The Privacy Pass HTTP Authentication Scheme
draft-ietf-privacypass-auth-scheme-09

Network Working Group                                           T. Pauly
Internet-Draft                                                Apple Inc.
Intended status: Standards Track                               S. Valdez
Expires: 7 September 2023                                     Google LLC
                                                              C. A. Wood
                                                              Cloudflare
                                                            6 March 2023

              The Privacy Pass HTTP Authentication Scheme
                 draft-ietf-privacypass-auth-scheme-09

Abstract

   This document defines an HTTP authentication scheme that can be used
   by clients to redeem Privacy Pass tokens with an origin.  It can also
   be used by origins to challenge clients to present an acceptable
   Privacy Pass token.

Status of This Memo

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   This Internet-Draft will expire on 7 September 2023.

Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   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
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  HTTP Authentication Scheme  . . . . . . . . . . . . . . . . .   4
     2.1.  Token Challenge . . . . . . . . . . . . . . . . . . . . .   4
       2.1.1.  Redemption Context Construction . . . . . . . . . . .   7
       2.1.2.  Token Caching . . . . . . . . . . . . . . . . . . . .   8
     2.2.  Token Redemption  . . . . . . . . . . . . . . . . . . . .   9
   3.  User Interaction  . . . . . . . . . . . . . . . . . . . . . .  10
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
     5.1.  Authentication Scheme . . . . . . . . . . . . . . . . . .  13
     5.2.  Token Type Registry . . . . . . . . . . . . . . . . . . .  13
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .  16
     6.2.  Informative References  . . . . . . . . . . . . . . . . .  17
   Appendix A.  Test Vectors . . . . . . . . . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  20

1.  Introduction

   Privacy Pass tokens are unlinkable authenticators that can be used to
   anonymously authorize a client (see [ARCHITECTURE]).  Tokens are
   generated by token issuers, on the basis of authentication,
   attestation, or some previous action such as solving a CAPTCHA.  A
   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 token (the 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],
   Section 11), PrivateToken, that allows clients to redeem various
   kinds of Privacy Pass tokens.

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   Clients and relying parties (origins) interact using this scheme to
   perform the token challenge and token redemption flow.  In
   particular, origins challenge clients for a token with an HTTP
   Authentication challenge (using the WWW-Authenticate response header
   field).  Clients then respond to that challenge with an HTTP
   authentiation response (using the Authorization request header
   field).  Clients produce an authentication response based on the
   origin's token challenge by running the token issuance protocol
   [ISSUANCE].  The act of presenting a token in an Authorization
   request header is referred to as token redemption.  This interaction
   between client and origin is shown below.

  +--------+                              +--------+
  | Origin |                              | Client |
  +---+----+                              +---+----+
      |                                       |
      +-- WWW-Authenticate: TokenChallenge -->|
      |                                       | // Run issuance protocol
      <------- Authorization: Token ----------+
      |                                       |

          Figure 1: Challenge-response redemption protocol flow

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

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

   *  Token challenge: A requirement for tokens sent from an origin to a
      client, using the "WWW-Authenticate" HTTP header field.  This
      challenge is bound to a specific token issuer and issuance
      protocol, and may be additionally bound to a specific context or
      origin name.

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

2.  HTTP Authentication Scheme

   Token redemption is performed using HTTP Authentication ([RFC9110],
   Section 11), with the scheme "PrivateToken".  Origins challenge
   clients to present a token from a specific issuer (Section 2.1).
   Once a client has received a token from that issuer, or already has a
   valid token available, it presents the token to the origin
   (Section 2.2).

   Unlike many authentication schemes in which a client will present the
   same credentials across multiple requests, tokens used with the
   "PrivateToken" scheme are single-use credentials, and are not reused.
   Spending the same token value more than once allows the origin to
   link multiple transactions to the same client.  In deployment
   scenarios where origins send token challenges to request tokens,
   origins ought to expect at most one request containing a token from
   the client in reaction to a particular challenge.

2.1.  Token Challenge

   Origins send a token challenge to clients in an "WWW-Authenticate"
   header field with the "PrivateToken" scheme.  This challenge includes
   a TokenChallenge message, along with information about what keys to
   use when requesting a token from the issuer.

