wdiff rfc9577v1.txt rfc9577.txt

Internet Engineering Task Force (IETF) T. Pauly
Request for Comments: 9577 Apple Inc.
Category: Standards Track S. Valdez
ISSN: 2070-1721 Google LLC
C. A. Wood
Cloudflare
May
June 2024

The Privacy Pass HTTP Authentication Scheme

Abstract

This document defines an HTTP authentication scheme for Privacy Pass,
a privacy-preserving authentication mechanism used for authorization.
The authentication scheme specified in this document can be used by
clients
Clients to redeem Privacy Pass tokens with an origin. Origin. It can also be
used by origins Origins to challenge clients Clients to present Privacy Pass tokens.

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/rfc9577.

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. Terminology
2. HTTP Authentication Scheme
2.1. Token Challenge
2.1.1. Token Challenge Structure
2.1.2. Sending Token Challenges
2.1.3. Processing Token Challenges
2.1.4. Token Caching
2.2. Token Redemption
2.2.1. Token Structure
2.2.2. Sending Tokens
2.2.3. Token Verification
3. Client Behavior
3.1. Choosing to Redeem Tokens
3.2. Choosing between Multiple Challenges
4. Origin Behavior
4.1. Greasing
5. Security Considerations
5.1. Randomness Requirements
5.2. Replay Attacks
5.3. Reflection Attacks
5.4. Token Exhaustion Attacks
5.5. Timing Correlation Attacks
5.6. Cross-Context Linkability Attacks
6. IANA Considerations
6.1. Authentication Scheme
6.2. Privacy Pass Token Type Types Registry
6.2.1. Reserved Values
7. References
7.1. Normative References
7.2. Informative References
Appendix A. Test Vectors
A.1. Challenge and Redemption Structure Test Vectors
A.2. HTTP Header Test Vectors
Authors' Addresses

1. Introduction

Privacy Pass tokens are unlinkable authenticators that can be used to
anonymously authorize a client Client (see [ARCHITECTURE]). Tokens are
generated by token issuers, Issuers, on the basis of authentication,
attestation, or some previous action such as solving a CAPTCHA. A
client
Client possessing such a token is able to prove that it was able to
get a token issued, without allowing the relying party redeeming the
client's
Client's token (the origin) Origin) to link it with the issuance flow.

Different types of authenticators, using different token issuance
protocols, can be used as Privacy Pass tokens.

This document defines a common HTTP authentication scheme ([RFC9110], ([HTTP],
Section 11), PrivateToken, "PrivateToken", that allows clients Clients to redeem various
kinds of Privacy Pass tokens.

Clients and relying parties (origins) (Origins) interact using this scheme to
perform the token challenge and token redemption flow. In
particular, origins Origins challenge clients Clients for a token with an HTTP
Authentication
authentication challenge (using the WWW-Authenticate response header
field). Clients can then react to that challenge by issuing a new
request with a corresponding token (using the Authorization request
header field). Clients generate tokens that match the origin's Origin's token
challenge by running one of the token issuance protocol protocols defined in
[ISSUANCE]. The act of presenting a token in an Authorization
request header field is referred to as "token redemption". This
interaction between the
client Client and origin Origin is shown below.

Figure 1: Challenge and Redemption Protocol Flow

In addition to working with different token issuance protocols, this
scheme optionally supports the use of tokens that are associated with
origin-chosen
Origin-chosen contexts and specific origin Origin names. Relying parties
that request and redeem tokens can choose a specific kind of token,
as appropriate for its use case. These options (1) allow for
different deployment models to prevent double-spending, double-spending and (2) allow
for both interactive (online challenges) and non-interactive (pre-fetched) (pre-
fetched) tokens.

1.1. 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.

Unless otherwise specified, this document encodes protocol messages
in TLS notation from [TLS13], Section 3.

This document uses the terms "Client", "Origin", "Issuer", "issuance
protocol", and "Token" as defined in [ARCHITECTURE]. It additionally
uses the following terms in more specific ways:

Issuer key: Keying material that can be used with an issuance
protocol to create a signed token.

Token challenge: A request for tokens sent from an origin Origin to a
client,
Client, using the "WWW-Authenticate" WWW-Authenticate HTTP header field. This
challenge identifies a specific token issuer Issuer and issuance
protocol. Token challenges optionally include one or both of the
following: a redemption context (see Section 2.1.1.2) and a list
of associated origins. Origins. These optional values can then be bound to
the token that is issued.

Token redemption: An action by which a client Client presents a token to an
origin
Origin in an HTTP request, using the "Authorization" Authorization HTTP header
field.

2. HTTP Authentication Scheme

Token redemption is performed using HTTP Authentication ([RFC9110], authentication ([HTTP],
Section 11), with the scheme "PrivateToken". Origins challenge
clients
Clients to present a token from a specific issuer Issuer (Section 2.1).
Once a client Client has received a token from that issuer Issuer or already has a
valid token available, it presents the token to the origin Origin
(Section 2.2). The process of presenting a token as authentication
to an origin Origin is also referred to as "spending" a token.

In order to prevent linkability across different transactions,
clients
Clients will often present a particular "PrivateToken" only once.
Origins can link multiple transactions to the same client Client if that
client
Client spends the same token value more than once. As such, origins Origins
ought to expect at most one unique token value, carried in one
request, for each challenge.

The rest of this section describes the token challenge and redemption
interactions in more detail.

2.1. Token Challenge

Origins send a token challenge to clients Clients in a "WWW-Authenticate" WWW-Authenticate
header field with the "PrivateToken" scheme. This authentication
scheme has two mandatory parameters: one containing a token challenge
and another containing the token-key used for producing (and
verifying) a corresponding token.

Origins that support the "PrivateToken" authentication scheme need to
handle the following tasks in constructing the WWW-Authenticate
header field:

1. Select which issuer Issuer to use, and configure the issuer Issuer name and
token-key to include in WWW-Authenticate token challenges. The
issuer
Issuer name is included in the token challenge, and the issuer Issuer
token-key is used to populate the WWW-Authenticate header
parameter.

