wdiff rfc8471.original rfc8471.txt

Internet Engineering Task Force (IETF) A. Popov, Ed.
Internet-Draft
Request for Comments: 8471 M. Nystroem
Intended status:
Category: Standards Track Microsoft Corp.
Expires: November 24, 2018
ISSN: 2070-1721 D. Balfanz
A. Langley
Google Inc.
J. Hodges
PayPal
May 23,
Kings Mountain Systems
September 2018

The Token Binding Protocol Version 1.0
draft-ietf-tokbind-protocol-19

Abstract

This document specifies Version version 1.0 of the Token Binding protocol.
The Token Binding protocol allows client/server applications to
create long-lived, uniquely identifiable TLS bindings spanning
multiple TLS sessions and connections. Applications are then enabled
to cryptographically bind security tokens to the TLS layer,
preventing token export and replay attacks. To protect privacy, the
Token Binding identifiers are only conveyed over TLS and can be reset
by the user at any time.

Status of This Memo

This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.

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This document is a product of the Internet Engineering Task Force
(IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid the IETF community. It has
received public review and has been approved for a maximum publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.

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This Internet-Draft will expire on November 24, 2018.
https://www.rfc-editor.org/info/rfc8471.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Token Binding Protocol Overview . . . . . . . . . . . . . . . 3
3. Token Binding Protocol Message . . . . . . . . . . . . . . . 4
3.1. TokenBinding.tokenbinding_type . . . . . . . . . . . . . 6
3.2. TokenBinding.tokenbindingid . . . . . . . . . . . . . . . 7 6
3.3. TokenBinding.signature . . . . . . . . . . . . . . . . . 7
3.4. TokenBinding.extensions . . . . . . . . . . . . . . . . . 8
4. Establishing a Token Binding . . . . . . . . . . . . . . . . 9
4.1. Client Processing Rules . . . . . . . . . . . . . . . . . 9
4.2. Server Processing Rules . . . . . . . . . . . . . . . . . 9
5. Bound Security Token Creation and Validation . . . . . . . . 10
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
6.1. Token Binding Key Parameters Registry . . . . . . . . . . 11
6.2. Token Binding Types Registry . . . . . . . . . . . . . . 12
6.3. Token Binding Extensions Registry . . . . . . . . . . . . 13
6.4. Registration of Token Binding TLS Exporter Label . . . . 13
7. Security Considerations . . . . . . . . . . . . . . . . . . . 14 13
7.1. Security Token Replay . . . . . . . . . . . . . . . . . . 14 13
7.2. Downgrade Attacks . . . . . . . . . . . . . . . . . . . . 14
7.3. Privacy Considerations . . . . . . . . . . . . . . . . . 14
7.4. Token Binding Key Sharing Between Key-Sharing between Applications . . . . . 15
7.5. 14
7.4. Triple Handshake Vulnerability in TLS 1.2 and Older TLS
Versions . . . . . . . . . . . . . . . . . . . . . . . . 15 14
8. Acknowledgements . . . Privacy Considerations . . . . . . . . . . . . . . . . . . . 15
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 15
9.1. Normative References . . . . . . . . . . . . . . . . . . 16 15
9.2. Informative References . . . . . . . . . . . . . . . . . 17
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 17

1. Introduction

Servers often generate various security tokens (e.g. (e.g., HTTP cookies,
OAuth [RFC6749] tokens) tokens [RFC6749]) for applications to present when accessing
protected resources. In general, any party in possession of bearer
security tokens gain gains access to certain protected resource(s).
Attackers take advantage of this by exporting bearer tokens from a
user's application connections or machines, presenting them to
application servers, and impersonating authenticated users. The idea
of Token Binding is to prevent such attacks by cryptographically
binding application security tokens to the underlying TLS [RFC5246]
layer. layer
[RFC5246]. (Note: This document deals with TLS 1.2 and therefore
refers to RFC 5246 (which has been obsoleted by RFC 8446);
[TOKENBIND-TLS13] addresses Token Binding in TLS 1.3.)