   Origins that support this authentication scheme need to handle the
   following tasks:

   1.  Select which issuer to use, and configure the issuer name and
       token-key to include in WWW-Authenticate challenges.

   2.  Determine a redemption context construction to include in the
       TokenChallenge, as discussed in Section 2.1.1.

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   3.  Select the origin information to include in the TokenChallenge.
       This can be empty to allow fully cross-origin tokens, a single
       origin name that matches the origin itself, or a list of origin
       names containing the origin itself.

   The 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" is a 2-octet integer, in network byte order.  This
      type indicates the issuance protocol used to generate the token.
      Values are registered in an IANA registry, Section 5.2.
      Challenges with unsupported token_type values MUST be ignored.
      This value determines the structure and semantics of the rest of
      the structure.

   *  "issuer_name" is a string containing the name of the issuer.  This
      is a hostname that is used to identify the issuer that is allowed
      to issue tokens that can be redeemed by this origin.  The string
      is prefixed with a 2-octet integer indicating the length, in
      network byte order.

   *  "redemption_context" is an optional field.  If present, it allows
      the origin to require that clients fetch tokens bound to a
      specific context, as opposed to reusing tokens that were fetched
      for other contexts.  See Section 2.1.1 for example contexts that
      might be useful in practice.  When present, this value is a
      32-byte context generated by the origin.  Valid lengths for this
      field are either 0 or 32 bytes.  The field is prefixed with a
      single octet indicating the length.  Challenges with
      redemption_context values of invalid lengths MUST be ignored.

   *  "origin_info" is an optional string containing one or more origin
      names, which allows a token to be scoped to a specific set of
      origins.  The string is prefixed with a 2-octet integer indicating
      the length, in network byte order.  If empty, any non-origin-
      specific token can be redeemed.  If the string contains multiple
      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.

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

   *  "challenge", which contains a base64url-encoded [RFC4648]
      TokenChallenge value.  Since the length of the challenge is not
      fixed, the base64url value MUST include padding.  As an
      Authentication Parameter (auth-param from [RFC9110],
      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 challenge value MUST be
      unique for every 401 HTTP response to prevent replay attacks.
      This parameter is required for all challenges.

   *  "token-key", which contains a base64url encoding of the public key
      for use with the issuance protocol indicated by the challenge.
      Since the length of the key is not fixed, the base64url value MUST
      include padding.  As an Authentication Parameter (auth-param from
      [RFC9110], 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 are able to retrieve the
      issuer key using an out-of-band mechanism.

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

   Clients can ignore the challenge if the token-key is invalid or
   otherwise untrusted.

   The header field MAY also include the standard "realm" parameter, if
   desired.  Issuance protocols MAY require other parameters.  Clients
   SHOULD ignore unknown parameters in challenges, except if otherwise
   specified by issuance protocols.

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

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

   Upon receipt of this challenge, a client uses the message and keys in
   the issuance protocol indicated by the token_type.  If the
   TokenChallenge has a token_type the client does not recognize or
   support, it MUST NOT parse or respond to the challenge.  If the
   TokenChallenge contains a non-empty origin_info field, the client
   MUST validate that the name of the origin that issued the
   authentication challenge is included in the list of origin names; if
   validation fails, the client MUST NOT process or respond to the

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   challenge.  Clients MAY have further restrictions and requirements
   around validating when a challenge is considered acceptable or valid.
   For example, clients can choose to ignore challenges that list origin
   names for which current connection is not authoritative (according to
   the TLS certificate).

   Caching and pre-fetching of tokens is discussed in Section 2.1.2.

   Note that it is possible for the WWW-Authenticate header field to
   include multiple challenges.  This allows the origin to indicate
   support for different token types, 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 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 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.

2.1.1.  Redemption Context Construction

   The TokenChallenge redemption context allows the 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 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 redemption context
      as SHA256(current time window).

   *  Context bound to a client location: Construct redemption context
      as SHA256(client IP address prefix).

   *  Context bound to a given time window and location: Construct
      redemption context as SHA256(current time window, client IP
      address prefix).