2. Determine a redemption context construction to include in the
token challenge, as discussed in Section 2.1.1.2.

3. Select the origin Origin information to include in the token challenge.
This can be empty to allow fully cross-origin cross-Origin tokens, a single
origin
Origin name that matches the origin Origin itself for per-origin per-Origin tokens,
or a list of origin Origin names containing the origin Origin itself. See
Section 3.4 of [ARCHITECTURE] for more information about the
difference between cross-origin cross-Origin and per-origin per-Origin tokens.

Once these decisions are made, origins Origins construct the WWW-Authenticate
header by first constructing the token challenge as described in
Section 2.1.1. Origins send challenges as described in
Section 2.1.2, and clients Clients process them as described in
Sections 2.1.3 and 2.1.4.

2.1.1. Token Challenge Structure

This document defines the default challenge structure that can be
used across token types, although future token types MAY extend or
modify the structure of the challenge; see Section 6.2 for the
registry information that establishes and defines the relationship
between "token_type" token_type and the contents of the TokenChallenge message.

All token challenges MUST begin with a 2-octet integer that defines
the token type, in network byte order. This type indicates the
issuance protocol used to generate the token and determines the
structure and semantics of the rest of the structure. Values are
registered in an IANA registry; see Section 6.2. Clients MUST ignore
challenges with token types they do not support.

Even when a given token type uses the default challenge structure,
the requirements on the presence or interpretation of the fields can
differ across token types. For example, some token types might
require that "origin_info" origin_info is non-empty, while others allow it to be
empty.

The default TokenChallenge message has the following structure:

struct {
uint16_t token_type;
opaque issuer_name<1..2^16-1>;
opaque redemption_context<0..32>;
opaque origin_info<0..2^16-1>;
} TokenChallenge;

The structure fields are defined as follows:

* "token_type" token_type is a 2-octet integer, in network byte order, as
described above.

* "issuer_name" issuer_name is an ASCII string that identifies the issuer, Issuer, using
the format of a server name as defined in Section 2.1.1.1. This
name identifies the issuer Issuer that is allowed to issue tokens that
can be redeemed by this origin. Origin. The field that stores this string
in the challenge is prefixed with a 2-octet integer indicating the
length, in network byte order.

* "redemption_context" redemption_context is a field that is either 0 or 32 bytes,
prefixed with a single octet indicating the length (either 0 or
32). If the value is non-empty, it is a 32-byte value generated
by the origin Origin that allows the origin Origin to require that clients Clients fetch
tokens bound to a specific context, as opposed to reusing tokens
that were fetched for other contexts. See Section 2.1.1.2 for
example contexts that might be useful in practice. Challenges
with redemption_context values of invalid lengths MUST be ignored.

* "origin_info" origin_info is an ASCII string that either is empty or contains
one or more origin Origin names that allow a token to be scoped to a
specific set of origins. Origins. Each origin Origin name uses the format of a
server name as defined in Section 2.1.1.1. The string is prefixed
with a 2-octet integer indicating the length, in network byte
order. If empty, any non-origin-specific non-Origin-specific token can be redeemed.
If the string contains multiple origin Origin names, they are delimited
with commas (",") without any whitespace. If this field is not
empty, the Origin MUST include its own name as one of the names in
the list.

If "origin_info" origin_info contains multiple origin Origin names, this means the
challenge is valid for any of the origins Origins in the list, including the
origin
Origin that issued the challenge (which must always be present in the
list if it is non-empty; see Section 2.1.3). This can be useful in
settings where clients Clients pre-fetch and cache tokens for a particular
challenge -- including the "origin_info" origin_info field -- and then later redeem
these tokens with one of the origins Origins in the list. See Section 2.1.4
for more discussion about token caching.

2.1.1.1. Server Name Encoding

Server names contained in a token challenge are ASCII strings that
contain a hostname and optional port, where the port is implied to be
"443" if missing. The names use the format of the authority portion
of a URI as defined in Section 3.2 of [URI]. The names MUST NOT
include a "userinfo" portion of an authority. For example, a valid
server name might be "issuer.example.com" or
"issuer.example.com:8443", but not "issuer@example.com".

2.1.1.2. Redemption Context Construction

The TokenChallenge redemption context allows the origin Origin to determine
the context in which a given token can be redeemed. This value can
be a unique per-request nonce, constructed from 32 freshly generated
random bytes. It can also represent state or properties of the
client
Client session. Some example properties and methods for constructing
the corresponding context are below. This list is not exhaustive.

Context bound to a given time window: Construct the redemption
context as F(current time window), where F is a pseudorandom
function.

Context bound to a client network: Client network based on Autonomous System
Number (ASN): Construct the redemption context as F(client F(Client ASN),
where F is a pseudorandom function.

Context bound to a given time window and client Client network: Construct
the redemption context as F(current time window, client Client ASN),
where F is a pseudorandom function.

Preventing double-spending on tokens requires the origin Origin to keep
state associated with the redemption context. An empty redemption
context is not bound to any property of the client Client request, so state
to prevent double-spending needs to be stored and shared across all
origin
Origin servers that can accept tokens until token-key expiration or
rotation. For a non-empty redemption context, the double-spend state
only needs to be stored across the set of origin Origin servers that will
accept tokens with that redemption context.

Origins that share redemption contexts, i.e., by using the same
redemption context, choosing the same issuer, Issuer, and providing the same
origin_info field in the TokenChallenge, must necessarily share state
required to enforce double-spend prevention. Origins should consider
the operational complexity of this shared state before choosing to
share redemption contexts. Failure to successfully synchronize this
state and use it for double-spend prevention can allow Clients to
redeem tokens to one Origin that were issued after an interaction
with another Origin that shares the context.