A Token Binding is established by a user agent User Agent generating a private-
public
private-public key pair (possibly within a secure hardware module,
such as a Trusted Platform Module) per target server, providing the
public key to the server, and proving possession of the corresponding
private key, on every TLS connection to the server. The proof of
possession involves signing the exported keying material Exported Keying Material (EKM)
[RFC5705] from the TLS connection with the private key. The
corresponding public key is included in the Token Binding identifier
structure (described in the Section 3.2 "TokenBinding.tokenbindingid"). ("TokenBinding.tokenbindingid")).
Token Bindings are long-lived, i.e., they encompass multiple TLS
connections and TLS sessions between a given client and server. To
protect privacy, Token Binding IDs are never conveyed over insecure
connections and can be reset by the user at any time, e.g., when
clearing browser cookies.

When issuing a security token to a client that supports Token
Binding, a server includes the client's Token Binding ID (or its
cryptographic hash) in the token. Later on, when a client presents a
security token containing a Token Binding ID, the server verifies
that the ID in the token matches the ID of the Token Binding
established with the client. In the case of a mismatch, the server
rejects the token (details are application-specific). application specific).

In order to successfully export and replay a bound security token, an
attacker needs to also be able to use the client's private key, which key; this
is hard to do if the key is specially protected, e.g., generated in a
secure hardware module.

1.1. Requirements Language

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.

2. Token Binding Protocol Overview

In the course of a TLS handshake, a client and server can use the
Token Binding Negotiation negotiation TLS Extension
[I-D.ietf-tokbind-negotiation] extension [RFC8472] to negotiate the
Token Binding protocol version and the parameters (signature
algorithm, length) of the Token Binding key. This negotiation does
not require additional
round-trips.

As described in [I-D.ietf-tokbind-negotiation], version round trips.

Version 1.0 of the Token Binding protocol is represented by
TB_ProtocolVersion.major = 1 and TB_ProtocolVersion.minor = 0 in the
Token Binding Negotiation negotiation TLS
Extension. extension; see [RFC8472] ("Transport
Layer Security (TLS) Extension for Token Binding Protocol
Negotiation").

The Token Binding protocol consists of one message sent by the client
to the server, proving possession of one or more client-generated
asymmetric private keys. This message is not sent if the Token
Binding Negotiation negotiation has been unsuccessful. The Token Binding message
is sent with the application protocol data over TLS.

A server receiving the Token Binding message verifies that the key
parameters in the message match the Token Binding parameters
negotiated (e.g., via [I-D.ietf-tokbind-negotiation]), [RFC8472]) and then validates the signatures
contained in the Token Binding message. If either of these checks
fails, the server rejects the binding, along with all associated
bound tokens. Otherwise, the Token Binding is successfully
established with the ID contained in the Token Binding message.

When a server supporting the Token Binding protocol receives a bound
token, the server compares the Token Binding ID in the token with the
Token Binding ID established with the client. If the bound token is
received on a TLS connection without a Token Binding, Binding or if the Token
Binding IDs do not match, the token is rejected.

This document defines the format of the Token Binding protocol
message, the process of establishing a Token Binding, the format of
the Token Binding ID, and the process of validating a bound token.
Token Binding Negotiation TLS Extension
[I-D.ietf-tokbind-negotiation]
[RFC8472] describes the negotiation of the Token Binding protocol and
key parameters. Token [RFC8473] ("Token Binding over HTTP
[I-D.ietf-tokbind-https] HTTP") explains how
the Token Binding message is encapsulated within HTTP/1.1 messages
[RFC7230] or HTTP/2 [RFC7540] messages.
[I-D.ietf-tokbind-https] messages [RFC7540]. [RFC8473] also describes
Token Binding between multiple communicating parties: User Agent,
Identity Provider Provider, and Relying Party.

3. Token Binding Protocol Message

The Token Binding message is sent by the client to prove possession
of one or more private keys held by the client. This message MUST be
sent if the client and server successfully negotiated the use of the
Token Binding protocol (e.g., via [I-D.ietf-tokbind-negotiation] [RFC8472] or a different mechanism), mechanism)
and MUST NOT be sent otherwise. This message MUST be sent in the
client's first application protocol message. This message MAY also
be sent in subsequent application protocol messages, proving
possession of additional private keys held by the same client, which client; this
information can be used to facilitate token binding Token Binding between more than
two communicating parties. For example, Token Binding over
HTTP [I-D.ietf-tokbind-https] [RFC8473] specifies an
encapsulation of the Token Binding message in HTTP application
protocol messages, as well as scenarios involving more than two
communicating parties.

