Multiple Signature Confusion

Since multiple signatures can be applied to one message
(Section 4.3), it is possible for an attacker to attach their own
signature to a captured message without modifying existing
signatures. This new signature could be completely valid based on
the attacker's key, or it could be an invalid signature for any
number of reasons. Each of these situations needs to be accounted
for.

A verifier processing a set of valid signatures needs to account for
all of the signers, identified by the signing keys. Only signatures
from expected signers should be accepted, regardless of the
cryptographic validity of the signature itself.

A verifier processing a set of signatures on a message also needs to
determine what to do when one or more of the signatures are not
valid. If a message is accepted when at least one signature is
valid, then a verifier could drop all invalid signatures from the
request before processing the message further. Alternatively, if the
verifier rejects a message for a single invalid signature, an
attacker could use this to deny service to otherwise valid messages
by injecting invalid signatures alongside the valid signatures.

7.2.7. Collision of Application-Specific Signature Tag

Multiple applications and protocols could apply HTTP signatures on
the same message simultaneously. In fact, this is a desired feature
in many circumstances; see Section 4.3. A naive verifier could
become confused while processing multiple signatures, either
accepting or rejecting a message based on an unrelated or irrelevant
signature. In order to help an application select which signatures
apply to its own processing, the application can declare a specific
value for the tag signature parameter as defined in Section 2.3. For
example, a signature targeting an application gateway could require
tag="app-gateway" as part of the signature parameters for that
application.

The use of the tag parameter does not prevent an attacker from also
using the same value as a target application, since the parameter's
value is public and otherwise unrestricted. As a consequence, a
verifier should only use a value of the tag parameter to limit which
signatures to check. Each signature still needs to be examined by
the verifier to ensure that sufficient coverage is provided, as
discussed in Section 7.2.1.

7.2.8. Message Content

On its own, this specification does not provide coverage for the
content of an HTTP message under the signature, in either a request
or a response. However, [DIGEST] defines a set of fields that allow
a cryptographic digest of the content to be represented in a field.
Once this field is created, it can be included just like any other
field as defined in Section 2.1.

For example, in the following response message:

HTTP/1.1 200 OK
Content-Type: application/json

{"hello": "world"}

The digest of the content can be added to the Content-Digest field as
follows:

NOTE: '\' line wrapping per RFC 8792

HTTP/1.1 200 OK
Content-Type: application/json
Content-Digest: \
sha-256=:X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=:

{"hello": "world"}

This field can be included in a signature base just like any other
field along with the basic signature parameters:

"@status": 200
"content-digest": \
sha-256=:X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=:
"@signature-input": ("@status" "content-digest")

From here, the signing process proceeds as usual.

Upon verification, it is important that the verifier validate not
only the signature but also the value of the Content-Digest field
itself against the actual received content. Unless the verifier
performs this step, it would be possible for an attacker to
substitute the message content but leave the Content-Digest field
value untouched to pass the signature. Since only the field value is
covered by the signature directly, checking only the signature is not
sufficient protection against such a substitution attack.

As discussed in [DIGEST], the value of the Content-Digest field is
dependent on the content encoding of the message. If an intermediary
changes the content encoding, the resulting Content-Digest value
would change. This would in turn invalidate the signature. Any
intermediary performing such an action would need to apply a new
signature with the updated Content-Digest field value, similar to the
reverse proxy use case discussed in Section 4.3.

Applications that make use of the req parameter (Section 2.4) also
need to be aware of the limitations of this functionality.
Specifically, if a client does not include something like a Content-
Digest header field in the request, the server is unable to include a
signature that covers the request's content.

7.3. Cryptographic Considerations

7.3.1. Cryptography and Signature Collision

This document does not define any of its own cryptographic primitives
and instead relies on other specifications to define such elements.
If the signature algorithm or key used to process the signature base
is vulnerable to any attacks, the resulting signature will also be
susceptible to these same attacks.

A common attack against signature systems is to force a signature
collision, where the same signature value successfully verifies
against multiple different inputs. Since this specification relies
on reconstruction of the signature base from an HTTP message and the
list of components signed is fixed in the signature, it is difficult
but not impossible for an attacker to effect such a collision. An
attacker would need to manipulate the HTTP message and its covered
message components in order to make the collision effective.

To counter this, only vetted keys and signature algorithms should be
used to sign HTTP messages. The "HTTP Signature Algorithms" registry
is one source of trusted signature algorithms for applications to
apply to their messages.

While it is possible for an attacker to substitute the signature
parameters value or the signature value separately, the signature
base generation algorithm (Section 2.5) always covers the signature
parameters as the final value in the signature base using a
deterministic serialization method. This step strongly binds the
signature base with the signature value in a way that makes it much
more difficult for an attacker to perform a partial substitution on
the signature base.

7.3.2. Key Theft

A foundational assumption of signature-based cryptographic systems is
that the signing key is not compromised by an attacker. If the keys
used to sign the message are exfiltrated or stolen, the attacker will
be able to generate their own signatures using those keys. As a
consequence, signers have to protect any signing key material from
exfiltration, capture, and use by an attacker.

To combat this, signers can rotate keys over time to limit the amount
of time that stolen keys are useful. Signers can also use key escrow
and storage systems to limit the attack surface against keys.
Furthermore, the use of asymmetric signing algorithms exposes key
material less than the use of symmetric signing algorithms
(Section 7.3.3).

7.3.3. Symmetric Cryptography

This document allows both asymmetric and symmetric cryptography to be
applied to HTTP messages. By their nature, symmetric cryptographic
methods require the same key material to be known by both the signer
and verifier. This effectively means that a verifier is capable of
generating a valid signature, since they have access to the same key
material. An attacker that is able to compromise a verifier would be
able to then impersonate a signer.

Where possible, asymmetric methods or secure key agreement mechanisms
should be used in order to avoid this type of attack. When symmetric
methods are used, distribution of the key material needs to be
protected by the overall system. One technique for this is the use
of separate cryptographic modules that separate the verification
process (and therefore the key material) from other code, minimizing
the vulnerable attack surface. Another technique is the use of key
derivation functions that allow the signer and verifier to agree on
unique keys for each message without having to share the key values
directly.

Additionally, if symmetric algorithms are allowed within a system,
special care must be taken to avoid key downgrade attacks
(Section 7.3.6).

7.3.4. Key Specification Mixup

The existence of a valid signature on an HTTP message is not
sufficient to prove that the message has been signed by the
appropriate party. It is up to the verifier to ensure that a given
key and algorithm are appropriate for the message in question. If
the verifier does not perform such a step, an attacker could
substitute their own signature using their own key on a message and
force a verifier to accept and process it. To combat this, the
verifier needs to ensure not only that the signature can be validated
for a message but that the key and algorithm used are appropriate.

7.3.5. Non-deterministic Signature Primitives

Some cryptographic primitives, such as RSA-PSS and ECDSA, have non-
deterministic outputs, which include some amount of entropy within
the algorithm. For such algorithms, multiple signatures generated in
succession will not match. A lazy implementation of a verifier could
ignore this distinction and simply check for the same value being
created by re-signing the signature base. Such an implementation
would work for deterministic algorithms such as HMAC and EdDSA but
fail to verify valid signatures made using non-deterministic
algorithms. It is therefore important that a verifier always use the
correctly defined verification function for the algorithm in question
and not do a simple comparison.

7.3.6. Key and Algorithm Specification Downgrades

Applications of this specification need to protect against key
specification downgrade attacks. For example, the same RSA key can
be used for both RSA-PSS and RSA v1.5 signatures. If an application
expects a key to only be used with RSA-PSS, it needs to reject
signatures for any key that uses the weaker RSA 1.5 specification.

Another example of a downgrade attack would be when an asymmetric
algorithm is expected, such as RSA-PSS, but an attacker substitutes a
signature using a symmetric algorithm, such as HMAC. A naive
verifier implementation could use the value of the public RSA key as
the input to the HMAC verification function. Since the public key is
known to the attacker, this would allow the attacker to create a
valid HMAC signature against this known key. To prevent this, the
verifier needs to ensure that both the key material and the algorithm
are appropriate for the usage in question. Additionally, while this
specification does allow runtime specification of the algorithm using
the alg signature parameter, applications are encouraged to use other
mechanisms such as static configuration or a higher-protocol-level
algorithm specification instead, preventing an attacker from
substituting the algorithm specified.