2.1.2. Sending Token Challenges

When used in an authentication challenge, the "PrivateToken" scheme
uses the following parameters:

* "challenge", challenge, which contains a base64url TokenChallenge value,
encoded per [RFC4648]. This document follows the default padding
behavior described in Section 3.2 of [RFC4648], so the base64url
value MUST include padding. As an authentication parameter (auth-
param from [RFC9110], [HTTP], Section 11.2), the value can be either a token
or a quoted-string and might be required to be a quoted-
string quoted-string if
the base64url string includes "=" characters. This parameter is
required for all challenges.

* "token-key", token-key, which contains a base64url encoding of the public key
for use with the issuance protocol indicated by the challenge.
See [ISSUANCE] for more information about how this public key is
used by the issuance protocols described in that specification.
The encoding of the public key is determined by the token type;
see Section 6.2. As with "challenge", challenge, the base64url value MUST
include padding. As an authentication parameter (auth-param from
[RFC9110],
[HTTP], Section 11.2), the value can be either a token or a
quoted-string and might be required to be a quoted-string if the
base64url string includes "=" characters. This parameter MAY be
omitted in deployments where clients Clients are able to retrieve the
issuer
Issuer key using an out-of-band mechanism.

* "max-age", max-age, which is an optional parameter that consists of the
number of seconds for which the challenge will be accepted by the
origin.
Origin.

The header field MAY also include the standard "realm" realm parameter, if
desired. Issuance protocols MAY define other parameters, some of
which might be required. Clients MUST ignore parameters in
challenges that are not defined for the issuance protocol
corresponding to the token type in the challenge.

As an example, the WWW-Authenticate header field could look like
this:

WWW-Authenticate:
PrivateToken challenge="abc...", token-key="123..."

2.1.2.1. Sending Multiple Token Challenges

It is possible for the WWW-Authenticate header field to include
multiple challenges ([RFC9110], ([HTTP], Section 11.6.1). This allows the
origin Origin
to indicate support for different token types, issuers, types or different Issuers,
or to include multiple redemption contexts. For example, the WWW-
Authenticate header field could look like this:

WWW-Authenticate:
PrivateToken challenge="abc...", token-key="123...",
PrivateToken challenge="def...", token-key="234..."

Origins should only include challenges for different types of
issuance protocols with functionally equivalent properties. For
instance, both issuance protocols in [ISSUANCE] have the same
functional properties, albeit with different mechanisms for verifying
the resulting tokens during redemption. Since clients Clients are free to
choose which challenge they want to consume when presented with
options, mixing multiple challenges with different functional
properties for one use case is nonsensical. If the origin Origin has a
preference for one challenge over another (for example, if one uses a
token type that is faster to verify), it can sort it to be first in
the list of challenges as a hint to the client. Client.

2.1.3. Processing Token Challenges

Upon receipt of a challenge, a client Client validates the TokenChallenge
structure before taking any action, such as fetching a new token or
redeeming a token in a new request. Validation requirements are as
follows:

* The token_type is recognized and supported by the client; Client;

* The TokenChallenge structure is well-formed; and

* If the origin_info field is non-empty, the name of the origin Origin that
issued the authentication challenge is included in the list of
origin
Origin names. Comparison of the origin Origin name that issued the
authentication challenge against elements in the origin_info list
is done via case-insensitive equality checks.

If validation fails, the client Client MUST NOT fetch or redeem a token
based on the challenge. Clients MAY have further restrictions and
requirements around validating when a challenge is considered
acceptable or valid. For example, clients Clients can choose to ignore
challenges that list origin Origin names for which the current connection is
not authoritative (according to the TLS certificate).

Caching and pre-fetching of tokens are discussed in Section 2.1.4.

2.1.4. Token Caching

Clients can generate multiple tokens from a single TokenChallenge and
cache them for future use. This improves privacy by separating the
time of token issuance from the time of token redemption, and also
allows clients Clients to avoid the overhead of receiving new tokens via the
issuance protocol.

Cached tokens can only be redeemed when they match all of the fields
in the TokenChallenge: token_type, issuer_name, redemption_context,
and origin_info. Clients ought to store cached tokens based on all
of these fields, to avoid trying to redeem a token that does not
match. Note that each token has a unique client Client nonce, which is sent
in token redemption (Section 2.2).

If a client Client fetches a batch of multiple tokens for future use that
are bound to a specific redemption context (the redemption_context in
the TokenChallenge was not empty), clients Clients SHOULD discard these
tokens upon flushing state such as HTTP cookies [COOKIES], or if
there is a network change and the client Client does not have any origin- Origin-
specific state like HTTP cookies. Using these tokens in a context
that otherwise would not be linkable to the original context could
allow the origin Origin to recognize a client. Client.

2.2. Token Redemption

The output of the issuance protocol is a token that corresponds to
the origin's Origin's challenge (see Section 2.1).

2.2.1. Token Structure

A token is a structure that begins with a two-octet 2-octet field that
indicates a token type, which MUST match the token_type in the
TokenChallenge structure. This value determines the structure and
semantics of the rest of the token structure.

This document defines the default token structure that can be used
across token types, although future token types MAY extend or modify
the structure of the token; see Section 6.2 for the registry
information that establishes and defines the relationship between
"token_type"
token_type and the contents of the Token token structure.

The default Token token message has the following structure:

struct {
uint16_t token_type;
uint8_t nonce[32];
uint8_t challenge_digest[32];
uint8_t token_key_id[Nid];
uint8_t authenticator[Nk];
} Token;

The structure fields are defined as follows:

* "token_type" token_type is a 2-octet integer, in network byte order, as
described above.

* "nonce" nonce is a 32-octet value containing a client-generated Client-generated random
nonce.

* "challenge_digest" challenge_digest is a 32-octet value containing the hash of the
original TokenChallenge, SHA-256(TokenChallenge), where SHA-256 is
as defined in [SHS]. Changing the hash function to something
other than SHA-256 would require defining a new token type and
token structure (since the contents of challenge_digest would be
computed differently), which can be done in a future
specification.

* "token_key_id" token_key_id is a Nid-octet identifier for the token
authentication key. The value of this field is defined by the
token_type and corresponding issuance protocol.