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   An empty redemption context is not bound to any property of the
   client session.  Preventing double spending on tokens requires the
   origin to keep state associated with the redemption context.  The
   size of this state varies based on the size of the redemption
   context.  For example, double spend state for unique, per-request
   redemption contexts does only needs to exist within the scope of the
   request connection or session.  In contrast, double spend state for
   empty redemption contexts must be stored and shared across all
   requests until token-key expiration or rotation.

   Origins that share redemption contexts, i.e., by using the same
   redemption context, choosing the same 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.  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 to avoid any 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 nonce, which is sent
   in token redemption (Section 2.2).

   If a 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 SHOULD discard these
   tokens upon flushing state such as HTTP cookies [COOKIES], or
   changing networks.  Using these tokens in a context that otherwise
   would not be linkable to the original context could allow the origin
   to recognize a client.

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2.2.  Token Redemption

   The output of the issuance protocol is a token that corresponds to
   the origin's challenge (see Section 2.1).  A token is a structure
   that begins with a two-octet field that indicates a token type, which
   MUST match the token_type in the TokenChallenge 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" is a 2-octet integer, in network byte order.  This
      value must match the value in the challenge (Section 2.1).  This
      value determines the structure and semantics of the rest of the
      structure.

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

   *  "challenge_digest" is a 32-octet message containing the hash of
      the original TokenChallenge, SHA256(TokenChallenge).

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

   *  "authenticator" is a Nk-octet authenticator that covers the
      preceding fields in the token.  The value of this field is defined
      by the token_type and corresponding issuance protocol.  The value
      of constant Nk depends on token_type, as defined in Section 5.2.

   The authenticator value in the Token structure is computed over the
   token_type, nonce, challenge_digest, and token_key_id fields.

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   When used for client authorization, the "PrivateToken" authentication
   scheme defines one parameter, "token", which contains the base64url-
   encoded Token struct.  Since the length of the Token struct is not
   fixed, the base64url value MUST include padding.  As an
   Authentication Parameter (auth-param from [RFC9110], 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 structure to origins in a new HTTP request
   using the Authorization header field as follows:

   Authorization: PrivateToken token="abc..."

   For token types that support public verifiability, origins verify the
   token authenticator using the public key of the issuer, and validate
   that the signed message matches the concatenation of the client nonce
   and the hash of a valid TokenChallenge.  For context-bound tokens,
   origins store or reconstruct the contexts of previous TokenChallenge
   structures in order to validate the token.  A TokenChallenge MAY be
   bound to a specific TLS session with a client, but 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.  This prevents clients
   from "replaying" tokens for previous challenges.  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.

   If a client is unable to fetch a token, it MUST react to the
   challenge as if it could not produce a valid Authorization response.

3.  User Interaction

   When used in contexts like websites, origins that challenge clients
   for tokens need to consider how to optimize their interaction model
   to ensure a good user experience.

   Tokens challenges can be performed without explicit user involvement,
   depending on the issuance protocol.  If tokens are scoped to a
   specific origin, there is no need for per-challenge user interaction.
   Note that the issuance protocol may separately involve user
   interaction if the client needs to be newly validated.

   If a client cannot use cached tokens to respond to a challenge
   (either because it has run out of cached tokens or the associated
   context is unique), the token issuance process can add user-
   perceivable latency.  Origins need not block useful work on token

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   authentication.  Instead, token authentication can be used in similar
   ways to CAPTCHA validation today, but without the need for user
   interaction.  If issuance is taking a long time, a website could show
   an indicator that it is waiting, or fall back to another method of
   user validation.

   An origin MUST NOT use more than one redemption context value for a
   given token type and issuer per client request.  If an origin issues
   a large number of challenges with unique contexts, such as more than
   once for each request, this can indicate that the origin is either
   not functioning correctly or is trying to attack or overload the
   client or issuance server.  In such cases, a client MUST ignore
   redundant token challenges for the same request and SHOULD alert the
   user if possible.

   Origins MAY include multiple challenges, where each challenge refers
   to a different issuer or a different token type, to allow clients to
   choose a preferred issuer or type.

