Batched Token Issuance Protocol
draft-ietf-privacypass-batched-tokens-03

Network Working Group                                          R. Robert
Internet-Draft                                               Phoenix R&D
Intended status: Standards Track                              C. A. Wood
Expires: 9 May 2025                                           Cloudflare
                                                         5 November 2024

                    Batched Token Issuance Protocol
                draft-ietf-privacypass-batched-tokens-03

Abstract

   This document specifies a variant of the Privacy Pass issuance
   protocol that allows for batched issuance of tokens.  This allows
   clients to request more than one token at a time and for issuers to
   issue more than one token at a time.

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

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   This Internet-Draft will expire on 9 May 2025.

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   Copyright (c) 2024 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

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Table of Contents

   1.  Change Log: . . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Motivation  . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Batched Privately Verifiable Token  . . . . . . . . . . . . .   4
     4.1.  Client-to-Issuer Request  . . . . . . . . . . . . . . . .   4
     4.2.  Issuer-to-Client Response . . . . . . . . . . . . . . . .   6
     4.3.  Finalization  . . . . . . . . . . . . . . . . . . . . . .   8
   5.  Arbitrary Batched Token Issuance  . . . . . . . . . . . . . .  10
     5.1.  Client-to-Issuer Request  . . . . . . . . . . . . . . . .  10
     5.2.  Issuer-to-Client Response . . . . . . . . . . . . . . . .  11
     5.3.  Finalization  . . . . . . . . . . . . . . . . . . . . . .  12
   6.  Security considerations . . . . . . . . . . . . . . . . . . .  12
     6.1.  Batched Privately Verifiable Tokens . . . . . . . . . . .  12
     6.2.  Arbitrary Batched Verifiable Tokens . . . . . . . . . . .  12
   7.  IANA considerations . . . . . . . . . . . . . . . . . . . . .  12
     7.1.  Token Type  . . . . . . . . . . . . . . . . . . . . . . .  13
     7.2.  Media Types . . . . . . . . . . . . . . . . . . . . . . .  13
       7.2.1.  "application/
               private-token-privately-verifiable-batch-request" media
               type  . . . . . . . . . . . . . . . . . . . . . . . .  14
       7.2.2.  "application/
               private-token-privately-verifiable-batch-response" media
               type  . . . . . . . . . . . . . . . . . . . . . . . .  14
       7.2.3.  "application/private-token-arbitrary-batch-request"
               media type  . . . . . . . . . . . . . . . . . . . . .  15
       7.2.4.  "application/private-token-arbitrary-batch-response"
               media type  . . . . . . . . . . . . . . . . . . . . .  15
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  16
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16

1.  Change Log:

   RFC EDITOR PLEASE DELETE THIS SECTION.

   draft-03

   *  Arbitrary token types

   *  Error code 400 aligned with RFC9578 and replaced by error code 422

   draft-02

   *  Renaming TokenRequest to BatchTokenRequest and TokenResponse to
      BatchTokenResponse

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   *  IANA: Media types for BatchTokenRequest and BatchTokenResponse

   *  IANA: Expand Token Type registry entry

   *  Various editorial fixes

   draft-01

   *  Initial WG document version

2.  Introduction

   This document specifies two Privacy Pass issuance protocols (as
   defined in [RFC9576]) that allow for batched issuance of tokens.
   This allows clients to request more than one token at a time and for
   issuers to issue more than one token at a time.

   The base Privacy Pass issuance protocol [RFC9578] defines stateless
   anonymous tokens, which can either be publicly verifiable or not.
   While it is possible to run multiple instances of the issuance
   protocol in parallel, e.g., over a multiplexed transport such as
   HTTP/3 [HTTP3] or by orchestrating multiple HTTP requests, these ad-
   hoc solutions vary based on transport protocol support.  In addition,
   in some cases, they cannot take advantage of cryptographic
   optimizations.

   The first variant of the issuance protocol builds upon the privately
   verifiable issuance protocol in [RFC9578] that uses VOPRF [OPRF], and
   allows for batched issuance of tokens.  This allows clients to
   request more than one token at a time and for issuers to issue more
   than one token at a time.  In effect, private batched issuance
   performance scales better than linearly.