* "authenticator" authenticator is a Nk-octet authenticator that is
cryptographically bound to the preceding fields in the token; see
Section 2.2.3 for more information about how this field is used in
verifying a token. The token_type and corresponding issuance
protocol determine the value of the authenticator field and how it
is computed. The value of constant Nk depends on token_type, as
defined in Section 6.2.

The authenticator value in the Token token structure is computed over the
token_type, nonce, challenge_digest, and token_key_id fields. A
token is considered a valid if token verification using succeeds; see
Section 2.2.3 for details about verifying the token and its
authenticator value.

2.2.2. Sending Tokens

When used for client Client authorization, the "PrivateToken" authentication
scheme defines one parameter, "token", token, which contains the base64url-
encoded Token token structure. As with the challenge parameters
(Section 2.1), the base64url value MUST include padding. As an
authentication parameter (auth-param from [RFC9110], [HTTP], Section 11.2), the
value can be either a token or a quoted-string and might be required
to be a quoted-string if the base64url string includes "="
characters. All unknown or unsupported parameters to "PrivateToken"
authentication credentials MUST be ignored.

Clients present this Token token structure to origins Origins in a new HTTP request
using the Authorization header field as follows:

Authorization: PrivateToken token="abc..."

For context-bound tokens, origins Origins store or reconstruct the contexts
of previous TokenChallenge structures in order to validate the token.
A TokenChallenge can be bound to a specific TLS session with a
client,
Client, but origins Origins can also accept tokens for valid challenges in
new sessions. Origins SHOULD implement some form of double-spend
prevention that prevents a token with the same nonce from being
redeemed twice. Double-spend prevention ensures that clients Clients cannot
replay tokens for previous challenges. See Section 5.2 for more
information about replay attacks. For context-bound tokens, this
double-spend prevention can require no state or minimal state, since
the context can be used to verify token uniqueness.

2.2.3. Token Verification

A token consists of some input cryptographically bound to an
authenticator value, such as a digital signature. Verifying a token
consists of checking that the authenticator value is correct.

The authenticator value is as computed when running and finalizing
the issuance protocol corresponding to the token type with the
following values as the input:

struct {
uint16_t token_type;
uint8_t nonce[32];
uint8_t challenge_digest[32];
uint8_t token_key_id[Nid];
} AuthenticatorInput;

The values of these fields are as described in Section 2.2.1. The
cryptographic verification check depends on the token type; see
Sections 5.4 and 6.4 of [ISSUANCE] for verification instructions for
the issuance protocols described in that specification. As such, the
security properties of the token, e.g., the probability that one can
forge an authenticator value without invoking the issuance protocol,
depend on the cryptographic algorithm used by the issuance protocol
as determined by the token type.

3. Client Behavior

When a client Client receives one or more token challenges in response to a
request, the client Client has a set of choices to make:

* Whether or not to redeem a token via a new request to the origin. Origin.

* Whether to redeem a previously issued and cached token or redeem a
token freshly issued from the issuance protocol.

* If multiple challenges were sent, which challenge to use for
redeeming a token on a subsequent request.

The approach to these choices depends on the use case of the
application, as well as the deployment model (see Section 4 of
[ARCHITECTURE] for discussion of the different deployment models).

3.1. Choosing to Redeem Tokens

Some applications of tokens might require clients Clients to always present a
token as authentication in order to successfully make requests. For
example, a restricted service that wants to only allow access to
valid users but wants to do so without learning specific user
credential information could use tokens that are based on attesting
user credentials. In these kinds of use cases, clients Clients will need to
always redeem a token in order to successfully make a request.

Many other use cases for Privacy Pass tokens involve open services
that must work with any client, Client, including those that either cannot
redeem tokens or can only sometimes redeem tokens. For example, a
service can use tokens as a way to reduce the incidence of presenting
CAPTCHAs to users. In such use cases, services will regularly
encounter clients Clients that cannot redeem a token or choose not to. In
order to mitigate the risk of these services relying on always
receiving tokens, clients Clients that are capable of redeeming tokens can
ignore token challenges (and instead behave as if they were a client Client
that either doesn't support redeeming tokens or is unable to generate
a new token, by not sending a new request that contains a token to
redeem) with some non-trivial probability. See Section 5.1 of
[ARCHITECTURE] for further considerations regarding avoiding
discriminatory behavior across clients Clients when using Privacy Pass
tokens.

Clients might also choose to not redeem tokens in subsequent requests
when the token challenges indicate erroneous or malicious behavior on
the part of the challenging origin. Origin. For example, if a client's Client's
ability to generate tokens via an Attester and issuer Issuer is limited to a
certain rate, a malicious origin Origin could send an excessive number of
token challenges with unique redemption contexts in order to
(1) cause the client Client to exhaust its ability to generate new tokens or
(2) overwhelm issuance servers. Based on the specific deployment,
the limits here will vary, but clients Clients SHOULD have some
implementation-specific policy to minimize the number of tokens that
can be retrieved by origins. Origins.

3.2. Choosing between Multiple Challenges

A single response from an origin Origin can include multiple token
challenges. For example, a set of challenges could include different
token types and issuers, Issuers, to allow clients Clients to choose a preferred
issuer
Issuer or type.

If clients Clients choose to respond, clients Clients should satisfy exactly one of
the challenges presented. The choice of which challenge to use for
redeeming tokens is up to client Client policy. This can involve which
token types are supported or preferred, which issuers Issuers are supported
or preferred, or whether or not the client Client is able to use cached
tokens based on the redemption context or origin Origin information in the
challenge. See Section 2.1.4 for more discussion on token caching.
Regardless of how the choice is made, it SHOULD be done in a
consistent manner to ensure that the choice does not reveal
information about the specific client; Client; see Section 6.2 of
[ARCHITECTURE] for more details on the privacy implications of
issuance consistency.