   An origin MUST NOT assume that token challenges will always yield a
   valid token.  Clients might experience issues running the issuance
   protocol, e.g., because the attester or issuer is unavailable, or
   clients might simply not support the requested token type.  Origins
   SHOULD account for such operational or interoperability failures by
   offering clients an alternative type of challenge such as CAPTCHA for
   accessing a resource.

   For example, consider a scenario in which the client is a web
   browser, and the origin can accept either a token or a solution to a
   puzzle intended to determine if the client is a real human user.  The
   origin would send clients a 401 HTTP response that contains a token
   challenge in a "WWW-Authenticate" header field along with content
   that contains the puzzle to display to the user.  Clients that are
   able to respond with a token will be able to automatically return the
   token and not show the puzzle, while clients that either do not
   support tokens or are unable to fetch tokens at a particular time can
   present the user with the puzzle.

   To mitigate the risk of deployments becoming dependent on tokens,
   clients and servers SHOULD grease their behavior unless explicitly
   configured not to.  In particular, clients SHOULD ignore token
   challenges with some non-zero probability.  Likewise, origins SHOULD
   randomly choose to not challenge clients for tokens with some non-
   zero probability.  Moreover, origins SHOULD include random token
   types, from the Reserved list of "greased" types (defined in
   Section 5.2), with some non-zero probability.

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4.  Security Considerations

   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 per-origin or not.  For example, cross-
   origin tokens with empty contexts can be replayed from one party by
   another, as shown below.

    Client          Attacker                  Origin
                          <------ TokenChallenge \
      <--- TokenChallenge                        |
       Token ---------->                         |
                          Token ---------------> /

                  Figure 2: Token Architectural Components

   Moreover, when a Client holds cross-origin tokens with empty
   contexts, it is possible for any Origin in the cross-origin set to
   deplete that Client 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, 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.  As described in Section 2.1, clients
   only accept such challenges from origin names listed in the
   origin_info string.  Even if multiple origins are listed, a token can
   only be redeemed for an origin if the challenge has an exact 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 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"
   or "b.example.com,a.example.com" or
   "a.example.com,b.example.com,c.example.com", the string would not
   match and the client would need to use a different token.

   Context-bound token challenges require 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 is challenged
   for a token with a unique context at time T1 and then subsequently

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   obtains a token at time T2, a colluding issuer and origin can link
   this to the same client if T2 is unique to the client.  This
   linkability is less feasible as the number of issuance events at time
   T2 increases.  Depending on the "max-age" token challenge parameter,
   clients MAY try to augment the time between getting challenged then
   redeeming a token so as to make this sort of linkability more
   difficult.  For more discussion on correlation risks between token
   issuance and redemption, see [ARCHITECTURE].

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

   Applications SHOULD constrain tokens to a single origin unless the
   use case can accommodate such replay attacks.  Replays are also
   possible if the client redeems a token sent as part of 0-RTT data.
   If successful token redemption produces side effects, origins SHOULD
   implement an anti-replay mechanism to mitigate the harm of such
   replays.  See [RFC8446], 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.

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

5.  IANA Considerations

5.1.  Authentication Scheme

   This document registers the "PrivateToken" authentication scheme in
   the "Hypertext Transfer Protocol (HTTP) Authentication Scheme
   Registry" defined in [RFC9110], Section 16.4.

   Authentication Scheme Name: PrivateToken

   Pointer to specification text: Section 2 of this document

5.2.  Token Type Registry

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

   Template:

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   *  Value: The two-byte identifier for the algorithm

   *  Name: Name of the issuance protocol

   *  Token Structure: The contents of the Token structure

   *  TokenChallenge Structure: The contents of the TokenChallenge
      structure

   *  Publicly Verifiable: A Y/N value indicating if the output tokens
      are publicly verifiable

   *  Public Metadata: A Y/N value indicating if the output tokens can
      contain public metadata.

   *  Private Metadata: A Y/N value indicating if the output tokens can
      contain private metadata.