The Token Binding message format is defined using the TLS Presentation
Language
presentation language (see Section 4 of [RFC5246]):

enum {
rsa2048_pkcs1.5(0), rsa2048_pss(1), ecdsap256(2), (255)
} TokenBindingKeyParameters;

struct {
opaque modulus<1..2^16-1>;
opaque publicexponent<1..2^8-1>;
} RSAPublicKey;

struct {
opaque point <1..2^8-1>;
} TB_ECPoint;

struct {
TokenBindingKeyParameters key_parameters;
uint16 key_length; /* Length (in bytes) of the following
TokenBindingID.TokenBindingPublicKey */
select (key_parameters) {
case rsa2048_pkcs1.5:
case rsa2048_pss:
RSAPublicKey rsapubkey;
case ecdsap256:
TB_ECPoint point;
} TokenBindingPublicKey;
} TokenBindingID;

enum {
(255) /* No initial TB_ExtensionType registrations */
} TB_ExtensionType;

struct {
TB_ExtensionType extension_type;
opaque extension_data<0..2^16-1>;
} TB_Extension;

enum {
provided_token_binding(0), referred_token_binding(1), (255)
} TokenBindingType;

struct {
TokenBindingType tokenbinding_type;
TokenBindingID tokenbindingid;
opaque signature<64..2^16-1>; /* Signature over the concatenation
of tokenbinding_type,
key_parameters
key_parameters, and exported
keying material (EKM) EKM */
TB_Extension extensions<0..2^16-1>;
} TokenBinding;

struct {
TokenBinding tokenbindings<132..2^16-1>;
} TokenBindingMessage;

The Token Binding message consists of a series of TokenBinding
structures, each containing the type of the token binding, Token Binding, the
TokenBindingID, and a signature using the Token Binding key,
optionally followed by TB_Extension structures.

3.1. TokenBinding.tokenbinding_type

This document defines two Token Binding types:

o provided_token_binding - used to establish a Token Binding when
connecting to a server.

o referred_token_binding - used when requesting tokens that are
intended to be presented to a different server.

Token Binding over HTTP [I-D.ietf-tokbind-https]

[RFC8473] describes a use case for referred_token_binding where Token
Bindings are established between multiple communicating parties:
User Agent, Identity Provider Provider, and Relying Party. The User Agent
sends referred_token_binding to the Identity Provider in order to
prove possession of the Token Binding key it uses with the Relying
Party. The Identity Provider can then bind the token it is supplying
(for presentation to the Relying Party) to the Token Binding ID
contained in the referred_token_binding.

An implementation MUST ignore any unknown Token Binding types.

3.2. TokenBinding.tokenbindingid

The ID of the Token Binding established as a result of Token Binding
message processing contains the identifier of the negotiated key
parameters, the length (in bytes) of the Token Binding public key,
and the Token Binding public key itself. The Token Binding ID can be
obtained from the TokenBinding structure by discarding the Token
Binding type, signature signature, and extensions.

When rsa2048_pkcs1.5 or rsa2048_pss is used, RSAPublicKey.modulus and
RSAPublicKey.publicexponent contain the modulus and exponent of a
2048-bit RSA public key represented in big-endian format, with
leading zero bytes omitted.

When ecdsap256 is used, TB_ECPoint.point contains the X coordinate
followed by the Y coordinate of a Curve P-256 key. The X and Y
coordinates are unsigned 32-byte integers encoded in big-endian
format, preserving any leading zero bytes. Future specifications may
define Token Binding keys using other elliptic curves with their
corresponding signature and point formats.

Token Binding protocol implementations SHOULD make Token Binding IDs
available to the application as opaque byte sequences, so that
applications do not rely on a particular Token Binding ID structure.
E.g.,
For example, server applications will use Token Binding IDs when
generating and verifying bound tokens.