4. Origin Behavior

Origins choose what token challenges to send to clients; Clients; these token
challenges will vary, depending on the use case and deployment model.
The origin Origin chooses which token types, issuers, Issuers, redemption contexts,
and origin Origin information to include in challenges. If an origin Origin sends
multiple challenges, each challenge SHOULD be equivalent in terms of
acceptability for token redemption, since clients Clients are free to choose
to generate tokens based on any of the challenges.

Origins ought to consider the time involved in token issuance.
Particularly, a challenge that includes a unique redemption context
will prevent a client Client from using cached tokens and thus can add more
delay before the client Client is able to redeem a token.

Origins SHOULD minimize the number of challenges sent to a particular
client
Client context (referred to as the "redemption context" in
Section 3.3 of [ARCHITECTURE]), to avoid overwhelming clients Clients and
issuers
Issuers with token requests that might cause clients Clients to hit rate
limits.

4.1. Greasing

In order to prevent clients Clients from becoming incompatible with new token
challenges, origins Origins SHOULD include random token types, from the
Reserved
reserved list of "greased" types (defined in Section 6.2), with some
non-trivial probability.

Additionally, for deployments where tokens are not required (such as
when tokens are used as a way to avoid showing CAPTCHAs), origins Origins
SHOULD randomly choose to not challenge clients Clients for tokens with some
non-trivial probability. This helps origins Origins ensure that their
behavior for handling clients Clients that cannot redeem tokens is maintained
and exercised consistently.

5. Security Considerations

This section contains security considerations for the PrivateToken "PrivateToken"
authentication scheme described in this document.

5.1. Randomness Requirements

All random values in the challenge and token MUST be generated using
a cryptographically secure source of randomness [RFC4086].

5.2. Replay Attacks

Applications SHOULD constrain tokens to a single origin Origin unless the
use case can accommodate replay attacks. Replaying tokens is not
necessarily a security or privacy problem. As an example, it is
reasonable for clients Clients to replay tokens in contexts where token
redemption does not induce side effects and in which client Client requests
are already linkable. One possible setting where this applies is
where tokens are sent as part of 0-RTT data.

If successful token redemption produces side effects, origins Origins SHOULD
implement an anti-replay mechanism to mitigate the harm of such
replays. See [TLS13], Section 8 and [RFC9001], Section 9.2 for
details about anti-replay mechanisms, as well as [RFC8470], Section 3
for discussion about safety considerations for 0-RTT HTTP data.

5.3. Reflection Attacks

The security properties of token challenges vary, depending on
whether the challenge contains a redemption context or not, as well
as whether the challenge is a per-origin per-Origin challenge or not. For
example, cross-origin cross-Origin tokens with empty contexts can be reflected
from one party by another, as shown below.

Figure 2: Replay Reflection Attack Example

5.4. Token Exhaustion Attacks

When a Client holds cross-origin cross-Origin tokens with empty contexts, it is
possible for any Origin in the cross-origin cross-Origin set to deplete that
Client's set of tokens. To prevent this from happening, tokens can
be scoped to single Origins (with non-empty origin_info) such that
they can only be redeemed for a single Origin. Alternatively, if
tokens are cross-Origin tokens, Clients can use alternate methods to
prevent many tokens from being redeemed at once. For example, if the
Origin requests an excess of tokens, the Client could choose to not
present any tokens for verification if a redemption had already
occurred in a given time window.

Token challenges that include non-empty origin_info bind tokens to
one or more specific origins. Origins. As described in Section 2.1, clients 2.1.3, Clients
only accept such challenges from origin Origin names listed in the
origin_info string. string if it is non-empty. Even if multiple origins Origins are
listed, a token can only be redeemed for an origin Origin if the challenge
has a match for the origin_info. For example, if "a.example.com"
issues a challenge with an origin_info string of
"a.example.com,b.example.com", a client Client could redeem a token fetched
for this challenge if and only if "b.example.com" also included an
origin_info string of "a.example.com,b.example.com". On the other
hand, if "b.example.com" had an origin_info string of
"b.example.com", "b.example.com,a.example.com", or
"a.example.com,b.example.com,c.example.com", the string would not
match, and the client Client would need to use a different token.

5.5. Timing Correlation Attacks

Context-bound token challenges require clients Clients to obtain matching
tokens when challenged, rather than presenting a token that was
obtained from a different context in the past. This can make it more
likely that issuance and redemption events will occur at
approximately the same time. For example, if a client Client is challenged
for a token with a unique context at time T1 and then subsequently
obtains a token at time T2, a colluding issuer Issuer and origin Origin can link
this to the same client Client if T2 is unique to the client. Client. This
linkability is less feasible as the number of issuance events at time
T2 increases. Depending on the "max-age" max-age token challenge parameter,
clients
Clients MAY try to add delay to the time between being challenged and
redeeming a token to make this sort of linkability more difficult.
For more discussion on correlation risks between token issuance and
redemption, see Section 6.3 of [ARCHITECTURE].

5.6. Cross-Context Linkability Attacks

As discussed in Section 2.1, clients Clients SHOULD discard any context-bound
tokens upon flushing cookies or changing networks, to prevent an
origin
Origin from using the redemption context state as a cookie to
recognize clients. Clients.

6. IANA Considerations

6.1. Authentication Scheme

IANA has registered the "PrivateToken" authentication scheme in the
"HTTP Authentication Schemes" subregistry of the "Hypertext Transfer
Protocol (HTTP) Authentication Scheme Registry" as defined in
[RFC9110], [HTTP],
Section 16.4.

Authentication Scheme Name: PrivateToken

Reference: RFC 9577, Section 2

6.2. Privacy Pass Token Type Types Registry

IANA has created a new "Privacy Pass Token Type" Types" registry in a new
"Privacy Pass" page to list identifiers for issuance protocols
defined for use with the Privacy Pass token authentication scheme.
These identifiers are two-byte 2-byte values, so the maximum possible value is
0xFFFF = 65535.