7.3.7. Signing Signature Values

When applying the req parameter (Section 2.4) or multiple signatures
(Section 4.3) to a message, it is possible to sign the value of an
existing Signature field, thereby covering the bytes of the existing
signature output in the new signature's value. While it would seem
that this practice would transitively cover the components under the
original signature in a verifiable fashion, the attacks described in
[JACKSON2019] can be used to impersonate a signature output value on
an unrelated message.

In this example, Alice intends to send a signed request to Bob, and
Bob wants to provide a signed response to Alice that includes a
cryptographic proof that Bob is responding to Alice's incoming
message. Mallory wants to intercept this traffic and replace Alice's
message with her own, without Alice being aware that the interception
has taken place.

1. Alice creates a message Req_A and applies a signature Sig_A
using her private key Key_A_Sign.

2. Alice believes she is sending Req_A to Bob.

3. Mallory intercepts Req_A and reads the value Sig_A from this
message.

4. Mallory generates a different message Req_M to send to Bob
instead.

5. Mallory crafts a signing key Key_M_Sign such that she can create
a valid signature Sig_M over her request Req_M using this key,
but the byte value of Sig_M exactly equals that of Sig_A.

6. Mallory sends Req_M with Sig_M to Bob.

7. Bob validates Sig_M against Mallory's verification key
Key_M_Verify. At no time does Bob think that he's responding to
Alice.

8. Bob responds with response message Res_B to Req_M and creates
signature Sig_B over this message using his key Key_B_Sign. Bob
includes the value of Sig_M under Sig_B's covered components but
does not include anything else from the request message.

9. Mallory receives the response Res_B from Bob, including the
signature Sig_B value. Mallory replays this response to Alice.

10. Alice reads Res_B from Mallory and verifies Sig_B using Bob's
verification key Key_B_Verify. Alice includes the bytes of her
original signature Sig_A in the signature base, and the
signature verifies.

11. Alice is led to believe that Bob has responded to her message
and believes she has cryptographic proof of this happening, but
in fact Bob responded to Mallory's malicious request and Alice
is none the wiser.

To mitigate this, Bob can sign more portions of the request message
than just the Signature field, in order to more fully differentiate
Alice's message from Mallory's. Applications using this feature,
particularly for non-repudiation purposes, can stipulate that any
components required in the original signature also be covered
separately in the second signature. For signed messages, requiring
coverage of the corresponding Signature-Input field of the first
signature ensures that unique items such as nonces and timestamps are
also covered sufficiently by the second signature.

7.4. Matching Signature Parameters to the Target Message

7.4.1. Modification of Required Message Parameters

An attacker could effectively deny a service by modifying an
otherwise benign signature parameter or signed message component.
While rejecting a modified message is the desired behavior,
consistently failing signatures could lead to (1) the verifier
turning off signature checking in order to make systems work again
(see Section 7.1.1) or (2) the application minimizing the
requirements related to the signed component.

If such failures are common within an application, the signer and
verifier should compare their generated signature bases with each
other to determine which part of the message is being modified. If
an expected modification is found, the signer and verifier can agree
on an alternative set of requirements that will pass. However, the
signer and verifier should not remove the requirement to sign the
modified component when it is suspected that an attacker is modifying
the component.

7.4.2. Matching Values of Covered Components to Values in the Target
Message

The verifier needs to make sure that the signed message components
match those in the message itself. For example, the @method derived
component requires that the value within the signature base be the
same as the HTTP method used when presenting this message. This
specification encourages this by requiring the verifier to derive the
signature base from the message, but lazy caching or conveyance of a
raw signature base to a processing subsystem could lead to downstream
verifiers accepting a message that does not match the presented
signature.

To counter this, the component that generates the signature base
needs to be trusted by both the signer and verifier within a system.

7.4.3. Message Component Source and Context

The signature context for deriving message component values includes
the target HTTP Message message itself, any associated messages (such as the
request that triggered a response), and additional information that
the signer or verifier has access to. Both signers and verifiers
need to carefully consider the source of all information when
creating component values for the signature base and take care not to
take information from untrusted sources. Otherwise, an attacker
could leverage such a loosely defined message context to inject their
own values into the signature base string, overriding or corrupting
the intended values.

For example, in most situations, the target URI of the message is as
defined in [HTTP], Section 7.1. However, let's say that there is an
application that requires signing of the @authority of the incoming
request, but the application doing the processing is behind a reverse
proxy. Such an application would expect a change in the @authority
value, and it could be configured to know the external target URI as
seen by the client on the other side of the proxy. This application
would use this configured value as its target URI for the purposes of
deriving message component values such as @authority instead of using
the target URI of the incoming message.

This approach is not without problems, as a misconfigured system
could accept signed requests intended for different components in the
system. For this scenario, an intermediary could instead add its own
signature to be verified by the application directly, as demonstrated
in Section 4.3. This alternative approach requires a more active
intermediary but relies less on the target application knowing
external configuration values.

As another example, Section 2.4 defines a method for signing response
messages and also including portions of the request message that
triggered the response. In this case, the context for component
value calculation is the combination of the response and request
messages, not just the single message to which the signature is
applied. For this feature, the req flag allows both signers to
explicitly signal which part of the context is being sourced for a
component identifier's value. Implementations need to ensure that
only the intended message is being referred to for each component;
otherwise, an attacker could attempt to subvert a signature by
manipulating one side or the other.

7.4.4. Multiple Message Component Contexts

It is possible that the context for deriving message component values
could be distinct for each signature present within a single message.
This is particularly the case when proxies mutate messages and
include signatures over the mutated values, in addition to any
existing signatures. For example, a reverse proxy can replace a
public hostname in a request to a service with the hostname for the
individual service host to which it is forwarding the request. If
both the client and the reverse proxy add signatures covering
@authority, the service host will see two signatures on the request,
each signing different values for the @authority message component,
reflecting the change to that component as the message made its way
from the client to the service host.

In such a case, it's common for the internal service to verify only
one of the signatures or to use externally configured information, as
discussed in Section 7.4.3. However, a verifier processing both
signatures has to use a different message component context for each
signature, since the component value for the @authority component
will be different for each signature. Verifiers like this need to be
aware of both the reverse proxy's context for incoming messages and
the target service's context for the message coming from the reverse
proxy. The verifier needs to take particular care to apply the
correct context to the correct signature; otherwise, an attacker
could use knowledge of this complex setup to confuse the inputs to
the verifier.

Such verifiers also need to ensure that any differences in message
component contexts between signatures are expected and permitted.
For example, in the above scenario, the reverse proxy could include
the original hostname in a Forwarded header field and could sign
@authority, forwarded, and the client's entry in the Signature field.
The verifier can use the hostname from the Forwarded header field to
confirm that the hostname was transformed as expected.

7.5. HTTP Processing

7.5.1. Processing Invalid HTTP Field Names as Derived Component Names

The definition of HTTP field names does not allow for the use of the
@ character anywhere in the name. As such, since all derived
component names start with the @ character, these namespaces should
be completely separate. However, some HTTP implementations are not
sufficiently strict about the characters accepted in HTTP field
names. In such implementations, a sender (or attacker) could inject
a header field starting with an @ character and have it passed
through to the application code. These invalid header fields could
be used to override a portion of the derived message content and
substitute an arbitrary value, providing a potential place for an
attacker to mount a signature collision (Section 7.3.1) attack or
other functional substitution attack (such as using the signature
from a GET request on a crafted POST request).

To combat this, when selecting values for a message component, if the
component name starts with the @ character, it needs to be processed
as a derived component and never processed as an HTTP field. Only if
the component name does not start with the @ character can it be
taken from the fields of the message. The algorithm discussed in
Section 2.5 provides a safe order of operations.

7.5.2. Semantically Equivalent Field Values

The signature base generation algorithm (Section 2.5) uses the value
of an HTTP field as its component value. In the common case, this
amounts to taking the actual bytes of the field value as the
component value for both the signer and verifier. However, some
field values allow for transformation of the values in semantically
equivalent ways that alter the bytes used in the value itself. For
example, a field definition can declare some or all of its values to
be case insensitive or to have special handling of internal
whitespace characters. Other fields have expected transformations
from intermediaries, such as the removal of comments in the Via
header field. In such cases, a verifier could be tripped up by using
the equivalent transformed field value, which would differ from the
byte value used by the signer. The verifier would have a difficult
time finding this class of errors, since the value of the field is
still acceptable for the application but the actual bytes required by
the signature base would not match.