3.3. TokenBinding.signature

When rsa2048_pkcs1.5 is used, TokenBinding.signature contains the
signature generated using the RSASSA-PKCS1-v1_5 signature scheme
defined in [RFC8017] with SHA256 [FIPS.180-4.2015] as the hash
function.

When rsa2048_pss is used, TokenBinding.signature contains the
signature generated using the RSASSA-PSS signature scheme RSA Probabilistic Signature Scheme
(RSASSA-PSS) defined in [RFC8017] with SHA256 as the hash function.
MGF1 with SHA256 MUST be used as the mask generation function, function (MGF),
and the salt length MUST equal 32 bytes.

When ecdsap256 is used, TokenBinding.signature contains a pair of
32-byte integers, R followed by S, generated with ECDSA the Elliptic Curve
Digital Signature Algorithm (ECDSA) using Curve P-256 and SHA256 as
defined in [ANSI.X9-62.2005] [FIPS.186-4.2013] and
[FIPS.186-4.2013]. [ANSI.X9-62.2005]. R and S are
encoded in big-endian format, preserving any leading zero bytes.

The signature is computed over the byte string representing the
concatenation of:

o The TokenBindingType value contained in the
TokenBinding.tokenbinding_type field; field,

o The TokenBindingKeyParameters value contained in the
TokenBindingID.key_parameters field; field, and

o Exported keying material (EKM) The EKM value obtained from the current TLS connection.

Please note that TLS 1.2 and earlier versions support renegotiation,
which produces a new TLS master secret for the same connection, with
the associated session keys and EKM value. TokenBinding.signature
MUST be a signature of the EKM value derived from the TLS master
secret that produced the session keys encrypting the TLS
application_data record(s) containing this TokenBinding. Such use of
the current EKM for the TLS connection makes replay of bound tokens
within renegotiated TLS sessions detectable, detectable but requires the
application to synchronize Token Binding message generation and
verification with the TLS handshake state.

Specifications defining the use of Token Binding with application
protocols, such as Token Binding over HTTP [I-D.ietf-tokbind-https], [RFC8473], MAY prohibit
the use of TLS renegotiation in combination with Token Binding,
obviating the need for such synchronization. Alternatively, such
specifications need to define (1) a way to determine which EKM value
corresponds to a given TokenBindingMessage, TokenBindingMessage and (2) a mechanism
preventing that
prevents a TokenBindingMessage from being split across TLS
renegotiation boundaries (i.e., due to TLS message fragmentation - fragmentation; see
Section 6.2.1 of [RFC5246]). [RFC5246]. Note that application layer application-layer messages
conveying a TokenBindingMessage may cross renegotiation boundaries in
ways that make processing difficult.

The EKM is obtained using the Keying Material Exporters keying material exporters for TLS as
defined in [RFC5705], by supplying the following input values:

o Label: The ASCII string "EXPORTER-Token-Binding" with no
terminating NUL.

o Context value: No application context supplied.

o Length: 32 bytes.

3.4. TokenBinding.extensions

A Token Binding message may optionally contain a series of
TB_Extension structures, each consisting of an extension_type and
extension_data. The structure and meaning of extension_data depends
on the specific extension_type.

Initially, no extension types are defined (see Section 6.3
"Token
("Token Binding Extensions Registry"). Registry")). One of the possible uses of
extensions envisioned at the time of this writing is attestation:
cryptographic proof that allows the server to verify that the Token
Binding key is hardware-bound. hardware bound. The definitions of such Token Binding
protocol extensions are outside the scope of this specification.

4. Establishing a Token Binding

4.1. Client Processing Rules

The client MUST include at least one TokenBinding structure in the
Token Binding message. The When a provided_token_binding is included,
the key parameters used in a provided_token_binding MUST match those
negotiated with the server (e.g., via [I-D.ietf-tokbind-negotiation] [RFC8472] or a different
mechanism).

The client MUST generate and store Token Binding keys in a secure
manner that prevents key export. In order to prevent cooperating
servers from linking user identities, the scope of the Token Binding
keys MUST NOT be broader than the scope of the tokens, as defined by
the application protocol.