   *  Nk: The length in bytes of an output authenticator

   *  Nid: The length of the token key identifier

   *  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 sufficiently clearly
   defined to be used for both token issuance and redemption, and meets
   the common security and privacy requirements for issuance protocols
   defined in Section 3.2 of [ARCHITECTURE].

   This registry also will also allow provisional registrations to allow
   for experimentation with protocols being developed.  Designated
   experts review, approve, and revoke provisional registrations.

   Values 0xFF00-0xFFFF are reserved for private use, to enable
   proprietary uses and limited experimentation.

   This document defines several Reserved values, which can be used by
   clients and servers to send "greased" values in token challenges and
   responses to ensure that implementations remain able to handle
   unknown token types gracefully (this technique is inspired by
   [RFC8701]).  Implemenations 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

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   challenges that include real values.  Clients can include these in
   Token structures when they are not able to present a real token
   response.  The contents of the Token structure SHOULD be filled with
   random bytes when using greased values.

   The initial contents for this registry consist of the following
   Values.  For each Value, the Name is "RESERVED", the Publicly
   Verifiable, Public Metadata, Private Metadata, Nk, and Nid attributes
   are all assigned "N/A", the Reference is this document, and the Notes
   attribute is "None".  The iniital list of Values is as follows:

   *  0x0000

   *  0x02AA

   *  0x1132

   *  0x2E96

   *  0x3CD3

   *  0x4473

   *  0x5A63

   *  0x6D32

   *  0x7F3F

   *  0x8D07

   *  0x916B

   *  0xA6A4

   *  0xBEAB

   *  0xC3F3

   *  0xDA42

   *  0xE944

   *  0xF057

   Additionally, the registry is to be initialized with the following
   entry for Private Use.

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   *  Value: 0xFF00-0xFFFF

   *  Name: Private Use

   *  Token Structure: As defined in Section 2.2

   *  TokenChallenge Structure: As defined in Section 2.1

   *  Publicly Verifiable: N/A

   *  Public Metadata: N/A

   *  Private Metadata: N/A

   *  Nk: N/A

   *  Nid: N/A

   *  Reference: This document

   *  Notes: None

6.  References

6.1.  Normative References

   [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/rfc/rfc2119>.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <https://www.rfc-editor.org/rfc/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/rfc/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/rfc/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/rfc/rfc9110>.

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   [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/rfc/rfc8446>.

6.2.  Informative References

   [ARCHITECTURE]
              Davidson, A., Iyengar, J., and C. A. Wood, "The Privacy
              Pass Architecture", Work in Progress, Internet-Draft,
              draft-ietf-privacypass-architecture-10, 30 January 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-
              privacypass-architecture-10>.

   [COOKIES]  Bingler, S., West, M., and J. Wilander, "Cookies: HTTP
              State Management Mechanism", Work in Progress, Internet-
              Draft, draft-ietf-httpbis-rfc6265bis-11, 7 November 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
              rfc6265bis-11>.

   [ISSUANCE] Celi, S., Davidson, A., Faz-Hernandez, A., Valdez, S., and
              C. A. Wood, "Privacy Pass Issuance Protocol", Work in
              Progress, Internet-Draft, draft-ietf-privacypass-protocol-
              08, 30 January 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-
              privacypass-protocol-08>.

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

   [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/rfc/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/rfc/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/rfc/rfc9001>.

Appendix A.  Test Vectors

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

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   *  token_type: The type of token issuance protocol, a value from
      Section 5.2.  For these test vectors, token_type is 0x0002,
      corresponding to the issuance protocol in [ISSUANCE].

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

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

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

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

   *  token_authenticator: The output Token authenticator which verifies
      under token_key, represented as a hexadecimal string.

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

   *  TokenChallenge with a single origin and non-empty redemption
      context

   *  TokenChallenge with a single origin and empty redemption context

   *  TokenChallenge with an empty origin and redemption context

   *  TokenChallenge with an empty origin and non-empty redemption
      context

   *  TokenChallenge with a multiple origins and non-empty redemption
      context

   These test vectors are below.