When processing such fields, the signer and verifier have to agree on
how to handle such transformations, if at all. One option is to not
sign problematic fields, but care must be taken to ensure that there
is still sufficient signature coverage (Section 7.2.1) for the
application. Another option is to define an application-specific
canonicalization value for the field before it is added to the HTTP
message, such as to always remove internal comments before signing or
to always transform values to lowercase. Since these transformations
are applied prior to the field being used as input to the signature
base generation algorithm, the signature base will still simply
contain the byte value of the field as it appears within the message.
If the transformations were to be applied after the value is
extracted from the message but before it is added to the signature
base, different attack surfaces such as value substitution attacks
could be launched against the application. All application-specific
additional rules are outside the scope of this specification, and by
their very nature these transformations would harm interoperability
of the implementation outside of this specific application. It is
recommended that applications avoid the use of such additional rules
wherever possible.

7.5.3. Parsing Structured Field Values

Several parts of this specification rely on the parsing of Structured
Field values [STRUCTURED-FIELDS] -- in particular, strict
serialization of HTTP Structured Field values (Section 2.1.1),
referencing members of a Dictionary Structured Field (Section 2.1.2),
and processing the @signature-input value when verifying a signature
(Section 3.2). While Structured Field values are designed to be
relatively simple to parse, a naive or broken implementation of such
a parser could lead to subtle attack surfaces being exposed in the
implementation.

For example, if a buggy parser of the @signature-input value does not
enforce proper closing of quotes around string values within the list
of component identifiers, an attacker could take advantage of this
and inject additional content into the signature base through
manipulating the Signature-Input field value on a message.

To counteract this, implementations should use fully compliant and
trusted parsers for all Structured Field processing, on both the
signer side and the verifier side.

7.5.4. HTTP Versions and Component Ambiguity

Some message components are expressed in different ways across HTTP
versions. For example, the authority of the request target is sent
using the Host header field in HTTP/1.1 but with the :authority
pseudo-header in HTTP/2. If a signer sends an HTTP/1.1 message and
signs the Host header field but the message is translated to HTTP/2
before it reaches the verifier, the signature will not validate, as
the Host header field could be dropped.

It is for this reason that HTTP Message Signatures message signatures define a set of
derived components that define a single way to get the value in
question, such as the @authority derived component (Section 2.2.3) in
lieu of the Host header field. Applications should therefore prefer
derived components for such options where possible.

7.5.5. Canonicalization Attacks

Any ambiguity in the generation of the signature base could provide
an attacker with leverage to substitute or break a signature on a
message. Some message component values, particularly HTTP field
values, are potentially susceptible to broken implementations that
could lead to unexpected and insecure behavior. Naive
implementations of this specification might implement HTTP field
processing by taking the single value of a field and using it as the
direct component value without processing it appropriately.

For example, if the handling of obs-fold field values does not remove
the internal line folding and whitespace, additional newlines could
be introduced into the signature base by the signer, providing a
potential place for an attacker to mount a signature collision
(Section 7.3.1) attack. Alternatively, if header fields that appear
multiple times are not joined into a single string value, as required
by this specification, similar attacks can be mounted, as a signed
component value would show up in the signature base more than once
and could be substituted or otherwise attacked in this way.

To counter this, the entire field value processing algorithm needs to
be implemented by all implementations of signers and verifiers.

7.5.6. Non-List Field Values

When an HTTP field occurs multiple times in a single message, these
values need to be combined into a single one-line string value to be
included in the HTTP signature base, as described in Section 2.5.
Not all HTTP fields can be combined into a single value in this way
and still be a valid value for the field. For the purposes of
generating the signature base, the message component value is never
meant to be read back out of the signature base string or used in the
application. Therefore, it is considered best practice to treat the
signature base generation algorithm separately from processing the
field values by the application, particularly for fields that are
known to have this property. If the field values that are being
signed do not validate, the signed message should also be rejected.

If an HTTP field allows for unquoted commas within its values,
combining multiple field values can lead to a situation where two
semantically different messages produce the same line in a signature
base. For example, take the following hypothetical header field with
an internal comma in its syntax, here used to define two separate
lists of values:

Example-Header: value, with, lots
Example-Header: of, commas

For this header field, sending all of these values as a single field
value results in a single list of values:

Example-Header: value, with, lots, of, commas

Both of these messages would create the following line in the
signature base:

"example-header": value, with, lots, of, commas

Since two semantically distinct inputs can create the same output in
the signature base, special care has to be taken when handling such
values.

Specifically, the Set-Cookie field [COOKIE] defines an internal
syntax that does not conform to the List syntax provided in
[STRUCTURED-FIELDS]. In particular, some portions allow unquoted
commas, and the field is typically sent as multiple separate field
lines with distinct values when sending multiple cookies. When
multiple Set-Cookie fields are sent in the same message, it is not
generally possible to combine these into a single line and be able to
parse and use the results, as discussed in [HTTP], Section 5.3.
Therefore, all the cookies need to be processed from their separate
header
field values, without being combined, while the signature base needs
to be processed from the special combined value generated solely for
this purpose. If the cookie value is invalid, the signed message
ought to be rejected, as this is a possible padding attack as
described in Section 7.5.7.

To deal with this, an application can choose to limit signing of
problematic fields like Set-Cookie, such as including the field in a
signature only when a single field value is present and the results
would be unambiguous. Similar caution needs to be taken with all
fields that could have non-deterministic mappings into the signature
base. Signers can also make use of the bs parameter to armor such
fields, as described in Section 2.1.3.

7.5.7. Padding Attacks with Multiple Field Values

Since HTTP field values need to be combined into a single string
value to be included in the HTTP signature base (see Section 2.5), it
is possible for an attacker to inject an additional value for a given
field and add this to the signature base of the verifier.

In most circumstances, this causes the signature validation to fail
as expected, since the new signature base value will not match the
one used by the signer to create the signature. However, it is
theoretically possible for the attacker to inject both a garbage
value into a field and a desired value into another field in order to
force a particular input. This is a variation of the collision
attack described in Section 7.3.1, where the attacker accomplishes
their change in the message by adding to existing field values.

To counter this, an application needs to validate the content of the
fields covered in the signature in addition to ensuring that the
signature itself validates. With such protections, the attacker's
padding attack would be rejected by the field value processor, even
in the case where the attacker could force a signature collision.

7.5.8. Ambiguous Handling of Query Elements

The HTML form parameters format defined in Section 5 ("application/
x-www-form-urlencoded") of [HTMLURL] is widely deployed and supported
by many application frameworks. For convenience, some of these
frameworks in particular combine query parameters that are found in
the HTTP query and those found in the message content, particularly
for POST messages with a Content-Type value of "application/x-www-
form-urlencoded". The @query-param derived component identifier
defined in Section 2.2.8 draws its values only from the query section
of the target URI of the request. As such, it would be possible for
an attacker to shadow or replace query parameters in a request by
overriding a signed query parameter with an unsigned form parameter,
or vice versa.

To counter this, an application needs to make sure that values used
for the signature base and the application are drawn from a
consistent context, in this case the query component of the target
URI. Additionally, when the HTTP request has content, an application
should sign the message content as well, as discussed in
Section 7.2.8.

8. Privacy Considerations

8.1. Identification through Keys

If a signer uses the same key with multiple verifiers or uses the
same key over time with a single verifier, the ongoing use of that
key can be used to track the signer throughout the set of verifiers
that messages are sent to. Since cryptographic keys are meant to be
functionally unique, the use of the same key over time is a strong
indicator that it is the same party signing multiple messages.

In many applications, this is a desirable trait, and it allows HTTP
Message Signatures
message signatures to be used as part of authenticating the signer to
the verifier. However, it could also result in unintentional
tracking that a signer might not be aware of. To counter this kind
of tracking, a signer can use a different key for each verifier that
it is in communication with. Sometimes, a signer could also rotate
their key when sending messages to a given verifier. These
approaches do not negate the need for other anti-tracking techniques
to be applied as necessary.

8.2. Signatures do not provide confidentiality

HTTP Message Signatures message signatures do not provide confidentiality for any of the
information protected by the signature. The content of the HTTP
message, including the value of all fields and the value of the
signature itself, is presented in plaintext to any party with access
to the message.

To provide confidentiality at the transport level, TLS or its
equivalent can be used, as discussed in Section 7.1.2.

8.3. Oracles

It is important to balance the need for providing useful feedback to
developers regarding error conditions without providing additional
information to an attacker. For example, a naive but helpful server
implementation might try to indicate the required key identifier
needed for requesting a resource. If someone knows who controls that
key, a correlation can be made between the resource's existence and
the party identified by the key. Access to such information could be
used by an attacker as a means to target the legitimate owner of the
resource for further attacks.