When the client needs to send a referred_token_binding to the
Identity Provider, the client SHALL construct the referred
TokenBinding structure in the following manner:

o Set TokenBinding.tokenbinding_type to referred_token_binding.

o Set TokenBinding.tokenbindingid to the Token Binding ID used with
the Relying Party.

o Generate TokenBinding.signature, using the EKM value of the TLS
connection to the Identity Provider, the Token Binding key
established with the Relying Party Party, and the signature algorithm
indicated by the associated key parameters. Note that these key
parameters may differ from the key parameters negotiated with the
Identity Provider.

Conveying referred Token Bindings in this fashion allows the Identity
Provider to verify that the client controls the Token Binding key
used with the Relying Party.

4.2. Server Processing Rules

The triple handshake vulnerability in TLS 1.2 and older TLS versions
affects the security of the Token Binding protocol, as described in
Section 7 "Security Considerations". ("Security Considerations"). Therefore, the server
MUST NOT negotiate the use of the Token Binding protocol with these
TLS versions, unless the server also negotiates the Extended Master
Secret extended master
secret TLS extension [RFC7627] and Renegotiation Indication [RFC5746] the renegotiation indication TLS
extensions.
extension [RFC5746].

If the use of the Token Binding protocol was not negotiated, negotiated but the
client sends the a Token Binding message, the server MUST reject any
contained bindings.

If the Token Binding type is "provided_token_binding", the server
MUST verify that the signature algorithm (including an elliptic curve
in the case of ECDSA) and key length in the Token Binding message
match those negotiated with this client (e.g., via
[I-D.ietf-tokbind-negotiation] [RFC8472] or a
different mechanism). In the case of a mismatch, the server MUST
reject the binding. Token Bindings of type "referred_token_binding"
may use different key parameters than those negotiated with this
client.

If the Token Binding message does not contain at least one
TokenBinding structure, structure or if a signature contained in any
TokenBinding structure is invalid, the server MUST reject the
binding.

Servers MUST ignore any unknown extensions. Initially, no extension
types are defined (see Section 6.3
"Token ("Token Binding Extensions Registry").
Registry")).

If all checks defined above have passed successfully, the Token
Binding between this client and server is established. The Token
Binding ID(s) conveyed in the Token Binding Message message can be provided
to the server-side application. The application may then use the
Token Binding IDs for bound security token creation and validation, validation;
see Section 5.

If a Token Binding is rejected, any associated bound tokens presented
on the current TLS connection MUST also be rejected by the server.
The effect of this is application-specific, application specific, e.g., failing requests, a
requirement for the client to re-authenticate and present a different
token, or connection termination.

5. Bound Security Token Creation and Validation

Security tokens can be bound to the TLS layer in a variety of ways: ways,
e.g., by embedding the Token Binding ID or its cryptographic hash in
the
token, token or by maintaining a database mapping tokens to Token
Binding IDs. The specific method of generating bound security tokens
is
application-defined defined by the application and is beyond the scope of this
document. Note that applicable security considerations are outlined
in Section 7.

Either or both clients and servers MAY create bound security tokens.
For example, HTTPS servers employing Token Binding for securing their
HTTP cookies will bind these cookies. In the case of a server-
initiated challenge-response protocol employing Token Binding and
TLS, the client can, for example, incorporate the Token Binding ID
within the signed object it returns, thus binding the object.

Upon receipt of a security token, the server attempts to retrieve
Token Binding ID information from the token and from the TLS
connection with the client. Application-provided policy determines
whether to honor non-bound (bearer) tokens. If the token is bound
and a Token Binding has not been established for the client
connection, the server MUST reject the token. If the Token Binding
ID for the token does not match the Token Binding ID established for
the client connection, the server MUST reject the token.

6. IANA Considerations

This section establishes three IANA registries on a new IANA registry page
entitled titled "Token Binding Protocol":
Protocol" with subregistries "Token Binding Key Parameters", "Token
Binding Types" Types", and "Token Binding Extensions". It also registers a
new TLS exporter label in the TLS "TLS Exporter Label
Registry. Labels" registry.

