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The Privacy Pass HTTP Authentication Scheme
RFC 9577

Document Type RFC - Proposed Standard (June 2024)
Authors Tommy Pauly , Steven Valdez , Christopher A. Wood
Last updated 2024-06-13
RFC stream Internet Engineering Task Force (IETF)
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RFC 9577


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

              The Privacy Pass HTTP Authentication Scheme

Abstract

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

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9577.

Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Revised BSD License text as described in Section 4.e of the
   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

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

1.  Introduction

   Privacy Pass tokens are unlinkable authenticators that can be used to
   anonymously authorize a Client (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 ([HTTP],
   Section 11), "PrivateToken", that allows Clients to redeem various
   kinds of Privacy Pass tokens.

   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 can then react to that challenge by issuing a new
   request with a corresponding token (using the Authorization request
   header field).  Clients generate tokens that match the Origin's token
   challenge by running one of the token issuance protocols defined in
   [ISSUANCE].  The act of presenting a token in an Authorization
   request header field is referred to as "token redemption".  This
   interaction between the Client and Origin is shown below.

   +--------+                              +--------+
   | Origin |                              | Client |
   +---+----+                              +---+----+
       |                                       |
       +-- WWW-Authenticate: TokenChallenge -->|
       |                                       |
       |                            (Run issuance protocol)
       |                                       |
       |<------ Authorization: token ----------+
       |                                       |

              Figure 1: Challenge and Redemption Protocol Flow

   In addition to working with different token issuance protocols, this
   scheme optionally supports the use of tokens that are associated with
   Origin-chosen 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 (1) allow for
   different deployment models to prevent double-spending and (2) 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:

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

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

   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 ([HTTP],
   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).  The process of presenting a token as authentication
   to an Origin is also referred to as "spending" a token.

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

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

2.1.  Token Challenge

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

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

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

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

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

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

2.1.1.  Token Challenge Structure

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

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

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

   The default TokenChallenge message has the following structure:

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

   The structure fields are defined as follows:

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

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

   *  redemption_context is a field that is either 0 or 32 bytes,
      prefixed with a single octet indicating the length (either 0 or
      32).  If the value is non-empty, it is a 32-byte value generated
      by the Origin that 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 2.1.1.2 for
      example contexts that might be useful in practice.  Challenges
      with redemption_context values of invalid lengths MUST be ignored.

   *  origin_info is an ASCII string that either is empty or contains
      one or more Origin names that allow a token to be scoped to a
      specific set of Origins.  Each Origin name uses the format of a
      server name as defined in Section 2.1.1.1.  The string is prefixed
      with a 2-octet integer indicating the length, in network byte
      order.  If empty, any non-Origin-specific 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.

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

2.1.1.1.  Server Name Encoding

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

2.1.1.2.  Redemption Context Construction

   The TokenChallenge redemption context allows the Origin 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 the redemption
      context as F(current time window), where F is a pseudorandom
      function.

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

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

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

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

2.1.2.  Sending Token Challenges

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

   *  challenge, which contains a base64url TokenChallenge value,
      encoded per [RFC4648].  This document follows the default padding
      behavior described in Section 3.2 of [RFC4648], so the base64url
      value MUST include padding.  As an authentication parameter (auth-
      param from [HTTP], Section 11.2), the value can be either a token
      or a quoted-string and might be required to be a quoted-string if
      the base64url string includes "=" characters.  This parameter 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.
      See [ISSUANCE] for more information about how this public key is
      used by the issuance protocols described in that specification.
      The encoding of the public key is determined by the token type;
      see Section 6.2.  As with challenge, the base64url value MUST
      include padding.  As an authentication parameter (auth-param from
      [HTTP], Section 11.2), the value can be either a token or a
      quoted-string and might be required to be a quoted-string if the
      base64url string includes "=" characters.  This parameter MAY be
      omitted in deployments where Clients are able to retrieve the
      Issuer key using an out-of-band mechanism.