   The second variant of the issuance protocol introduces a new Client-
   Issuer communication method, which allows for batched issuance of
   arbitrary token types.  This allows clients to request more than one
   token at a time and for issuers to issue more than one token at a
   time.  This variant has no other effect than batching requests and
   responses and the issuance performance remains linear.

   This batched issuance protocol registers one new token type
   (Section 7.1), to be used with the PrivateToken HTTP authentication
   scheme defined in [AUTHSCHEME].

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3.  Motivation

   Privacy Pass tokens (as defined in [RFC9576] and [RFC9578]) are
   unlinkable during issuance and redemption.  The basic issuance
   protocols defined in [RFC9578] however only allow for a single token
   to be issued at a time for every challenge.  In some cases,
   especially where a large number of clients need to fetch a large
   number of tokens, this may introduce performance bottlenecks.
   Batched token issuance improves upon the basic Privately Verifiable
   Token issuance protocol in the following key ways:

   1.  Issuing multiple tokens at once in response to a single
       TokenChallenge, thereby reducing the size of the proofs required
       for multiple tokens.

   2.  Improving server and client issuance efficiency by amortizing the
       cost of the VOPRF proof generation and verification,
       respectively.

   For all Verifiable Token issuance protocol, it allows for a single
   TokenRequest to be sent that encompasses multiple token requests.
   This enables the issuance of tokens for more than one key in one
   round trip between the Client and the Issuer.  The cost remains
   linear.

4.  Batched Privately Verifiable Token

   This section describes a batched issuance protocol for select token
   types, including 0x0001 (defined in [RFC9578]) and 0xF91A (defined in
   this document).  This variant is more efficient than Arbitary Batch
   Token Issuance defined below.  It does so by requiring the same key
   to be used by all token requests.

4.1.  Client-to-Issuer Request

   Except where specified otherwise, the client follows the same
   protocol as described in [RFC9578], Section 5.1.

   The Client first creates a context as follows:

   client_context = SetupVOPRFClient(ciphersuiteID, pkI)

   ciphersuiteID is the ciphersuite identifier from [OPRF] corresponding
   to the ciphersuite being used for this token version.
   SetupVOPRFClient is defined in [OPRF], Section 3.2.

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   Nr denotes the number of tokens the clients wants to request.  For
   every token, the Client then creates an issuance request message for
   a random value nonce with the input challenge and Issuer key
   identifier as described below:

   nonce_i = random(32)
   challenge_digest = SHA256(challenge)
   token_input = concat(token_type, nonce_i, challenge_digest,
                   token_key_id)
   blind_i, blinded_element_i = client_context.Blind(token_input)

   token_type corresponds to the 2-octet integer in the challenge.

   The above is repeated for each token to be requested.  Importantly, a
   fresh nonce MUST be sampled each time.

   The Client then creates a BatchTokenRequest structured as follows:

   struct {
       uint8_t blinded_element[Ne];
   } BlindedElement;

   struct {
      uint16_t token_type;
      uint8_t truncated_token_key_id;
      BlindedElement blinded_elements<0..2^16-1>;
   } BatchTokenRequest;

   The structure fields are defined as follows:

   *  "token_type" is a 2-octet integer, which matches the type in the
      challenge.

   *  "truncated_token_key_id" is the least significant byte of the
      token_key_id in network byte order (in other words, the last 8
      bits of token_key_id).

   *  "blinded_elements" is a list of Nr serialized elements, each of
      length Ne bytes and computed as
      SerializeElement(blinded_element_i), where blinded_element_i is
      the i-th output sequence of Blind invocations above.  Ne is as
      defined in [OPRF], Section 4.

   The Client then generates an HTTP POST request to send to the Issuer
   Request URL, with the BatchTokenRequest as the content.  The media
   type for this request is "application/private-token-privately-
   verifiable-batch-request".  An example request for the Issuer Request
   URL "https://issuer.example.net/request" is shown below.

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POST /request HTTP/1.1
Host: issuer.example.net
Accept: application/private-token-privately-verifiable-batch-response
Content-Type: application/private-token-privately-verifiable-batch-request
Content-Length: <Length of BatchTokenRequest>

<Bytes containing the BatchTokenRequest>

4.2.  Issuer-to-Client Response

   Except where specified otherwise, the client follows the same
   protocol as described in [RFC9578], Section 5.2.