6.1. Token Binding Key Parameters Registry

This document establishes a registry subregistry for identifiers of Token
Binding key parameters entitled titled "Token Binding Key Parameters" under
the "Token Binding Protocol" heading. registry.

Entries in this registry require the following fields:

o Value: The octet value that identifies a set of Token Binding key
parameters (0-255).

o Description: The description of the Token Binding key parameters.

o Specification: Reference: A reference to a specification that defines the Token
Binding key parameters.

This registry operates under the "Specification Required" policy as
defined in [RFC8126]. The designated expert will require the
inclusion of a reference to a permanent and readily available
specification that enables the creation of interoperable
implementations using the identified set of Token Binding key
parameters.

An initial set of registrations for this registry follows:

Value: 0
Description: rsa2048_pkcs1.5
Specification: this This document

---------------------------------

Value: 1
Description: rsa2048_pss
Specification: this This document

---------------------------------

Value: 2
Description: ecdsap256
Specification: this This document

6.2. Token Binding Types Registry

This document establishes a registry subregistry for Token Binding type
identifiers entitled titled "Token Binding Types" under the "Token Binding
Protocol" heading. registry.

Entries in this registry require the following fields:

o Value: The octet value that identifies the Token Binding type
(0-255).

o Description: The description of the Token Binding type.

o Specification: Reference: A reference to a specification that defines the Token
Binding type.

An initial set of registrations for this registry follows:

Value: 0
Description: provided_token_binding
Specification: this This document

---------------------------------

Value: 1
Description: referred_token_binding
Specification: this This document

6.3. Token Binding Extensions Registry

This document establishes a registry subregistry for Token Binding extensions
entitled
titled "Token Binding Extensions" under the "Token Binding Protocol" heading.
registry.

Entries in this registry require the following fields:

o Value: The octet value that identifies the Token Binding extension
(0-255).

o Description: The description of the Token Binding extension.

o Specification: Reference: A reference to a specification that defines the Token
Binding extension.

6.4. Registration of Token Binding TLS Exporter Label

This document adds the following registration in the TLS "TLS Exporter
Label Registry:
Labels" registry:

Value: EXPORTER-Token-Binding
DTLS-OK: Y
Recommended: Y
Reference: this This document

7. Security Considerations

7.1. Security Token Replay

The goal of the Token Binding protocol is to prevent attackers from
exporting and replaying security tokens, tokens and from thereby
impersonating legitimate users and gaining access to protected
resources. Bound tokens can be replayed by malware present in
User Agents, which Agents; this may be undetectable by to a server. However, in order
to export bound tokens to other machines and successfully replay
them, attackers also need to export corresponding Token Binding
private keys. Token Binding private keys are therefore high-value
assets and SHOULD be strongly protected, ideally by generating them
in a hardware security module that prevents key export.

The manner in which a token is bound to the TLS layer is application- defined by
the application and is beyond the scope of this document. However,
the resulting bound token needs to be integrity-protected, so that an
attacker cannot remove the binding or substitute a Token Binding ID
of their choice without detection.

The Token Binding protocol does not prevent cooperating clients from
sharing a bound token. A client could intentionally export a bound
token with the corresponding Token Binding private key, key or perform
signatures using this key on behalf of another client.

7.2. Downgrade Attacks

The Token Binding protocol MUST be negotiated using a mechanism that
prevents downgrade. E.g., [I-D.ietf-tokbind-negotiation] uses downgrade attacks. For example, [RFC8472] specifies a TLS
extension for Token Binding negotiation. TLS detects handshake
message modification by active attackers, therefore attackers; therefore, it is not
possible for an attacker to remove or modify the "token_binding"
extension without breaking the TLS handshake. The signature
algorithm and key length used in the TokenBinding of type
"provided_token_binding" MUST match the negotiated parameters.