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   token_type: 2
   issuer_name: 6973737565722e6578616d706c65
   redemption_context:
   9d262778b3dc2be365d667b03f9cca99efd049e76eb53a6de37120ca34da373b
   origin_info: 6f726967696e2e6578616d706c65
   nonce:
   86ed4bb9f76ab1107a05a9af4aa4eec84dd02f390f9bf5ef14730e0ee15aa92d
   token_key_id:
   f861220ad4241ee0e33eb4a486a32f05af05ee33fcfdd1104c665eb827c20621
   token_authenticator_input: 000286ed4bb9f76ab1107a05a9af4aa4eec84d
   d02f390f9bf5ef14730e0ee15aa92df099ea46d6c892cdbc8513586fa8518a6d6
   3f28fe4da6f8ddd2a46a405c14488f861220ad4241ee0e33eb4a486a32f05af05
   ee33fcfdd1104c665eb827c20621

   token_type: 2
   issuer_name: 6973737565722e6578616d706c65
   redemption_context:
   origin_info: 6f726967696e2e6578616d706c65
   nonce:
   86ed4bb9f76ab1107a05a9af4aa4eec84dd02f390f9bf5ef14730e0ee15aa92d
   token_key_id:
   f861220ad4241ee0e33eb4a486a32f05af05ee33fcfdd1104c665eb827c20621
   token_authenticator_input: 000286ed4bb9f76ab1107a05a9af4aa4eec84d
   d02f390f9bf5ef14730e0ee15aa92d11e15c91a7c2ad02abd66645802373db1d8
   23bea80f08d452541fb2b62b5898bf861220ad4241ee0e33eb4a486a32f05af05
   ee33fcfdd1104c665eb827c20621

   token_type: 2
   issuer_name: 6973737565722e6578616d706c65
   redemption_context:
   origin_info:
   nonce:
   86ed4bb9f76ab1107a05a9af4aa4eec84dd02f390f9bf5ef14730e0ee15aa92d
   token_key_id:
   f861220ad4241ee0e33eb4a486a32f05af05ee33fcfdd1104c665eb827c20621
   token_authenticator_input: 000286ed4bb9f76ab1107a05a9af4aa4eec84d
   d02f390f9bf5ef14730e0ee15aa92db741ec1b6fd05f1e95f8982906aec161289
   6d9ca97d53eef94ad3c9fe023f7a4f861220ad4241ee0e33eb4a486a32f05af05
   ee33fcfdd1104c665eb827c20621

   token_type: 2
   issuer_name: 6973737565722e6578616d706c65
   redemption_context:
   9d262778b3dc2be365d667b03f9cca99efd049e76eb53a6de37120ca34da373b
   origin_info:
   nonce:
   86ed4bb9f76ab1107a05a9af4aa4eec84dd02f390f9bf5ef14730e0ee15aa92d
   token_key_id:

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   f861220ad4241ee0e33eb4a486a32f05af05ee33fcfdd1104c665eb827c20621
   token_authenticator_input: 000286ed4bb9f76ab1107a05a9af4aa4eec84d
   d02f390f9bf5ef14730e0ee15aa92dda5366799e0facc5cf9ea3dfc4a6f57072c
   31fff84e7331919ebdb06445b2c50f861220ad4241ee0e33eb4a486a32f05af05
   ee33fcfdd1104c665eb827c20621

   token_type: 2
   issuer_name: 6973737565722e6578616d706c65
   redemption_context:
   9d262778b3dc2be365d667b03f9cca99efd049e76eb53a6de37120ca34da373b
   origin_info:
   6f726967696e2e6578616d706c652c6f726967696e322e6578616d706c65
   nonce:
   86ed4bb9f76ab1107a05a9af4aa4eec84dd02f390f9bf5ef14730e0ee15aa92d
   token_key_id:
   f861220ad4241ee0e33eb4a486a32f05af05ee33fcfdd1104c665eb827c20621
   token_authenticator_input: 000286ed4bb9f76ab1107a05a9af4aa4eec84d
   d02f390f9bf5ef14730e0ee15aa92df7eeba1bd7c550a8184bee32ce66e6fb527
   17aa67da7e0ca32f4cdca9dec7130f861220ad4241ee0e33eb4a486a32f05af05
   ee33fcfdd1104c665eb827c20621

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

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