8.4. Required Content

A core design tenet of this specification is that all message
components covered by the signature need to be available to the
verifier in order to recreate the signature base and verify the
signature. As a consequence, if an application of this specification
requires that a particular field be signed, the verifier will need
access to the value of that field.

For example, in some complex systems with intermediary processors,
this could cause surprising behavior where, for fear of breaking the
signature, an intermediary cannot remove privacy-sensitive
information from a message before forwarding it on for processing.
One way to mitigate this specific situation would be for the
intermediary to verify the signature itself and then modify the
message to remove the privacy-sensitive information. The
intermediary can add its own signature at this point to signal to the
next destination that the incoming signature was validated, as shown
in the example in Section 4.3.

9. References

9.1. Normative References

[ABNF] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.

[ASCII] Cerf, V., "ASCII format for network interchange", STD 80,
RFC 20, DOI 10.17487/RFC0020, October 1969,
<https://www.rfc-editor.org/info/rfc20>.

[FIPS186-5]
NIST, "Digital Signature Standard (DSS)",
DOI 10.6028/NIST.FIPS.186-5, February 2023,
<https://doi.org/10.6028/NIST.FIPS.186-5>.

[HTMLURL] WHATWG, "URL (Living Standard)", September 2023, January 2024,
<https://url.spec.whatwg.org/>.

[HTTP] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Semantics", STD 97, RFC 9110,
DOI 10.17487/RFC9110, June 2022,
<https://www.rfc-editor.org/info/rfc9110>.

[HTTP/1.1] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP/1.1", STD 99, RFC 9112, DOI 10.17487/RFC9112,
June 2022, <https://www.rfc-editor.org/info/rfc9112>.

[POSIX.1] IEEE, "The Open Group Base Specifications Issue 7, 2018
edition", 2018,
<https://pubs.opengroup.org/onlinepubs/9699919799/>.

[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
DOI 10.17487/RFC2104, February 1997,
<https://www.rfc-editor.org/info/rfc2104>.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
(SHA and SHA-based HMAC and HKDF)", RFC 6234,
DOI 10.17487/RFC6234, May 2011,
<https://www.rfc-editor.org/info/rfc6234>.

[RFC7517] Jones, M., "JSON Web Key (JWK)", RFC 7517,
DOI 10.17487/RFC7517, May 2015,
<https://www.rfc-editor.org/info/rfc7517>.

[RFC7518] Jones, M., "JSON Web Algorithms (JWA)", RFC 7518,
DOI 10.17487/RFC7518, May 2015,
<https://www.rfc-editor.org/info/rfc7518>.

[RFC8017] Moriarty, K., Ed., Kaliski, B., Jonsson, J., and A. Rusch,
"PKCS #1: RSA Cryptography Specifications Version 2.2",
RFC 8017, DOI 10.17487/RFC8017, November 2016,
<https://www.rfc-editor.org/info/rfc8017>.

[RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
Signature Algorithm (EdDSA)", RFC 8032,
DOI 10.17487/RFC8032, January 2017,
<https://www.rfc-editor.org/info/rfc8032>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[STRUCTURED-FIELDS]
Nottingham, M. and P. Kamp, "Structured Field Values for
HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021,
<https://www.rfc-editor.org/info/rfc8941>.

[URI] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/info/rfc3986>.

9.2. Informative References

[AWS-SIGv4]
Amazon Simple Storage Service, "Authenticating Requests
(AWS Signature Version 4)", March 2006,
<https://docs.aws.amazon.com/AmazonS3/latest/API/sig-v4-
authenticating-requests.html>.

[BCP195] Moriarty, K. and S. Farrell, "Deprecating TLS 1.0 and TLS
1.1", BCP 195, RFC 8996, March 2021.

Sheffer, Y., Saint-Andre, P., and T. Fossati,
"Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 9325, November 2022.

<https://www.rfc-editor.org/info/bcp195>

[CLIENT-CERT]
Campbell, B. and M. Bishop, "Client-Cert HTTP Header
Field", RFC 9440, DOI 10.17487/RFC9440, July 2023,
<https://www.rfc-editor.org/info/rfc9440>.

[COOKIE] Barth, A., "HTTP State Management Mechanism", RFC 6265,
DOI 10.17487/RFC6265, April 2011,
<https://www.rfc-editor.org/info/rfc6265>.

[DIGEST] Polli, R. and L. Pardue, "Digest Fields", Work in
Progress, Internet-Draft, draft-ietf-httpbis-digest-
headers-13, 10 July 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
digest-headers-13>. RFC 9530,
DOI 10.17487/RFC9530, February 2024,
<https://www.rfc-editor.org/info/rfc9530>.

[JACKSON2019]
Jackson, D., Cremers, C., Cohn-Gordon, K., and R. Sasse,
"Seems Legit: Automated Analysis of Subtle Attacks on
Protocols that Use Signatures", CCS '19: Proceedings of
the 2019 ACM SIGSAC Conference on Computer and
Communications Security, pp. 2165-2180,
DOI 10.1145/3319535.3339813, November 2019,
<https://dl.acm.org/doi/10.1145/3319535.3339813>.

[JWS] Jones, M., Bradley, J., and N. Sakimura, "JSON Web
Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
2015, <https://www.rfc-editor.org/info/rfc7515>.

[RFC7239] Petersson, A. and M. Nilsson, "Forwarded HTTP Extension",
RFC 7239, DOI 10.17487/RFC7239, June 2014,
<https://www.rfc-editor.org/info/rfc7239>.

[RFC7468] Josefsson, S. and S. Leonard, "Textual Encodings of PKIX,
PKCS, and CMS Structures", RFC 7468, DOI 10.17487/RFC7468,
April 2015, <https://www.rfc-editor.org/info/rfc7468>.

[RFC7807] Nottingham, M. and E. Wilde, "Problem Details for HTTP
APIs", RFC 7807, DOI 10.17487/RFC7807, March 2016,
<https://www.rfc-editor.org/info/rfc7807>.

[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.

[RFC8792] Watsen, K., Auerswald, E., Farrel, A., and Q. Wu,
"Handling Long Lines in Content of Internet-Drafts and
RFCs", RFC 8792, DOI 10.17487/RFC8792, June 2020,
<https://www.rfc-editor.org/info/rfc8792>.

[RFC9457] Nottingham, M., Wilde, E., and S. Dalal, "Problem Details
for HTTP APIs", RFC 9457, DOI 10.17487/RFC9457, July 2023,
<https://www.rfc-editor.org/info/rfc9457>.

[SIGNING-HTTP-MESSAGES]
Cavage, M. and M. Sporny, "Signing HTTP Messages", Work in
Progress, Internet-Draft, draft-cavage-http-signatures-12,
21 October 2019, <https://datatracker.ietf.org/doc/html/
draft-cavage-http-signatures-12>.

[SIGNING-HTTP-REQS-OAUTH]
Richer, J., Ed., Bradley, J., and H. Tschofenig, "A Method
for Signing HTTP Requests for OAuth", Work in Progress,
August 2016, <https://datatracker.ietf.org/doc/html/draft-
ietf-oauth-signed-http-request-03>.

[TLS] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.

Appendix A. Detecting HTTP Message Signatures

There have been many attempts to create signed HTTP messages in the
past, including other non-standardized definitions of the Signature
field that is used within this specification. It is recommended that
developers wishing to support this specification, other published
documents, or other historical drafts do so carefully and
deliberately, as incompatibilities between this specification and
other documents or various versions of other drafts could lead to
unexpected problems.

It is recommended that implementors first detect and validate the
Signature-Input field defined in this specification to detect that
the mechanism described in this standard, document is in use and not an alternative, is in use.
alternative. If the Signature-
Input Signature-Input field is present, all Signature
fields can be parsed and interpreted in the context of this
specification.

Appendix B. Examples

The following non-normative examples are provided as a means of
testing implementations of HTTP Message Signatures. message signatures. The signed
messages given can be used to create the signature base with the
stated parameters, creating signatures using the stated algorithms
and keys.

The private keys given can be used to generate signatures, though
since several of the demonstrated algorithms are non-deterministic,
the results of a signature are expected to be different from the
exact bytes of the examples. The public keys given can be used to
validate all signed examples.

B.1. Example Keys

This section provides cryptographic keys that are referenced in
example signatures throughout this document. These keys MUST NOT be
used for any purpose other than testing.

The key identifiers for each key are used throughout the examples in
this specification. It is assumed for these examples that the signer
and verifier can unambiguously dereference all key identifiers used
here and that the keys and algorithms used are appropriate for the
context in which the signature is presented.