7.3. Privacy Considerations

The Token Binding protocol uses persistent, long-lived Key-Sharing between Applications

Existing systems provide a variety of platform-specific mechanisms
for certain applications to share tokens, e.g., to enable "single
sign-on" scenarios. For these scenarios to keep working with bound
tokens, the applications that are allowed to share tokens will need
to also share Token Binding
IDs. To protect privacy, keys. Care must be taken to restrict the
sharing of Token Binding IDs are never transmitted in
clear text and can be reset by keys to the user at any time, e.g. when
clearing browser cookies. Some same group(s) of applications offer a special privacy
mode where they don't store or use tokens supplied by the server,
e.g., "in private" browsing. When operating in this special privacy
mode, applications SHOULD use newly generated Token Binding keys and
delete them when exiting this mode, or else SHOULD NOT negotiate
Token Binding at all.

In order to prevent cooperating servers from linking user identities,
the scope of the Token Binding keys MUST NOT be broader than the
scope of the tokens, as defined by the application protocol.

A server can use tokens and Token Binding IDs to track clients.
Client applications that automatically limit the lifetime or scope of
tokens to maintain user privacy SHOULD apply the same validity time
and scope limits to Token Binding keys.

7.4. Token Binding Key Sharing Between Applications

Existing systems provide a variety of platform-specific mechanisms
for certain applications to share tokens, e.g. to enable single sign-
on scenarios. For these scenarios to keep working with bound tokens,
the applications that are allowed to share tokens will need to also
share Token Binding keys. Care must be taken to restrict the sharing
of Token Binding keys to the same group(s) of applications that share
that shares the same tokens.

7.5.

7.4. Triple Handshake Vulnerability in TLS 1.2 and Older TLS Versions

The Token Binding protocol relies on the TLS Exporters exporters [RFC5705] to
associate a TLS connection with a Token Binding. The triple
handshake attack [TRIPLE-HS] is a known vulnerability in TLS 1.2 and
older TLS versions, allowing the attacker to synchronize keying
material between TLS connections. The attacker can then successfully
replay bound tokens. For this reason, the Token Binding protocol
MUST NOT be negotiated with these TLS versions, unless the Extended
Master Secret extended
master secret TLS extension [RFC7627] and Renegotiation Indication [RFC5746] the renegotiation
indication TLS
extensions extension [RFC5746] have also been negotiated.

8. Acknowledgements

This document incorporates comments Privacy Considerations

The Token Binding protocol uses persistent, long-lived Token Binding
IDs. To protect privacy, Token Binding IDs are never transmitted in
clear text and suggestions offered can be reset by the user at any time, e.g., when
clearing browser cookies. Some applications offer a special privacy
mode where they don't store or use tokens supplied by the server,
e.g., "in private" browsing. When operating in this special privacy
mode, applications SHOULD use newly generated Token Binding keys and
delete them when exiting this mode; otherwise, they SHOULD NOT
negotiate Token Binding at all.

In order to prevent cooperating servers from linking user identities,
the scope of the Token Binding keys MUST NOT be broader than the
scope of the tokens, as defined by Eric
Rescorla, Gabriel Montenegro, Martin Thomson, Vinod Anupam, Anthony
Nadalin, Michael B. Jones, Bill Cox, Nick Harper, Brian Campbell,
Benjamin Kaduk, Alexey Melnikov the application protocol.

A server can use tokens and others.

This document was produced under Token Binding IDs to track clients.
Client applications that automatically limit the chairmanship lifetime or scope of John Bradley and
Leif Johansson. The area directors included Eric Rescorla, Kathleen
Moriarty
tokens to maintain user privacy SHOULD apply the same validity time
and Stephen Farrell. scope limits to Token Binding keys.

9. References

9.1. Normative References

[ANSI.X9-62.2005]
American National Standards Institute, "Public Key
Cryptography for the Financial Services Industry, Industry: The
Elliptic Curve Digital Signature Algorithm (ECDSA)",
ANSI X9.62, November 2005.

[FIPS.180-4.2015]
National Institute of Standards and Technology, "Secure
Hash Standard (SHS)", FIPS 180-4,
DOI 10.6028/NIST.FIPS.180-4, August 2015,
<https://nvlpubs.nist.gov/nistpubs/FIPS/
NIST.FIPS.180-4.pdf>.

[FIPS.186-4.2013]
National Institute of Standards and Technology, "Digital
Signature Standard (DSS)", FIPS 186-4, 2013.