New registrations need to list the following attributes:

Value: The two-byte 2-byte identifier for the algorithm.
Name: Name of the issuance protocol.
Token Structure: The contents of the Token token structure; see
Section 2.2.
Token Key Encoding: The encoding of the "token-key" token-key parameter; see
Section 2.2. 2.1.2.
TokenChallenge Structure: The contents of the TokenChallenge
structure; see Section 2.1.
Public Verifiability:
Publicly Verifiable: A Y/N value indicating if the output tokens
have the public verifiability property; see Section 3.5 of
[ARCHITECTURE] for more details about this property.
Public Metadata: A Y/N value indicating if the output tokens can
contain public metadata; see Section 3.5 of [ARCHITECTURE] for
more details about this property.
Private Metadata: A Y/N value indicating if the output tokens can
contain private metadata; see Section 3.5 of [ARCHITECTURE] for
more details about this property.
Nk: The length in bytes of an output authenticator.
Nid: The length of the token key identifier.
Change Controller: The entity that is responsible for the definition
of the registration.
Reference: Where this algorithm is defined.
Notes: Any notes associated with the entry.

New entries in this registry are subject to the Specification
Required registration policy ([RFC8126], Section 4.6). Designated
experts need to ensure that the token type is defined to be used for
both token issuance and redemption. Additionally, the experts can
reject registrations on the basis that they do not meet the security
and privacy requirements for issuance protocols defined in
Section 3.2 of [ARCHITECTURE].

[ISSUANCE] defines entries for this registry.

6.2.1. Reserved Values

This document defines several Reserved reserved values, which can be used by
clients
Clients and servers to send "greased" values in token challenges and
redemptions to ensure that implementations remain able to handle
unknown token types gracefully (this technique is inspired by
[RFC8701]). Implementations SHOULD select reserved values at random
when including them in greased messages. Servers can include these
in TokenChallenge structures, either as the only challenge when no
real token type is desired or as one challenge in a list of
challenges that include real values. Clients can include these in
Token
token structures when they are not able to present a real token. The
contents of the Token token structure SHOULD be filled with random bytes
when using greased values.

The initial contents of this registry consist of multiple reserved
values, with the following attributes, which are repeated for each
registration:

Value: 0x0000, 0x02AA, 0x1132, 0x2E96, 0x3CD3, 0x4473, 0x5A63,
0x6D32, 0x7F3F, 0x8D07, 0x916B, 0xA6A4, 0xBEAB, 0xC3F3, 0xDA42,
0xE944, 0xF057
Name: RESERVED
Token Structure: Random bytes
Token Key Encoding: Random bytes
TokenChallenge Structure: Random bytes
Publicly Verifiable: N/A
Public Metadata: N/A
Private Metadata: N/A
Nk: N/A
Nid: N/A
Change Controller: IETF
Reference: RFC 9577
Notes: None

7. References

7.1. Normative References

[ARCHITECTURE]
Davidson, A., Iyengar, J., and C. A. Wood, "The Privacy
Pass Architecture", RFC 9576, DOI 10.17487/RFC9576, May June
2024, <https://www.rfc-editor.org/info/rfc9576>.

[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>.

[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>.

[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/info/rfc4648>.

[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>.

[RFC9110] 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>.

[SHS] National Institute of Standards and Technology, "Secure
Hash Standard (SHS)", NIST FIPS Publication 180-4,
DOI 10.6028/NIST.FIPS.180-4, August 2015,
<https://doi.org/10.6028/nist.fips.180-4>.

[TLS13] 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>.

[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>.

7.2. Informative References

[COOKIES] Bingler, S., Ed., West, M., Ed., and J. Wilander, Ed.,
"Cookies: HTTP State Management Mechanism", Work in
Progress, Internet-Draft, draft-ietf-httpbis-rfc6265bis-
13, 15 November 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
rfc6265bis-13>.
14, 2 May 2024, <https://datatracker.ietf.org/doc/html/
draft-ietf-httpbis-rfc6265bis-14>.

[ISSUANCE] Celi, S., Davidson, A., Valdez, S., and C. A. Wood,
"Privacy Pass Issuance Protocol", Protocols", RFC 9578,
DOI 10.17487/RFC9578, May June 2024,
<https://www.rfc-editor.org/info/rfc9578>.

[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC4086, June 2005,
<https://www.rfc-editor.org/info/rfc4086>.

[RFC8470] Thomson, M., Nottingham, M., and W. Tarreau, "Using Early
Data in HTTP", RFC 8470, DOI 10.17487/RFC8470, September
2018, <https://www.rfc-editor.org/info/rfc8470>.

[RFC8701] Benjamin, D., "Applying Generate Random Extensions And
Sustain Extensibility (GREASE) to TLS Extensibility",
RFC 8701, DOI 10.17487/RFC8701, January 2020,
<https://www.rfc-editor.org/info/rfc8701>.

[RFC9001] Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure
QUIC", RFC 9001, DOI 10.17487/RFC9001, May 2021,
<https://www.rfc-editor.org/info/rfc9001>.

Appendix A. Test Vectors

This section includes test vectors for the HTTP authentication scheme
specified in this document. It consists of the following types of
test vectors:

1. Test vectors for the challenge and redemption protocols.
Implementations can use these test vectors for verifying code
that builds and encodes TokenChallenge structures, as well as
code that produces a well-formed Token token bound to a TokenChallenge.

2. Test vectors for the HTTP headers used for authentication.
Implementations can use these test vectors for validating whether
they parse HTTP authentication headers correctly to produce
TokenChallenge structures and the other associated parameters,
such as the token-key and max-age values.

A.1. Challenge and Redemption Structure Test Vectors

This section includes test vectors for the challenge and redemption
functionalities described in Sections 2.1 and 2.2. Each test vector
lists the following values:

token_type: The type of token issuance protocol -- a value from
Section 6.2. For these test vectors, token_type is 0x0002,
corresponding to the issuance protocol discussed in Section 6
("Issuance Protocol for Publicly Verifiable Tokens") of
[ISSUANCE].

issuer_name: The name of the issuer Issuer in the TokenChallenge structure,
represented as a hexadecimal string.

redemption_context: The redemption context in the TokenChallenge
structure, represented as a hexadecimal string.

origin_info: The origin Origin information in the TokenChallenge structure,
represented as a hexadecimal string.

nonce: The nonce in the Token token structure, represented as a
hexadecimal string.

token_key:

token_key_id: The public token-key, token key, encoded based on the
corresponding token type, represented as a hexadecimal string.

token_authenticator_input: The values in the Token token structure used to
compute the Token token authenticator value, represented as a
hexadecimal string.

