The Privacy Pass HTTP Authentication Scheme
draft-ietf-privacypass-auth-scheme-02

Network Working Group                                           T. Pauly
Internet-Draft                                                Apple Inc.
Intended status: Standards Track                               S. Valdez
Expires: 6 October 2022                                       Google LLC
                                                              C. A. Wood
                                                              Cloudflare
                                                            4 April 2022

              The Privacy Pass HTTP Authentication Scheme
                 draft-ietf-privacypass-auth-scheme-02

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

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 6 October 2022.

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

Pauly, et al.            Expires 6 October 2022                 [Page 1]
Internet-Draft         Privacy Pass Authentication            April 2022

   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  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  HTTP Authentication Scheme  . . . . . . . . . . . . . . . . .   4
     2.1.  Token Challenge . . . . . . . . . . . . . . . . . . . . .   4
       2.1.1.  Redemption Context Construction . . . . . . . . . . .   6
       2.1.2.  Token Caching . . . . . . . . . . . . . . . . . . . .   7
     2.2.  Token Redemption  . . . . . . . . . . . . . . . . . . . .   7
   3.  Issuance Protocol Requirements  . . . . . . . . . . . . . . .   9
   4.  User Interaction  . . . . . . . . . . . . . . . . . . . . . .   9
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
     6.1.  Authentication Scheme . . . . . . . . . . . . . . . . . .  11
     6.2.  Token Type Registry . . . . . . . . . . . . . . . . . . .  12
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  12
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   Privacy Pass tokens are unlinkable authenticators that can be used to
   anonymously authorize a client (see
   [I-D.ietf-privacypass-architecture]).  A client possessing such a
   token is able to prove that it was able to get a token issued by a
   token issuer -- based on some check from a token issuer, such as
   authentication or solving a CAPTCHA -- without allowing the relying
   party redeeming the client's token (the origin) to link it with
   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
   ([RFC7235]), PrivateToken, that allows clients to redeem various
   kinds of Privacy Pass tokens.

Pauly, et al.            Expires 6 October 2022                 [Page 2]
Internet-Draft         Privacy Pass Authentication            April 2022

   Clients and relying parties interact using this scheme to perform the
   token challenge and token redemption flow.  Clients use a token
   issuance protocol to actually fetch tokens to redeem.

    Client                             Relying Party (Origin)

       <------------------------------ Challenge \
                                                 |
   +----------------------------------\          |
   |                                  |          |
   |  Issuance Protocol               |          |
   |                                  |          |
   +----------------------------------/          |
                                                 |
        Redemption -------------------------- >  /

                  Figure 1: Token Architectural Components

   In addition to working with different token issuance protocols, this
   scheme supports optionally associating tokens 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
   [I-D.ietf-privacypass-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.

Pauly, et al.            Expires 6 October 2022                 [Page 3]
Internet-Draft         Privacy Pass Authentication            April 2022

   *  Token challenge: A requirement for tokens sent from an origin to a
      client, using the "WWW-Authenticate" HTTP header.  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, using the "Authorization" HTTP header.

2.  HTTP Authentication Scheme

   Token redemption is performed using HTTP Authentication ([RFC7235]),
   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).

2.1.  Token Challenge

   Origins send a token challenge to clients in an "WWW-Authenticate"
   header 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.

   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.

   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:

Pauly, et al.            Expires 6 October 2022                 [Page 4]
Internet-Draft         Privacy Pass Authentication            April 2022

   *  "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 6.2.
      Challenges with unsupported token_type values MUST be ignored.

   *  "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.

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

   *  "challenge", which contains a base64url-encoded [RFC4648]
      TokenChallenge value.  Since the length of the challenge is not
      fixed, the base64url data MUST include padding.  This MUST be
      unique for every 401 HTTP response to prevent replay attacks.
      This attribute 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 data MUST
      include padding.  This attribute MAY be omitted in deployments
      where clients are able to retrieve the issuer key using an out-of-
      band mechanism.

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

Pauly, et al.            Expires 6 October 2022                 [Page 5]
Internet-Draft         Privacy Pass Authentication            April 2022

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

   The header MAY also include the standard "realm" attribute, if
   desired.  Issuance protocols MAY require other attributes.

   As an example, the WWW-Authenticate header 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.
   Clients MAY have further restrictions and requirements around
   validating when a challenge is considered acceptable or valid.  For
   example, clients can choose to reject 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 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 could look like
   this:

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

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).

Pauly, et al.            Expires 6 October 2022                 [Page 6]
Internet-Draft         Privacy Pass Authentication            April 2022

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

   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.