[I-D.ietf-tokbind-https]
Popov, A., Nystrom, M., Balfanz, D., Langley, A., Harper,
N., and J. Hodges, "Token Binding over HTTP", draft-ietf-
tokbind-https-15 (work in progress), May 2018.

[I-D.ietf-tokbind-negotiation]
Popov, A., Nystrom, M., Balfanz, D., and A. Langley,
"Transport Layer Security (TLS) Extension for Token
Binding Protocol Negotiation", draft-ietf-tokbind-
negotiation-13 (work in progress), May 2018.
DOI 10.6028/NIST.FIPS.186-4, July 2013,
<https://nvlpubs.nist.gov/nistpubs/fips/
nist.fips.186-4.pdf>.

[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>.

[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<https://www.rfc-editor.org/info/rfc5246>.

[RFC5705] Rescorla, E., "Keying Material Exporters for Transport
Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705,
March 2010, <https://www.rfc-editor.org/info/rfc5705>.

[RFC5746] Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
"Transport Layer Security (TLS) Renegotiation Indication
Extension", RFC 5746, DOI 10.17487/RFC5746, February 2010,
<https://www.rfc-editor.org/info/rfc5746>.

[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<https://www.rfc-editor.org/info/rfc7230>.

[RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
DOI 10.17487/RFC7540, May 2015,
<https://www.rfc-editor.org/info/rfc7540>.

[RFC7627] Bhargavan, K., Ed., Delignat-Lavaud, A., Pironti, A.,
Langley, A., and M. Ray, "Transport Layer Security (TLS)
Session Hash and Extended Master Secret Extension",
RFC 7627, DOI 10.17487/RFC7627, September 2015,
<https://www.rfc-editor.org/info/rfc7627>.

[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>.

[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>.

[RFC8472] Popov, A., Ed., Nystroem, M., and D. Balfanz, "Transport
Layer Security (TLS) Extension for Token Binding Protocol
Negotiation", RFC 8472, DOI 10.17487/RFC8472, September
2018, <https://www.rfc-editor.org/info/rfc8472>.

[RFC8473] Popov, A., Nystroem, M., Balfanz, D., Ed., Harper, N., and
J. Hodges, "Token Binding over HTTP", RFC 8473,
DOI 10.17487/RFC8473, September 2018,
<https://www.rfc-editor.org/info/rfc8473>.

9.2. Informative References

[RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
RFC 6749, DOI 10.17487/RFC6749, October 2012,
<https://www.rfc-editor.org/info/rfc6749>.

[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>.

[TOKENBIND-TLS13]
Harper, N., "Token Binding for Transport Layer Security
(TLS) Version 1.3 Connections", Work in Progress,
draft-ietf-tokbind-tls13-01, May 2018.

[TRIPLE-HS]
Bhargavan, K., Delignat-Lavaud, A., Fournet, C., Pironti,
A., and P. Strub, "Triple Handshakes and Cookie Cutters:
Breaking and Fixing Authentication over TLS. TLS",
IEEE Symposium on Security and Privacy", Privacy,
DOI 10.1109/SP.2014.14, May 2014.

Acknowledgements

This document incorporates comments and suggestions offered by Eric
Rescorla, Gabriel Montenegro, Martin Thomson, Vinod Anupam, Anthony
Nadalin, Michael B. Jones, Bill Cox, Nick Harper, Brian Campbell,
Benjamin Kaduk, Alexey Melnikov, and others.

This document was produced under the chairmanship of John Bradley and
Leif Johansson. The area directors included Eric Rescorla, Kathleen
Moriarty, and Stephen Farrell.

Authors' Addresses

Andrei Popov (editor)
Microsoft Corp.
USA
United States of America

Email: andreipo@microsoft.com
Magnus Nystroem
Microsoft Corp.
USA
United States of America

Email: mnystrom@microsoft.com

Dirk Balfanz
Google Inc.
USA
United States of America

Email: balfanz@google.com

Adam Langley
Google Inc.
USA

Email: agl@google.com

Jeff Hodges
PayPal
USA
Kings Mountain Systems
United States of America

Email: Jeff.Hodges@paypal.com Jeff.Hodges@KingsMountain.com