The components for each private key, in PEM format [RFC7468], can be
displayed by executing the following OpenSSL command:

openssl pkey -text

This command was tested with all the example keys on OpenSSL version
1.1.1m. Note that some systems cannot produce or use all of these
keys directly and may require additional processing. All keys are
also made available in JWK format.

B.1.1. Example Key RSA Test Key

The following key is a 2048-bit RSA public and private key pair,
referred to in this document as test-key-rsa. This key is encoded in
PEM format, with no encryption.

The same public and private key pair in JWK format:

NOTE: '\' line wrapping per RFC 8792

{
"kty": "RSA",
"kid": "test-key-rsa",
"p": "sqeUJmqXE3LP8tYoIjMIAKiTm9o6psPlc8CrLI9CH0UbuaA2JCOMcCNq8Sy\
YbTqgnWlB9ZfcAm_cFpA8tYci9m5vYK8HNxQr-8FS3Qo8N9RJ8d0U5CswDzMYfRgh\
AfUGwmlWj5hp1pQzAuhwbOXFtxKHVsMPhz1IBtF9Y8jvgqgYHLbmyiu1mw",
"q": "vSlgXQbvHzWmuUBFRHAejRh_naQTDV3GnH4lcRHuFBFZCSLn82xQS2_7xFO\
qfabqq17kNcvKfzdvWpGxxJ2cILAq0pZS6DmrZlvBU4IkK2ZHCac_XfWVZFh-PrsH\
_EnVkDpfcYR_iw1F40C1q5w8R6WBHaew3SAp",
"d": "b8lm5JZ2hUduLnq-OAKCSODeWQ7Uqs7eet2bqeuAD0_2po-PG4qhZoo7VwF\
CUTWlJan9wqdxiAPlbEQKkCdFRcbakbjN2TMJjMCHWL5zfgvqhmgeyKsrqg1wSce9\
7J1_Mkvn3fh6CbqnwNb6bVFDvTJS3i5FzRhKiv6rUsYm8ZAdF4XRaYkFkeuHPl7rc\
-ruUTSAjC4GovxIxoDJFe0r4kbFmkiZOr40e8RZYK7T1IKrSvzfxx5AjnlK_OZOTC\
q0L7wBPbMW-IxmQpFCjpI-yuoi3FlZG3LaLNrBMXQF_lLZUDHs77q3fAGxDWwum2h\
KBfdBuUQtjlqwjQlgXPsskQ",
"e": "AQAB",
"qi": "PkbARLOwU_LcZrQy9mmfcPoQlAuCyeu1Q9nH7PYSnbHTFzmiud4Hl8bIXU\
9a0_58blDoOl3PctF-b4rAEJYUpCODu5PFyN6uEFYRg-YQwpjBMkXk8Eb39128ctA\
RB40Lx8caDhRdTyaEedIG3cQDXSpAl9EOzXkzfx4bZxjAHU9mkMdJwOcMDQ",
"dp": "aiodZsrWpi8HFfZfeRs8OS_0L5x6WBl3Y9btoZgsIeruc9uZ8NXTIdxaM6\
FdnyNEyOYA1VH94tDYR-xEt1br1ud_dkPslLV_Aac7d7EaYc7cdkb7oC9t6sphVg0\
dqE0UTDlOwBxBYMtGmQbJsFzGpmjzVgKqWqJ3B947li2U7t63HXEvKprY2w",
"dq": "b0DzpSMb5p42dcQgOTU8Mr4S6JOEhRr_YjErMkpaXUEqvZ3jEB9HRmcRi5\
Gtt4NBiBMiY6V9br8a5gjEpiAQoIUcWokBMAYjEeurU8M6JLBd3YaZVVjISaFmdty\
nwLFoQxCh6_EC1rSywwrfDpSwO29S9i8Xbaap",
"n": "hAKYdtoeoy8zcAcR874L8cnZxKzAGwd7v36APp7Pv6Q2jdsPBRrwWEBnez6\
d0UDKDwGbc6nxfEXAy5mbhgajzrw3MOEt8uA5txSKobBpKDeBLOsdJKFqMGmXCQvE\
G7YemcxDTRPxAleIAgYYRjTSd_QBwVW9OwNFhekro3RtlinV0a75jfZgkne_YiktS\
vLG34lw2zqXBDTC5NHROUqGTlML4PlNZS5Ri2U4aCNx2rUPRcKIlE0PuKxI4T-HIa\
Fpv8-rdV6eUgOrB2xeI1dSFFn_nnv5OoZJEIB-VmuKn3DCUcCZSFlQPSXSfBDiUGh\
wOw76WuSSsf1D4b_vLoJ10w"
}

B.1.2. Example RSA-PSS Key

The following key is a 2048-bit RSA public and private key pair,
referred to in this document as test-key-rsa-pss. This key is PKCS
#8 encoded in PEM format, with no encryption.

The same public and private key pair in JWK format:

NOTE: '\' line wrapping per RFC 8792

{
"kty": "RSA",
"kid": "test-key-rsa-pss",
"p": "5V-6ISI5yEaCFXm-fk1EM2xwAWekePVCAyvr9QbTlFOCZwt9WwjUjhtKRus\
i5Uq-IYZ_tq2WRE4As4b_FHEMtp2AER43IcvmXPqKFBoUktVDS7dThIHrsnRi1U7d\
HqVdwiMEMe5jxKNgnsKLpnq-4NyhoS6OeWu1SFozG9J9xQk",
"q": "w-wIde17W5Y0Cphp3ZZ0uM8OUq1AkrV2IKauqYHaDxAT32EM4ci2MMER2nI\
UEo4g_42lW0zYouFFqONwv0-HyOsgPpdSqKRC5WLgn0VXabjaNcy6KhNPXeJ0Agtq\
diDwPeJ2_L_eKwNWQ43RfdQBUquAwSd7SEmmQ8sViqB628M",
"d": "lAfIqfpCYomVShfAKnwf2lD9I0wKjkHsCtZCif4kAlwQqqW6N-tIL3bdOR-\
VWf0Q1ZBIDtpO91UrG7pansyrPERbNrRJlPiYEyPTHkCT1nD-l2isuiyGLNBNnFoK\
fBgA4KAbPJZQatFIV9Cn34JSHnpN5-2ehreGBYHtkwHFtlmzeF3yu5bqRcqOhx8lk\
YmBzDAEUFyyXjknU5-WjAT9DzuG0MpOTkcU1EnjnIjyVBZLUB5Lxm8puyq8hH8B_E\
5LNC-1oc8j-tDy98UvRTTiYvZvs87cGCFxg0LijNhg7CE3g9piNqB6DzMgA9MHSOw\
cElVtfKdYfo4H3OHZXsSmEQ",
"e": "AQAB",
"qi": "jRAqfYi_tKCjhP9eM0N2XaRlNeoYCTx06GlSLD8d0zc4ZZuEePY10LMGWI\
6Y_JC0CvvvQYhNa9sAj4hFjIVLsWeTplVVUezGO1ofLW4kYWVpnMpHgAY1pRM4kyz\
o1p3MKYY8DE1BA4KqhSOfhdGs6Ov3Dfj0migZeE7Fu7yc7Fc",
"dp": "otDolkxtJ7Sk8gmRJqZCGx6GAvlGznWJfibXPv6xgUAl-G83dD84YgcNGn\
oeMxRzEekfDtT5LVMRPF4_AoucsqPqHDyOdfb-dlGBYfOBVxj6w-xF5HE0lV_4J-H\
rI63Od9fTSn4lY5d1JjyCVJIcnBEAyiD6EUZbUBh23vDzRcE",
"dq": "iZE1S6CpqmBoQDxOsXGQmaeBdhoCqkDSJhEDuS_dLhBq88FQa0UkcE1QvO\
K3J2Q21VnfDqGBx7SH1hOFOj-cpz45kNluB832ztxDvnHQ9AIA7h_HY_3VD6YPMNR\
VN4bfSYS3abdLR0Z7jsmInGJ9X0_fA0E2tkZIgXeas5EFU0M",
"n": "r4tmm3r20Wd_PbqvP1s2-QEtvpuRaV8Yq40gjUR8y2Rjxa6dpG2GXHbPfvM\
s8ct-Lh1GH45x28Rw3Ry53mm-oAXjyQ86OnDkZ5N8lYbggD4O3w6M6pAvLkhk95An\
dTrifbIFPNU8PPMO7OyrFAHqgDsznjPFmTOtCEcN2Z1FpWgchwuYLPL-Wokqltd11\
nqqzi-bJ9cvSKADYdUAAN5WUtzdpiy6LbTgSxP7ociU4Tn0g5I6aDZJ7A8Lzo0KSy\
ZYoA485mqcO0GVAdVw9lq4aOT9v6d-nb4bnNkQVklLQ3fVAvJm-xdDOp9LCNCN48V\
2pnDOkFV6-U9nV5oyc6XI2w"
}

B.1.3. Example ECC P-256 Test Key

The following key is a public and private elliptical curve key pair
over the curve P-256, referred to in this document as test-key-ecc-
p256. This key is encoded in PEM format, with no encryption.