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.

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.

Pauly, et al.            Expires 6 October 2022                 [Page 7]
Internet-Draft         Privacy Pass Authentication            April 2022

   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).

   *  "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 authenticator value in the Token structure is computed over the
   token_type, nonce, context, and token_key_id fields.

   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 data MUST include padding.  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 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

Pauly, et al.            Expires 6 October 2022                 [Page 8]
Internet-Draft         Privacy Pass Authentication            April 2022

   bound to a specific HTTP session with 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.  Issuance Protocol Requirements

   Clients initiate the issuance protocol using a challenge, a randomly
   generated nonce, and a public key for the issuer.  The issuance
   protocol itself can be any interactive protocol between client,
   issuer, or other parties that produces a valid authenticator over the
   client's input, subject to the following security requirements.

   1.  Unconditional input secrecy.  The issuance protocol MUST NOT
       reveal anything about the client's private input, including the
       challenge and nonce.  The issuance protocol can reveal the issuer
       public key for the purposes of determining which private key to
       use in producing the issuance protocol.  A result of this
       property is that the redemption flow is unlinkable from the
       issuance flow.

   2.  One-more forgery security.  The issuance protocol MUST NOT allow
       malicious clients to forge tokens without interacting with the
       issuer directly.

   3.  Concurrent security.  The issuance protocol MUST be safe to run
       concurrently with arbitrarily many clients.

4.  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.

Pauly, et al.            Expires 6 October 2022                 [Page 9]
Internet-Draft         Privacy Pass Authentication            April 2022

   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
   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.

5.  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

                          <----------- Challenge \
                                                 |
      <--------- Challenge                       |
                                                 |
      Redemption ---->                           |
                                                 |
                          Redemption ----------> /

                  Figure 2: Token Architectural Components

   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

Pauly, et al.            Expires 6 October 2022                [Page 10]
Internet-Draft         Privacy Pass Authentication            April 2022

   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
   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 attribute,
   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 [I-D.ietf-privacypass-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.

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

6.  IANA Considerations

6.1.  Authentication Scheme

   This document registers the "PrivateToken" authentication scheme in
   the "Hypertext Transfer Protocol (HTTP) Authentication Scheme
   Registry" established by [RFC7235].

   Authentication Scheme Name: PrivateToken

   Pointer to specification text: Section 2 of this document

Pauly, et al.            Expires 6 October 2022                [Page 11]
Internet-Draft         Privacy Pass Authentication            April 2022

6.2.  Token Type Registry

   The "Token Type" registry lists 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:

   *  Value: The two-byte identifier for the algorithm

   *  Name: Name of the issuance protocol

   *  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

   The initial contents for this registry are defined in the table
   below.

   +======+============+============+========+========+==+===+=========+
   |Value | Name       | Publicly   |Public  |Private |Nk|Nid|Reference|
   |      |            | Verifiable |Metadata|Metadata|  |   |         |
   +======+============+============+========+========+==+===+=========+
   |0x0000| (reserved) | N/A        |N/A     |N/A     |N/|N/A|N/A      |
   |      |            |            |        |        |A |   |         |
   +------+------------+------------+--------+--------+--+---+---------+

                            Table 1: Token Types

7.  References

7.1.  Normative References

Pauly, et al.            Expires 6 October 2022                [Page 12]
Internet-Draft         Privacy Pass Authentication            April 2022

   [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>.

   [RFC7235]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Authentication", RFC 7235,
              DOI 10.17487/RFC7235, June 2014,
              <https://www.rfc-editor.org/rfc/rfc7235>.

   [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>.

   [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>.

7.2.  Informative References

   [COOKIES]  Chen, L., Englehardt, S., West, M., and J. Wilander,
              "Cookies: HTTP State Management Mechanism", Work in
              Progress, Internet-Draft, draft-ietf-httpbis-rfc6265bis-
              09, 19 October 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
              rfc6265bis-09>.

   [I-D.ietf-privacypass-architecture]
              Davidson, A., Iyengar, J., and C. A. Wood, "Privacy Pass
              Architectural Framework", Work in Progress, Internet-
              Draft, draft-ietf-privacypass-architecture-03, 7 March
              2022, <https://datatracker.ietf.org/doc/html/draft-ietf-
              privacypass-architecture-03>.

Authors' Addresses

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

Pauly, et al.            Expires 6 October 2022                [Page 13]
Internet-Draft         Privacy Pass Authentication            April 2022

   Steven Valdez
   Google LLC
   Email: svaldez@chromium.org

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

Pauly, et al.            Expires 6 October 2022                [Page 14]