   Upon receipt of the request, the Issuer validates the following
   conditions:

   *  The BatchTokenRequest contains a supported token_type equal to one
      of the batched token types defined in this document.

   *  The BatchTokenRequest.truncated_token_key_id corresponds to a key
      ID of a Public Key owned by the issuer.

   *  Nr, as determined based on the size of
      BatchTokenRequest.blinded_elements, is less than or equal to the
      number of tokens that the issuer can issue in a single batch.

   If any of these conditions is not met, the Issuer MUST return an HTTP
   422 (Unprocessable Content) error to the client.

   The Issuer then tries to deseralize the i-th element of
   BatchTokenRequest.blinded_elements using DeserializeElement from
   Section 2.1 of [OPRF], yielding blinded_element_i of type Element.
   If this fails for any of the BatchTokenRequest.blinded_elements
   values, the Issuer MUST return an HTTP 422 (Unprocessable Content)
   error to the client.  Otherwise, if the Issuer is willing to produce
   a token to the Client, the issuer forms a list of Element values,
   denoted blinded_elements, and computes a blinded response as follows:

   server_context = SetupVOPRFServer(ciphersuiteID, skI, pkI)
   evaluated_elements, proof =
     server_context.BlindEvaluateBatch(skI, blinded_elements)

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   ciphersuiteID is the ciphersuite identifier from [OPRF] corresponding
   to the ciphersuite being used for this token version.
   SetupVOPRFServer is defined in [OPRF], Section 3.2.  The issuer uses
   a list of blinded elements to compute in the proof generation step.
   The BlindEvaluateBatch function is a batch-oriented version of the
   BlindEvaluate function described in [OPRF], Section 3.3.2.  The
   description of BlindEvaluateBatch is below.

   Input:

     Element blindedElements[Nr]

   Output:

     Element evaluatedElements[Nr]
     Proof proof

   Parameters:

     Group G
     Scalar skS
     Element pkS

   def BlindEvaluateBatch(blindedElements):
     evaluatedElements = []
     for blindedElement in blindedElements:
       evaluatedElements.append(skS * blindedElement)

     proof = GenerateProof(skS, G.Generator(), pkS,
                           blindedElements, evaluatedElements)
     return evaluatedElements, proof

   The Issuer then creates a BatchTokenResponse structured as follows:

   struct {
       uint8_t evaluated_element[Ne];
   } EvaluatedElement;

   struct {
      EvaluatedElement evaluated_elements<0..2^16-1>;
      uint8_t evaluated_proof[Ns + Ns];
   } BatchTokenResponse;

   The structure fields are defined as follows:

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   *  "evaluated_elements" is a list of Nr serialized elements, each of
      length Ne bytes and computed as
      SerializeElement(evaluate_element_i), where evaluate_element_i is
      the i-th output of BlindEvaluate.

   *  "evaluated_proof" is the (Ns+Ns)-octet serialized proof, which is
      a pair of Scalar values, computed as
      concat(SerializeScalar(proof[0]), SerializeScalar(proof[1])),
      where Ns is as defined in [OPRF], Section 4.

   The Issuer generates an HTTP response with status code 200 whose
   content consists of TokenResponse, with the content type set as
   "application/private-token-privately-verifiable-batch-response".

HTTP/1.1 200 OK
Content-Type: application/private-token-privately-verifiable-batch-response
Content-Length: <Length of BatchTokenResponse>

<Bytes containing the BatchTokenResponse>

4.3.  Finalization

   Upon receipt, the Client handles the response and, if successful,
   deserializes the body values TokenResponse.evaluate_response and
   TokenResponse.evaluate_proof, yielding evaluated_elements and proof.
   If deserialization of either value fails, the Client aborts the
   protocol.  Otherwise, the Client processes the response as follows:

 authenticator_values = client_context.FinalizeBatch(token_input, blind,
                          evaluated_elements, blinded_elements, proof)

   The FinalizeBatch function is a batched variant of the Finalize
   function as defined in [OPRF], Section 3.3.2.  FinalizeBatch accepts
   lists of evaluated elements and blinded elements as input parameters,
   and is implemented as described below:

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  Input:

    PrivateInput input
    Scalar blind
    Element evaluatedElements[Nr]
    Element blindedElements[Nr]
    Proof proof

  Output:

    opaque output[Nh * Nr]

  Parameters:

    Group G
    Element pkS

  Errors: VerifyError

  def FinalizeBatch(input, blind,
    evaluatedElements, blindedElements, proof):
    if VerifyProof(G.Generator(), pkS, blindedElements,
                   evaluatedElements, proof) == false:
      raise VerifyError

    output = nil
    for evaluatedElement in evaluatedElements:
      N = G.ScalarInverse(blind) * evaluatedElement
      unblindedElement = G.SerializeElement(N)
      hashInput = I2OSP(len(input), 2) || input ||
                  I2OSP(len(unblindedElement), 2) || unblindedElement ||
                  "Finalize"
      output = concat(output, Hash(hashInput))

    return output

   If this succeeds, the Client then constructs Nr Token values, where
   authenticator is the i-th Nh-byte length slice of
   authenticator_values that corresponds to nonce, the i-th nonce that
   was sampled in Section 4.1:

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

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   If the FinalizeBatch function fails, the Client aborts the protocol.
   Token verification works exactly as specified in [RFC9578].

5.  Arbitrary Batched Token Issuance

   This section describes an issuance protocol mechanism for issuing
   multiple tokens in one round trip between Client and Issuer.  An
   arbitrary batched token request can contain token requests for any
   token type.

5.1.  Client-to-Issuer Request

   The Client first creates all TokenRequest it wants to batch.  To do
   so, the client follows protocol describing issuance, such as
   [RFC9578], Section 5.1 or [RFC9578], Section 6.1.

   The Client then creates a BatchedTokenRequest structured as follows:

   struct {
      uint16_t token_type;
      select (token_type) {
         case (0x0001): /* Type VOPRF(P-384, SHA-384), RFC 9578 */
            uint8_t truncated_token_key_id;
            uint8_t blinded_msg[Ne];
         case (0x0002): /* Type Blind RSA (2048-bit), RFC 9578 */
            uint8_t truncated_token_key_id;
            uint8_t blinded_msg[Nk];
      }
   } TokenRequest;

   struct {
     TokenRequest token_requests<0..2^16-1>;
   } BatchTokenRequest

   The structure fields are defined as follows:

   *  "token_type" is a 2-octet integer.  TokenRequest MUST be prefixed
      with a uint16 "token_type" indicating the token type.  The rest of
      the structure follows based on that type, within the inner opaque
      token_request attribute.  The above definition corresponds to
      TokenRequest from [RFC9578].  For TokenRequest not defined in
      [RFC9578], they MAY be used as long as they are prefixed with a
      2-octet token_type.

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   *  "token_requests" are serialized TokenRequests, in network byte
      order.  The number of token_requests, as a 2-octet integer, is
      prepended to the serialized TokenRequests.  In addition, the
      2-octet integer length of each TokenRequest is prepended to the
      serialized TokenRequests.

   The Client then generates an HTTP POST request to send to the Issuer
   Request URL, with the BatchTokenRequest as the content.  The media
   type for this request is "application/private-token-arbitrary-batch-
   request".  An example request for the Issuer Request URL
   "https://issuer.example.net/request" is shown below.

   POST /request HTTP/1.1
   Host: issuer.example.net
   Accept: application/private-token-arbitrary-batch-response
   Content-Type: application/private-token-arbitrary-batch-request
   Content-Length: <Length of BatchTokenRequest>

   <Bytes containing the BatchTokenRequest>

5.2.  Issuer-to-Client Response

   Upon receipt of the request, the Issuer validates the following
   conditions:

   *  The Content-Type is application/private-token-arbitrary-batch-
      request as registered with IANA.

   If this condition is not met, the Issuer MUST return an HTTP 422
   (Unprocessable Content) error to the client.