Test vectors are provided for each of the following TokenChallenge
configurations:

1. TokenChallenge with a single origin Origin and a non-empty redemption
context.

2. TokenChallenge with a single origin Origin and empty redemption context.

3. TokenChallenge with an empty origin Origin and redemption context.

4. TokenChallenge with an empty origin Origin and a non-empty redemption
context.

5. TokenChallenge with multiple origins Origins and a non-empty redemption
context.

6. TokenChallenge for greasing.

These test vectors are below.

// Test vector 1:
// token_type(0002), issuer_name(issuer.example),
// origin_info(origin.example), redemption_context(non-empty)
token_type: 0002
issuer_name: 6973737565722e6578616d706c65
redemption_context:
476ac2c935f458e9b2d7af32dacfbd22dd6023ef5887a789f1abe004e79bb5bb
origin_info: 6f726967696e2e6578616d706c65
nonce:
e01978182c469e5e026d66558ee186568614f235e41ef7e2378e6f202688abab
token_key_id:
ca572f8982a9ca248a3056186322d93ca147266121ddeb5632c07f1f71cd2708
token_authenticator_input: 0002e01978182c469e5e026d66558ee1865686
14f235e41ef7e2378e6f202688abab8e1d5518ec82964255526efd8f9db88205a
8ddd3ffb1db298fcc3ad36c42388fca572f8982a9ca248a3056186322d93ca147
266121ddeb5632c07f1f71cd2708

// Test vector 2:
// token_type(0002), issuer_name(issuer.example),
// origin_info(origin.example), redemption_context(empty)
token_type: 0002
issuer_name: 6973737565722e6578616d706c65
redemption_context:
origin_info: 6f726967696e2e6578616d706c65
nonce:
e01978182c469e5e026d66558ee186568614f235e41ef7e2378e6f202688abab
token_key_id:
ca572f8982a9ca248a3056186322d93ca147266121ddeb5632c07f1f71cd2708
token_authenticator_input: 0002e01978182c469e5e026d66558ee1865686
14f235e41ef7e2378e6f202688abab11e15c91a7c2ad02abd66645802373db1d8
23bea80f08d452541fb2b62b5898bca572f8982a9ca248a3056186322d93ca147
266121ddeb5632c07f1f71cd2708

// Test vector 3:
// token_type(0002), issuer_name(issuer.example),
// origin_info(), redemption_context(empty)
token_type: 0002
issuer_name: 6973737565722e6578616d706c65
redemption_context:
origin_info:
nonce:
e01978182c469e5e026d66558ee186568614f235e41ef7e2378e6f202688abab
token_key_id:
ca572f8982a9ca248a3056186322d93ca147266121ddeb5632c07f1f71cd2708
token_authenticator_input: 0002e01978182c469e5e026d66558ee1865686
14f235e41ef7e2378e6f202688ababb741ec1b6fd05f1e95f8982906aec161289
6d9ca97d53eef94ad3c9fe023f7a4ca572f8982a9ca248a3056186322d93ca147
266121ddeb5632c07f1f71cd2708

// Test vector 4:
// token_type(0002), issuer_name(issuer.example),
// origin_info(), redemption_context(non-empty)
token_type: 0002
issuer_name: 6973737565722e6578616d706c65
redemption_context:
476ac2c935f458e9b2d7af32dacfbd22dd6023ef5887a789f1abe004e79bb5bb
origin_info:
nonce:
e01978182c469e5e026d66558ee186568614f235e41ef7e2378e6f202688abab
token_key_id:
ca572f8982a9ca248a3056186322d93ca147266121ddeb5632c07f1f71cd2708
token_authenticator_input: 0002e01978182c469e5e026d66558ee1865686
14f235e41ef7e2378e6f202688ababb85fb5bc06edeb0e8e8bdb5b3bea8c4fa40
837c82e8bcaf5882c81e14817ea18ca572f8982a9ca248a3056186322d93ca147
266121ddeb5632c07f1f71cd2708

// Test vector 5:
// token_type(0002), issuer_name(issuer.example),
// origin_info(foo.example,bar.example),
// redemption_context(non-empty)
token_type: 0002
issuer_name: 6973737565722e6578616d706c65
redemption_context:
476ac2c935f458e9b2d7af32dacfbd22dd6023ef5887a789f1abe004e79bb5bb
origin_info: 666f6f2e6578616d706c652c6261722e6578616d706c65
nonce:
e01978182c469e5e026d66558ee186568614f235e41ef7e2378e6f202688abab
token_key_id:
ca572f8982a9ca248a3056186322d93ca147266121ddeb5632c07f1f71cd2708
token_authenticator_input: 0002e01978182c469e5e026d66558ee1865686
14f235e41ef7e2378e6f202688ababa2a775866b6ae0f98944910c8f48728d8a2
735b9157762ddbf803f70e2e8ba3eca572f8982a9ca248a3056186322d93ca147
266121ddeb5632c07f1f71cd2708

// Test vector 6:
// token_type(0000), structure(random_bytes)
token_type: 0000
token_authenticator_input: 000058405ad31e286e874cb42d0ef9d50461ae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A.2. HTTP Header Test Vectors

This section includes test vectors for the contents of the HTTP
authentication headers. Each test vector consists of one or more
challenges that comprise a WWW-Authenticate header; see Section 3.2.
For each challenge, the token-type, token-key, max-age, and token-
challenge parameters are listed. Each challenge also includes an
unknown (unspecified) parameter that implementations are meant to
ignore.