-----BEGIN PUBLIC KEY-----
MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEqIVYZVLCrPZHGHjP17CTW0/+D9Lf
w0EkjqF7xB4FivAxzic30tMM4GF+hR6Dxh71Z50VGGdldkkDXZCnTNnoXQ==
-----END PUBLIC KEY-----

The same public and private key pair in JWK format:

{
"kty": "EC",
"crv": "P-256",
"kid": "test-key-ecc-p256",
"d": "UpuF81l-kOxbjf7T4mNSv0r5tN67Gim7rnf6EFpcYDs",
"x": "qIVYZVLCrPZHGHjP17CTW0_-D9Lfw0EkjqF7xB4FivA",
"y": "Mc4nN9LTDOBhfoUeg8Ye9WedFRhnZXZJA12Qp0zZ6F0"
}

B.1.4. Example Ed25519 Test Key

The following key is an elliptical curve key over the Edwards curve
ed25519, referred to in this document as test-key-ed25519. This key
is PKCS #8 encoded in PEM format, with no encryption.

-----BEGIN PUBLIC KEY-----
MCowBQYDK2VwAyEAJrQLj5P/89iXES9+vFgrIy29clF9CC/oPPsw3c5D0bs=
-----END PUBLIC KEY-----

-----BEGIN PRIVATE KEY-----
MC4CAQAwBQYDK2VwBCIEIJ+DYvh6SEqVTm50DFtMDoQikTmiCqirVv9mWG9qfSnF
-----END PRIVATE KEY-----

The same public and private key pair in JWK format:

{
"kty": "OKP",
"crv": "Ed25519",
"kid": "test-key-ed25519",
"d": "n4Ni-HpISpVObnQMW0wOhCKROaIKqKtW_2ZYb2p9KcU",
"x": "JrQLj5P_89iXES9-vFgrIy29clF9CC_oPPsw3c5D0bs"
}

B.1.5. Example Shared Secret

The following shared secret is 64 randomly generated bytes encoded in
Base64, referred to in this document as test-shared-secret:

NOTE: '\' line wrapping per RFC 8792

uzvJfB4u3N0Jy4T7NZ75MDVcr8zSTInedJtkgcu46YW4XByzNJjxBdtjUkdJPBt\
bmHhIDi6pcl8jsasjlTMtDQ==

B.2. Test Cases

This section provides non-normative examples that may be used as test
cases to validate implementation correctness. These examples are
based on the following HTTP messages:

For requests, this test-request message is used:

NOTE: '\' line wrapping per RFC 8792

{"hello": "world"}

For responses, this test-response message is used:

NOTE: '\' line wrapping per RFC 8792

HTTP/1.1 200 OK
Date: Tue, 20 Apr 2021 02:07:56 GMT
Content-Type: application/json
Content-Digest: sha-512=:JlEy2bfUz7WrWIjc1qV6KVLpdr/7L5/L4h7Sxvh6sN\
HpDQWDCL+GauFQWcZBvVDhiyOnAQsxzZFYwi0wDH+1pw==: sha-512=:mEWXIS7MaLRuGgxOBdODa3xqM1XdEvxoYhvlCFJ41Q\
JgJc4GTsPp29l5oGX69wWdXymyU0rjJuahq4l5aGgfLQ==:
Content-Length: 23

{"message": "good dog"}

B.2.1. Minimal Signature Using rsa-pss-sha512

This example presents a minimal signature using the rsa-pss-sha512
algorithm over test-request, covering none of the components of the
HTTP message but providing a timestamped signature proof of
possession of the key with a signer-provided nonce.

The corresponding signature base is:

NOTE: '\' line wrapping per RFC 8792

"@signature-params": ();created=1618884473;keyid="test-key-rsa-pss"\
;nonce="b3k2pp5k7z-50gnwp.yemd"

This results in the following Signature-Input and Signature header
fields being added to the message under the signature label sig-b21:

NOTE: '\' line wrapping per RFC 8792

Signature-Input: sig-b21=();created=1618884473\
;keyid="test-key-rsa-pss";nonce="b3k2pp5k7z-50gnwp.yemd"
Signature: sig-b21=:d2pmTvmbncD3xQm8E9ZV2828BjQWGgiwAaw5bAkgibUopem\
LJcWDy/lkbbHAve4cRAtx31Iq786U7it++wgGxbtRxf8Udx7zFZsckzXaJMkA7ChG\
52eSkFxykJeNqsrWH5S+oxNFlD4dzVuwe8DhTSja8xxbR/Z2cOGdCbzR72rgFWhzx\
2VjBqJzsPLMIQKhO4DGezXehhWwE56YCE+O6c0mKZsfxVrogUvA4HELjVKWmAvtl6\
UnCh8jYzuVG5WSb/QEVPnP5TmcAnLH1g+s++v6d4s8m0gCw1fV5/SITLq9mhho8K3\
+7EPYTU8IU1bLhdxO5Nyt8C8ssinQ98Xw9Q==:

Note that since the covered components list is empty, this signature
could be applied by an attacker to an unrelated HTTP message. In
this example, the nonce parameter is included to prevent the same
signature from being replayed more than once, but if an attacker
intercepts the signature and prevents its delivery to the verifier,
the attacker could apply this signature to another message.
Therefore, the use of an empty covered components set is discouraged.
See Section 7.2.1 for more discussion.

B.2.2. Selective Covered Components Using rsa-pss-sha512

This example covers additional components (the authority, the
Content-Digest header field, and a single named query parameter) in
test-request using the rsa-pss-sha512 algorithm. This example also
adds a tag parameter with the application-specific value of header-
example.

The corresponding signature base is:

NOTE: '\' line wrapping per RFC 8792

"@authority": example.com
"content-digest": sha-512=:WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX\
+TaPm+AbwAgBWnrIiYllu7BNNyealdVLvRwEmTHWXvJwew==:
"@query-param";name="Pet": dog
"@signature-params": ("@authority" "content-digest" \
"@query-param";name="Pet")\
;created=1618884473;keyid="test-key-rsa-pss"\
;tag="header-example"

This results in the following Signature-Input and Signature header
fields being added to the message under the label sig-b22:

NOTE: '\' line wrapping per RFC 8792

Signature-Input: sig-b22=("@authority" "content-digest" \
"@query-param";name="Pet");created=1618884473\
;keyid="test-key-rsa-pss";tag="header-example"
Signature: sig-b22=:LjbtqUbfmvjj5C5kr1Ugj4PmLYvx9wVjZvD9GsTT4F7GrcQ\
EdJzgI9qHxICagShLRiLMlAJjtq6N4CDfKtjvuJyE5qH7KT8UCMkSowOB4+ECxCmT\
8rtAmj/0PIXxi0A0nxKyB09RNrCQibbUjsLS/2YyFYXEu4TRJQzRw1rLEuEfY17SA\
RYhpTlaqwZVtR8NV7+4UKkjqpcAoFqWFQh62s7Cl+H2fjBSpqfZUJcsIk4N6wiKYd\
4je2U/lankenQ99PZfB4jY3I5rSV2DSBVkSFsURIjYErOs0tFTQosMTAoxk//0RoK\
UqiYY8Bh0aaUEb0rQl3/XaVe4bXTugEjHSw==:

B.2.3. Full Coverage Using rsa-pss-sha512

This example covers all applicable message components in test-request
(including the content type and length) plus many derived components,
again using the rsa-pss-sha512 algorithm. Note that the Host header
field is not covered because the @authority derived component is
included instead.