   The Issuer then tries to deserialize the first 2 bytes of the i-th
   element of BatchTokenRequest.token_requests.  If this is not a token
   type registered with IANA, the Issuer MUST return an HTTP 422
   (Unprocessable Content) error to the client.  The issuer creates a
   BatchTokenResponse structured as follows:

   struct {
     TokenResponse token_response<0..2^16-1>; /* Defined by token_type */
   } OptionalTokenResponse;

   struct {
     OptionalTokenResponse token_responses<0..2^16-1>;
   } BatchTokenResponse

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   BatchTokenResponse.token_responses is a vector of
   OptionalTokenResponses, length prefixed with two bytes.
   OptionalTokenResponse.token_response is a length-prefix-encoded
   TokenResponse, where a length of 0 indicates that the Issuer failed
   or refused to issue the associated TokenRequest.

   The Issuer generates an HTTP response with status code 200 whose
   content consists of TokenResponse, with the content type set as
   "application/private-token-arbitrary-batch-response".

   If the Issuer issues some tokens but not all, it MUST return an HTTP
   206 to the client and continue processing further requests.

   HTTP/1.1 200 OK
   Content-Type: application/private-token-arbitrary-batch-response
   Content-Length: <Length of BatchTokenResponse>

   <Bytes containing the BatchTokenResponse>

5.3.  Finalization

   The Client tries to deserialize the i-th element of
   BatchTokenResponse.token_responses using the protocol associated to
   BatchTokenRequest.token_type.  If the element has a size of 0, the
   Client MUST ignore this token, and continue processing the next
   token.  The Client finalizes each deserialized TokenResponse using
   the matching TokenRequest according to the corresponding finalization
   procedure defined by the token type.

6.  Security considerations

6.1.  Batched Privately Verifiable Tokens

   Implementors SHOULD be aware of the security considerations described
   in [OPRF], Section 6.2.3 and implement mitigation mechanisms.
   Application can mitigate this issue by limiting the number of clients
   and limiting the number of token requests per client per key.

6.2.  Arbitrary Batched Verifiable Tokens

   Implementors SHOULD be aware of the inherent linear cost of this
   token type.  An Issuer MAY ignore TokenRequest if the number of
   tokens per request past a limit.

7.  IANA considerations

   This section contains IANA codepoint allocation requests.

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7.1.  Token Type

   This document updates the "Token Type" Registry ([AUTHSCHEME]) with
   the following entry:

   *  Value: 0xF91A

   *  Name: VOPRF (ristretto255, SHA-512)

   *  Token Structure: As defined in Section 2.2 of [AUTHSCHEME]

   *  Token Key Encoding: Serialized using SerializeElement from
      Section 2.1 of [OPRF]

   *  TokenChallenge Structure: As defined in Section 2.1 of
      [AUTHSCHEME]

   *  Publicly Verifiable: N

   *  Public Metadata: N

   *  Private Metadata: N

   *  Nk: 32

   *  Nid: 32

   *  Reference: [RFC9578], Section 5

   *  Notes: None

7.2.  Media Types

   The following entries should be added to the IANA "media types"
   registry:

   *  "application/private-token-privately-verifiable-batch-request"

   *  "application/private-token-privately-verifiable-batch-response"

   *  "application/private-token-arbitrary-batch-request"

   *  "application/private-token-arbitrary-batch-response"

   The templates for these entries are listed below and the reference
   should be this RFC.

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7.2.1.  "application/private-token-privately-verifiable-batch-request"
        media type

   Type name:  application
   Subtype name:  private-token-request
   Required parameters:  N/A
   Optional parameters:  N/A
   Encoding considerations:  "binary"
   Security considerations:  see Section 6
   Interoperability considerations:  N/A
   Published specification:  this specification
   Applications that use this media type:  Applications that want to
      issue or facilitate issuance of Privacy Pass tokens, including
      Privacy Pass issuer applications themselves.
   Fragment identifier considerations:  N/A
   Additional information:  Magic number(s):  N/A
                            Deprecated alias names for this type:  N/A
                            File extension(s):  N/A
                            Macintosh file type code(s):  N/A
   Person and email address to contact for further information:  see Aut
      hors' Addresses section
   Intended usage:  COMMON
   Restrictions on usage:  N/A
   Author:  see Authors' Addresses section
   Change controller:  IETF

7.2.2.  "application/private-token-privately-verifiable-batch-response"
        media type