The parameters for each challenge are indexed by their position in
the WWW-Authentication WWW-Authenticate challenge list. For example, token-key-0
denotes the token-key parameter for the first challenge in the list,
whereas token-key-1 denotes the token-key for the second challenge in
the list.

The resulting wire-encoded WWW-Authentication WWW-Authenticate header based on this list
of challenges is then listed at the end. Line folding is only used
to fit the document-formatting constraints and is not supported in
actual requests.

The last challenge in this list includes Basic authentication, a
grease challenge, and a valid challenge for token type 0x0001.
Correct client Client implementations will ignore the Basic and grease
challenges.

token-type-0: 0x0002
token-key-0: 30820152303d06092a864886f70d01010a3030a00d300b060960864
8016503040202a11a301806092a864886f70d010108300b060960864801650304020
2a2030201300382010f003082010a0282010100cb1aed6b6a95f5b1ce013a4cfcab2
5b94b2e64a23034e4250a7eab43c0df3a8c12993af12b111908d4b471bec31d4b6c9
ad9cdda90612a2ee903523e6de5a224d6b02f09e5c374d0cfe01d8f529c500a78a2f
67908fa682b5a2b430c81eaf1af72d7b5e794fc98a3139276879757ce453b526ef9b
f6ceb99979b8423b90f4461a22af37aab0cf5733f7597abe44d31c732db68a181c6c
bbe607d8c0e52e0655fd9996dc584eca0be87afbcd78a337d17b1dba9e828bbd81e2
91317144e7ff89f55619709b096cbb9ea474cead264c2073fe49740c01f00e109106
066983d21e5f83f086e2e823c879cd43cef700d2a352a9babd612d03cad02db134b7
e225a5f0203010001
max-age-0: 10
token-challenge-0: 0002000e6973737565722e6578616d706c65208a3e83a33d9
8005d2f30bef419fa6bf4cd5c6005e36b1285bbb4ccd40fa4b383000e6f726967696
e2e6578616d706c65

WWW-Authenticate: PrivateToken challenge="AAIADmlzc3Vlci5leGFtcGxlII
o-g6M9mABdLzC-9Bn6a_TNXGAF42sShbu0zNQPpLODAA5vcmlnaW4uZXhhbXBsZQ==",
token-key="MIIBUjA9BgkqhkiG9w0BAQowMKANMAsGCWCGSAFlAwQCAqEaMBgGCSqG
SIb3DQEBCDALBglghkgBZQMEAgKiAwIBMAOCAQ8AMIIBCgKCAQEAyxrta2qV9bHOATpM
_KsluUsuZKIwNOQlCn6rQ8DfOowSmTrxKxEZCNS0cb7DHUtsmtnN2pBhKi7pA1I-beWi
JNawLwnlw3TQz-Adj1KcUAp4ovZ5CPpoK1orQwyB6vGvcte155T8mKMTknaHl1fORTtS
bvm_bOuZl5uEI7kPRGGiKvN6qwz1cz91l6vkTTHHMttooYHGy75gfYwOUuBlX9mZbcWE
7KC-h6-814ozfRex26noKLvYHikTFxROf_ifVWGXCbCWy7nqR0zq0mTCBz_kl0DAHwDh
CRBgZpg9IeX4PwhuLoI8h5zUPO9wDSo1Kpur1hLQPK0C2xNLfiJaXwIDAQAB",unknow
nChallengeAttribute="ignore-me", max-age="10"

token-type-0: 0x0000
token-key-0: 856de3c710b892e7cca1ae5eb121af42ca8e779137a11224228c9b9
9b0729bf84d5057d030000309b8f0d06ccffa17561f9eacd4c312e985a6bc60ffbea
0610264dcb1726255313da81d665692686a1d8644f1516bf612cea009e6dff6d9a9a
959fb538e1b5b2343c092992942382bdde22d5b324b1e4618ed21d7806286c2ce
token-challenge-0: 0000acc3b25795c636fd9dd8b12982394abba8777d35978e8
77fc8848892a217233045ac25a3d55c07c54efe6372973fee0073e77fc61bf19ab88
0f20edf5d627502
token-type-1: 0x0001
token-key-1: ebb1fed338310361c08d0c7576969671296e05e99a17d7926dfc28a
53fabd489fac0f82bca86249a668f3a5bfab374c9
max-age-1: 10
token-challenge-1: 0001000e6973737565722e6578616d706c65208a3e83a33d9
8005d2f30bef419fa6bf4cd5c6005e36b1285bbb4ccd40fa4b383000e6f726967696
e2e6578616d706c65

WWW-Authenticate: Basic realm="grease", PrivateToken challenge="AACs
w7JXlcY2_Z3YsSmCOUq7qHd9NZeOh3_IhIiSohcjMEWsJaPVXAfFTv5jcpc_7gBz53_G
G_GauIDyDt9dYnUC",token-key="hW3jxxC4kufMoa5esSGvQsqOd5E3oRIkIoybmbB
G_GauIDyDt9dYnUC", token-key="hW3jxxC4kufMoa5esSGvQsqOd5E3oRIkIoybmbB
ym_hNUFfQMAADCbjw0GzP-hdWH56s1MMS6YWmvGD_vqBhAmTcsXJiVTE9qB1mVpJoah2
GRPFRa_YSzqAJ5t_22ampWftTjhtbI0PAkpkpQjgr3eItWzJLHkYY7SHXgGKGws4=",
PrivateToken challenge="AAEADmlzc3Vlci5leGFtcGxlIIo-g6M9mABdLzC-9Bn6
a_TNXGAF42sShbu0zNQPpLODAA5vcmlnaW4uZXhhbXBsZQ==", token-key="67H-0z
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Authors' Addresses

Tommy Pauly
Apple Inc.
One Apple Park Way
Cupertino, California 95014
United States of America
Email: tpauly@apple.com

Steven Valdez
Google LLC
Email: svaldez@chromium.org

Christopher A. Wood
Cloudflare
Email: caw@heapingbits.net