The corresponding signature base is:

NOTE: '\' line wrapping per RFC 8792

"date": Tue, 20 Apr 2021 02:07:55 GMT
"@method": POST
"@path": /foo
"@query": ?param=Value&Pet=dog
"@authority": example.com
"content-type": application/json
"content-digest": sha-512=:WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX\
+TaPm+AbwAgBWnrIiYllu7BNNyealdVLvRwEmTHWXvJwew==:
"content-length": 18
"@signature-params": ("date" "@method" "@path" "@query" \
"@authority" "content-type" "content-digest" "content-length")\
;created=1618884473;keyid="test-key-rsa-pss"

This results in the following Signature-Input and Signature header
fields being added to the message under the label sig-b23:

NOTE: '\' line wrapping per RFC 8792

Signature-Input: sig-b23=("date" "@method" "@path" "@query" \
"@authority" "content-type" "content-digest" "content-length")\
;created=1618884473;keyid="test-key-rsa-pss"
Signature: sig-b23=:bbN8oArOxYoyylQQUU6QYwrTuaxLwjAC9fbY2F6SVWvh0yB\
iMIRGOnMYwZ/5MR6fb0Kh1rIRASVxFkeGt683+qRpRRU5p2voTp768ZrCUb38K0fU\
xN0O0iC59DzYx8DFll5GmydPxSmme9v6ULbMFkl+V5B1TP/yPViV7KsLNmvKiLJH1\
pFkh/aYA2HXXZzNBXmIkoQoLd7YfW91kE9o/CCoC1xMy7JA1ipwvKvfrs65ldmlu9\
bpG6A9BmzhuzF8Eim5f8ui9eH8LZH896+QIF61ka39VBrohr9iyMUJpvRX2Zbhl5Z\
JzSRxpJyoEZAFL2FUo5fTIztsDZKEgM4cUA==:

Note in this example that the value of the Date header field and the
value of the created signature parameter need not be the same. This
is due to the fact that the Date header field is added when creating
the HTTP Message message and the created parameter is populated when creating
the signature over that message, and these two times could vary. If
the Date header field is covered by the signature, it is up to the
verifier to determine whether its value has to match that of the
created parameter or not. See Section 7.2.4 for more discussion.

B.2.4. Signing a Response Using ecdsa-p256-sha256

This example covers portions of the test-response message using the
ecdsa-p256-sha256 algorithm and the key test-key-ecc-p256.

The corresponding signature base is:

NOTE: '\' line wrapping per RFC 8792

"@status": 200
"content-type": application/json
"content-digest": sha-512=:mEWXIS7MaLRuGgxOBdODa3xqM1XdEvxoYhvlCFJ4\
1QJgJc4GTsPp29l5oGX69wWdXymyU0rjJuahq4l5aGgfLQ==:
"content-length": 23
"@signature-params": ("@status" "content-type" "content-digest" \
"content-length");created=1618884473;keyid="test-key-ecc-p256"

This results in the following Signature-Input and Signature header
fields being added to the message under the label sig-b24:

NOTE: '\' line wrapping per RFC 8792

Signature-Input: sig-b24=("@status" "content-type" \
"content-digest" "content-length");created=1618884473\
;keyid="test-key-ecc-p256"
Signature: sig-b24=:wNmSUAhwb5LxtOtOpNa6W5xj067m5hFrj0XQ4fvpaCLx0NK\
ocgPquLgyahnzDnDAUy5eCdlYUEkLIj+32oiasw==:

B.2.5. Signing a Request Using hmac-sha256

This example covers portions of the test-request message using the
hmac-sha256 algorithm and the secret test-shared-secret.

The corresponding signature base is:

NOTE: '\' line wrapping per RFC 8792

"date": Tue, 20 Apr 2021 02:07:55 GMT
"@authority": example.com
"content-type": application/json
"@signature-params": ("date" "@authority" "content-type")\
;created=1618884473;keyid="test-shared-secret"

This results in the following Signature-Input and Signature header
fields being added to the message under the label sig-b25:

NOTE: '\' line wrapping per RFC 8792

Signature-Input: sig-b25=("date" "@authority" "content-type")\
;created=1618884473;keyid="test-shared-secret"
Signature: sig-b25=:pxcQw6G3AjtMBQjwo8XzkZf/bws5LelbaMk5rGIGtE8=:

Before using symmetric signatures in practice, see the discussion
regarding security trade-offs in Section 7.3.3.

B.2.6. Signing a Request Using ed25519

This example covers portions of the test-request message using the
Ed25519 algorithm and the key test-key-ed25519.

The corresponding signature base is:

NOTE: '\' line wrapping per RFC 8792

"date": Tue, 20 Apr 2021 02:07:55 GMT
"@method": POST
"@path": /foo
"@authority": example.com
"content-type": application/json
"content-length": 18
"@signature-params": ("date" "@method" "@path" "@authority" \
"content-type" "content-length");created=1618884473\
;keyid="test-key-ed25519"

This results in the following Signature-Input and Signature header
fields being added to the message under the label sig-b26:

NOTE: '\' line wrapping per RFC 8792

Signature-Input: sig-b26=("date" "@method" "@path" "@authority" \
"content-type" "content-length");created=1618884473\
;keyid="test-key-ed25519"
Signature: sig-b26=:wqcAqbmYJ2ji2glfAMaRy4gruYYnx2nEFN2HN6jrnDnQCK1\
u02Gb04v9EDgwUPiu4A0w6vuQv5lIp5WPpBKRCw==:

B.3. TLS-Terminating Proxies

In this example, there is a TLS-terminating reverse proxy sitting in
front of the resource. The client does not sign the request but
instead uses mutual TLS to make its call. The terminating proxy
validates the TLS stream and injects a Client-Cert header field
according to [CLIENT-CERT], and then applies a signature to this
field. By signing this header field, a reverse proxy not only can
attest to its own validation of the initial request's TLS parameters
but can also authenticate itself to the backend system independently
of the client's actions.

The client makes the following request to the TLS-terminating proxy
using mutual TLS:

POST /foo?param=Value&Pet=dog HTTP/1.1
Host: example.com
Date: Tue, 20 Apr 2021 02:07:55 GMT
Content-Type: application/json
Content-Length: 18

{"hello": "world"}

The proxy processes the TLS connection and extracts the client's TLS
certificate to a Client-Cert header field and passes it along to the
internal service hosted at service.internal.example. This results in
the following unsigned request:

NOTE: '\' line wrapping per RFC 8792

POST /foo?param=Value&Pet=dog HTTP/1.1
Host: service.internal.example
Date: Tue, 20 Apr 2021 02:07:55 GMT
Content-Type: application/json
Content-Length: 18
Client-Cert: :MIIBqDCCAU6gAwIBAgIBBzAKBggqhkjOPQQDAjA6MRswGQYDVQQKD\
BJMZXQncyBBdXRoZW50aWNhdGUxGzAZBgNVBAMMEkxBIEludGVybWVkaWF0ZSBDQT\
AeFw0yMDAxMTQyMjU1MzNaFw0yMTAxMjMyMjU1MzNaMA0xCzAJBgNVBAMMAkJDMFk\
wEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAE8YnXXfaUgmnMtOXU/IncWalRhebrXmck\
C8vdgJ1p5Be5F/3YC8OthxM4+k1M6aEAEFcGzkJiNy6J84y7uzo9M6NyMHAwCQYDV\
R0TBAIwADAfBgNVHSMEGDAWgBRm3WjLa38lbEYCuiCPct0ZaSED2DAOBgNVHQ8BAf\
8EBAMCBsAwEwYDVR0lBAwwCgYIKwYBBQUHAwIwHQYDVR0RAQH/BBMwEYEPYmRjQGV\
4YW1wbGUuY29tMAoGCCqGSM49BAMCA0gAMEUCIBHda/r1vaL6G3VliL4/Di6YK0Q6\
bMjeSkC3dFCOOB8TAiEAx/kHSB4urmiZ0NX5r5XarmPk0wmuydBVoU4hBVZ1yhk=:

{"hello": "world"}

Without a signature, the internal service would need to trust that
the incoming connection has the right information. By signing the
Client-Cert header field and other portions of the internal request,
the internal service can be assured that the correct party, the
trusted proxy, has processed the request and presented it to the
correct service. The proxy's signature base consists of the
following:

NOTE: '\' line wrapping per RFC 8792

"@path": /foo
"@query": ?param=Value&Pet=dog
"@method": POST
"@authority": service.internal.example
"client-cert": :MIIBqDCCAU6gAwIBAgIBBzAKBggqhkjOPQQDAjA6MRswGQYDVQQ\
KDBJMZXQncyBBdXRoZW50aWNhdGUxGzAZBgNVBAMMEkxBIEludGVybWVkaWF0ZSBD\
QTAeFw0yMDAxMTQyMjU1MzNaFw0yMTAxMjMyMjU1MzNaMA0xCzAJBgNVBAMMAkJDM\
FkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAE8YnXXfaUgmnMtOXU/IncWalRhebrXm\
ckC8vdgJ1p5Be5F/3YC8OthxM4+k1M6aEAEFcGzkJiNy6J84y7uzo9M6NyMHAwCQY\
DVR0TBAIwADAfBgNVHSMEGDAWgBRm3WjLa38lbEYCuiCPct0ZaSED2DAOBgNVHQ8B\
Af8EBAMCBsAwEwYDVR0lBAwwCgYIKwYBBQUHAwIwHQYDVR0RAQH/BBMwEYEPYmRjQ\
GV4YW1wbGUuY29tMAoGCCqGSM49BAMCA0gAMEUCIBHda/r1vaL6G3VliL4/Di6YK0\
Q6bMjeSkC3dFCOOB8TAiEAx/kHSB4urmiZ0NX5r5XarmPk0wmuydBVoU4hBVZ1yhk=:
"@signature-params": ("@path" "@query" "@method" "@authority" \
"client-cert");created=1618884473;keyid="test-key-ecc-p256"

This results in the following signature:

NOTE: '\' line wrapping per RFC 8792

xVMHVpawaAC/0SbHrKRs9i8I3eOs5RtTMGCWXm/9nvZzoHsIg6Mce9315T6xoklyy0y\
zhD9ah4JHRwMLOgmizw==

which results in the following signed request sent from the proxy to
the internal service with the proxy's signature under the label ttrp:

NOTE: '\' line wrapping per RFC 8792

{"hello": "world"}

The internal service can validate the proxy's signature and therefore
be able to trust that the client's certificate has been appropriately
processed.

B.4. HTTP Message Transformations

HTTP allows intermediaries and applications to transform an HTTP
message without affecting the semantics of the message itself. HTTP
message signatures are designed to be robust against many of these
transformations in different circumstances.

For example, the following HTTP request message has been signed using
the Ed25519 algorithm and the key test-key-ed25519:

NOTE: '\' line wrapping per RFC 8792

GET /demo?name1=Value1&Name2=value2 HTTP/1.1
Host: example.org
Date: Fri, 15 Jul 2022 14:24:55 GMT
Accept: application/json
Accept: */*
Signature-Input: transform=("@method" "@path" "@authority" \
"accept");created=1618884473;keyid="test-key-ed25519"
Signature: transform=:ZT1kooQsEHpZ0I1IjCqtQppOmIqlJPeo7DHR3SoMn0s5J\
Z1eRGS0A+vyYP9t/LXlh5QMFFQ6cpLt2m0pmj3NDA==:

The signature base string for this message is:

"@method": GET
"@path": /demo
"@authority": example.org
"accept": application/json, */*
"@signature-params": ("@method" "@path" "@authority" "accept")\
;created=1618884473;keyid="test-key-ed25519"

The following message has been altered by adding the Accept-Language
header field as well as adding a query parameter. However, since
neither the Accept-Language header field nor the query is covered by
the signature, the same signature is still valid:

NOTE: '\' line wrapping per RFC 8792

GET /demo?name1=Value1&Name2=value2&param=added HTTP/1.1
Host: example.org
Date: Fri, 15 Jul 2022 14:24:55 GMT
Accept: application/json
Accept: */*
Accept-Language: en-US,en;q=0.5
Signature-Input: transform=("@method" "@path" "@authority" \
"accept");created=1618884473;keyid="test-key-ed25519"
Signature: transform=:ZT1kooQsEHpZ0I1IjCqtQppOmIqlJPeo7DHR3SoMn0s5J\
Z1eRGS0A+vyYP9t/LXlh5QMFFQ6cpLt2m0pmj3NDA==:

The following message has been altered by removing the Date header
field, adding a Referer header field, and collapsing the Accept
header field into a single line. The Date and Referrer Referer header fields
are not covered by the signature, and the collapsing of the Accept
header field is an allowed transformation that is already accounted
for by the canonicalization algorithm for HTTP field values. The
same signature is still valid:

NOTE: '\' line wrapping per RFC 8792

GET /demo?name1=Value1&Name2=value2 HTTP/1.1
Host: example.org
Referer: https://developer.example.org/demo
Accept: application/json, */*
Signature-Input: transform=("@method" "@path" "@authority" \
"accept");created=1618884473;keyid="test-key-ed25519"
Signature: transform=:ZT1kooQsEHpZ0I1IjCqtQppOmIqlJPeo7DHR3SoMn0s5J\
Z1eRGS0A+vyYP9t/LXlh5QMFFQ6cpLt2m0pmj3NDA==:

The following message has been altered by reordering the field values
of the original message but not reordering the individual Accept
headers.
header fields. The same signature is still valid:

NOTE: '\' line wrapping per RFC 8792

GET /demo?name1=Value1&Name2=value2 HTTP/1.1
Accept: application/json
Accept: */*
Date: Fri, 15 Jul 2022 14:24:55 GMT
Host: example.org
Signature-Input: transform=("@method" "@path" "@authority" \
"accept");created=1618884473;keyid="test-key-ed25519"
Signature: transform=:ZT1kooQsEHpZ0I1IjCqtQppOmIqlJPeo7DHR3SoMn0s5J\
Z1eRGS0A+vyYP9t/LXlh5QMFFQ6cpLt2m0pmj3NDA==:

The following message has been altered by changing the method to POST
and the authority to "example.com" (inside the Host header). header field).
Since both the method and authority are covered by the signature, the
same signature is NOT still valid:

NOTE: '\' line wrapping per RFC 8792

POST /demo?name1=Value1&Name2=value2 HTTP/1.1
Host: example.com
Date: Fri, 15 Jul 2022 14:24:55 GMT
Accept: application/json
Accept: */*
Signature-Input: transform=("@method" "@path" "@authority" \
"accept");created=1618884473;keyid="test-key-ed25519"
Signature: transform=:ZT1kooQsEHpZ0I1IjCqtQppOmIqlJPeo7DHR3SoMn0s5J\
Z1eRGS0A+vyYP9t/LXlh5QMFFQ6cpLt2m0pmj3NDA==:

The following message has been altered by changing the order of the
two instances of the Accept header. header field. Since the order of fields
with the same name is semantically significant in HTTP, this changes
the value used in the signature base, and the same signature is NOT
still valid:

NOTE: '\' line wrapping per RFC 8792

GET /demo?name1=Value1&Name2=value2 HTTP/1.1
Host: example.org
Date: Fri, 15 Jul 2022 14:24:55 GMT
Accept: */*
Accept: application/json
Signature-Input: transform=("@method" "@path" "@authority" \
"accept");created=1618884473;keyid="test-key-ed25519"
Signature: transform=:ZT1kooQsEHpZ0I1IjCqtQppOmIqlJPeo7DHR3SoMn0s5J\
Z1eRGS0A+vyYP9t/LXlh5QMFFQ6cpLt2m0pmj3NDA==:

Acknowledgements

This specification was initially based on [SIGNING-HTTP-MESSAGES].
The editors would like to thank the authors of
[SIGNING-HTTP-MESSAGES] -- Mark Cavage and Manu Sporny -- for their
work on that Internet-Draft and their continuing contributions. This
specification also includes contributions from
[SIGNING-HTTP-REQS-OAUTH] and other similar efforts.

The editors would also like to thank the following individuals
(listed in alphabetical order) for feedback, insight, and
implementation of this document and its predecessors: Mark Adamcin,
Mark Allen, Paul Annesley, Karl Böhlmark, Stéphane Bortzmeyer, Sarven
Capadisli, Liam Dennehy, Stephen Farrell, Phillip Hallam-Baker, Tyler
Ham, Eric Holmes, Andrey Kislyuk, Adam Knight, Dave Lehn, Ilari
Liusvaara, Dave Longley, James H. Manger, Kathleen Moriarty, Yoav
Nir, Mark Nottingham, Adrian Palmer, Lucas Pardue, Roberto Polli,
Julian Reschke, Michael Richardson, Wojciech Rygielski, Rich Salz,
Adam Scarr, Cory J. Slep, Dirk Stein, Henry Story, Lukasz Szewc,
Chris Webber, and Jeffrey Yasskin.

Authors' Addresses

Annabelle Backman (editor)
Amazon
P.O. Box 81226
Seattle, WA 98108-1226
United States of America
Email: richanna@amazon.com
URI: https://www.amazon.com/

Justin Richer (editor)
Bespoke Engineering
Email: ietf@justin.richer.org
URI: https://bspk.io/

Manu Sporny
Digital Bazaar
203 Roanoke Street W.
Blacksburg, VA 24060
United States of America
Email: msporny@digitalbazaar.com