   Type name:  application
   Subtype name:  private-token-response
   Required parameters:  N/A
   Optional parameters:  N/A
   Encoding considerations:  "binary"
   Security considerations:  see Section 6
   Interoperability considerations:  N/A
   Published specification:  this specification
   Applications that use this media type:  Applications that want to
      issue or facilitate issuance of Privacy Pass tokens, including
      Privacy Pass issuer applications themselves.
   Fragment identifier considerations:  N/A
   Additional information:  Magic number(s):  N/A
                            Deprecated alias names for this type:  N/A
                            File extension(s):  N/A
                            Macintosh file type code(s):  N/A
   Person and email address to contact for further information:  see Aut
      hors' Addresses section
   Intended usage:  COMMON

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   Restrictions on usage:  N/A
   Author:  see Authors' Addresses section
   Change controller:  IETF

7.2.3.  "application/private-token-arbitrary-batch-request" media type

   Type name:  application
   Subtype name:  private-token-request
   Required parameters:  N/A
   Optional parameters:  N/A
   Encoding considerations:  "binary"
   Security considerations:  see Section 6
   Interoperability considerations:  N/A
   Published specification:  this specification
   Applications that use this media type:  Applications that want to
      issue or facilitate issuance of Privacy Pass tokens, including
      Privacy Pass issuer applications themselves.
   Fragment identifier considerations:  N/A
   Additional information:  Magic number(s):  N/A
                            Deprecated alias names for this type:  N/A
                            File extension(s):  N/A
                            Macintosh file type code(s):  N/A
   Person and email address to contact for further information:  see Aut
      hors' Addresses section
   Intended usage:  COMMON
   Restrictions on usage:  N/A
   Author:  see Authors' Addresses section
   Change controller:  IETF

7.2.4.  "application/private-token-arbitrary-batch-response" media type

   Type name:  application
   Subtype name:  private-token-response
   Required parameters:  N/A
   Optional parameters:  N/A
   Encoding considerations:  "binary"
   Security considerations:  see Section 6
   Interoperability considerations:  N/A
   Published specification:  this specification
   Applications that use this media type:  Applications that want to
      issue or facilitate issuance of Privacy Pass tokens, including
      Privacy Pass issuer applications themselves.
   Fragment identifier considerations:  N/A
   Additional information:  Magic number(s):  N/A
                            Deprecated alias names for this type:  N/A
                            File extension(s):  N/A
                            Macintosh file type code(s):  N/A
   Person and email address to contact for further information:  see Aut

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      hors' Addresses section
   Intended usage:  COMMON
   Restrictions on usage:  N/A
   Author:  see Authors' Addresses section
   Change controller:  IETF

8.  References

8.1.  Normative References

   [AUTHSCHEME]
              Pauly, T., Valdez, S., and C. A. Wood, "The Privacy Pass
              HTTP Authentication Scheme", Work in Progress, Internet-
              Draft, draft-ietf-privacypass-auth-scheme-15, 23 October
              2023, <https://datatracker.ietf.org/doc/html/draft-ietf-
              privacypass-auth-scheme-15>.

   [OPRF]     Davidson, A., Faz-Hernandez, A., Sullivan, N., and C. A.
              Wood, "Oblivious Pseudorandom Functions (OPRFs) using
              Prime-Order Groups", Work in Progress, Internet-Draft,
              draft-irtf-cfrg-voprf-21, 21 February 2023,
              <https://datatracker.ietf.org/doc/html/draft-irtf-cfrg-
              voprf-21>.

   [RFC9576]  Davidson, A., Iyengar, J., and C. A. Wood, "The Privacy
              Pass Architecture", Work in Progress, Internet-Draft,
              draft-ietf-privacypass-architecture-16, 25 September 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-
              privacypass-architecture-16>.

   [RFC9578]  Celi, S., Davidson, A., Valdez, S., and C. A. Wood,
              "Privacy Pass Issuance Protocol", Work in Progress,
              Internet-Draft, draft-ietf-privacypass-protocol-16, 3
              October 2023, <https://datatracker.ietf.org/doc/html/
              draft-ietf-privacypass-protocol-16>.

8.2.  Informative References

   [HTTP3]    Bishop, M., Ed., "HTTP/3", RFC 9114, DOI 10.17487/RFC9114,
              June 2022, <https://www.rfc-editor.org/rfc/rfc9114>.

Authors' Addresses

   Raphael Robert
   Phoenix R&D
   Email: ietf@raphaelrobert.com

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

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