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

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

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

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

2.1.2.1.  Sending Multiple Token Challenges

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

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

   Origins should only include challenges for different types of
   issuance protocols with functionally equivalent properties.  For
   instance, both issuance protocols in [ISSUANCE] have the same
   functional properties, albeit with different mechanisms for verifying
   the resulting tokens during redemption.  Since Clients 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.

2.1.3.  Processing Token Challenges

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

   *  The token_type is recognized and supported by the Client;

   *  The TokenChallenge structure is well-formed; and

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

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

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

2.1.4.  Token Caching

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

   Cached tokens can only be redeemed when they match all of the fields
   in the TokenChallenge: token_type, issuer_name, redemption_context,
   and origin_info.  Clients ought to store cached tokens based on all
   of these fields, to avoid trying to redeem a token that does not
   match.  Note that each token has a unique Client 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 if
   there is a network change and the Client does not have any Origin-
   specific state like HTTP cookies.  Using these tokens in a context
   that otherwise would not be linkable to the original context could
   allow the Origin 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).

2.2.1.  Token Structure

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

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

   The default token message has the following structure:

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

   The structure fields are defined as follows:

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

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

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

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

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

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

2.2.2.  Sending Tokens

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

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

   Authorization: PrivateToken token="abc..."

   For context-bound tokens, Origins store or reconstruct the contexts
   of previous TokenChallenge structures in order to validate the token.
   A TokenChallenge can be bound to a specific TLS session with a
   Client, 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.  Double-spend prevention ensures that Clients cannot
   replay tokens for previous challenges.  See Section 5.2 for more
   information about replay attacks.  For context-bound tokens, this
   double-spend prevention can require no state or minimal state, since
   the context can be used to verify token uniqueness.

2.2.3.  Token Verification

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

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

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

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

3.  Client Behavior

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

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

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

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

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

3.1.  Choosing to Redeem Tokens

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

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

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

3.2.  Choosing between Multiple Challenges

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

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

4.  Origin Behavior

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

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

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

4.1.  Greasing

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

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

5.  Security Considerations

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

5.1.  Randomness Requirements

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

5.2.  Replay Attacks

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

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

5.3.  Reflection Attacks

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

   +--------+            +----------+                +--------+
   | Origin |            | Attacker |                | Client |
   +---+----+            +----+-----+                +---+----+
       |                      |                          |
       +--- TokenChallenge -->|                          |
       |                      +-- (reflect challenge) -->|
       |                      |<-------- Token ----------+
       |<-- (reflect token) --+                          |
       |                      |                          |

                    Figure 2: Reflection Attack Example

5.4.  Token Exhaustion Attacks

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

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

5.5.  Timing Correlation Attacks

   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 parameter,
   Clients MAY try to add delay to the time between being challenged and
   redeeming a token to make this sort of linkability more difficult.
   For more discussion on correlation risks between token issuance and
   redemption, see Section 6.3 of [ARCHITECTURE].

5.6.  Cross-Context Linkability Attacks

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

6.  IANA Considerations

6.1.  Authentication Scheme

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

   Authentication Scheme Name:  PrivateToken

   Reference:  RFC 9577, Section 2

6.2.  Privacy Pass Token Types Registry

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

   New registrations need to list the following attributes:

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

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

   [ISSUANCE] defines entries for this registry.

6.2.1.  Reserved Values

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

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

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

7.  References

7.1.  Normative References

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

   [HTTP]     Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Semantics", STD 97, RFC 9110,
              DOI 10.17487/RFC9110, June 2022,
              <https://www.rfc-editor.org/info/rfc9110>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

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

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

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

   [TLS13]    Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

   [URI]      Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

7.2.  Informative References

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

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

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

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

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

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

Appendix A.  Test Vectors

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

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

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

A.1.  Challenge and Redemption Structure Test Vectors

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

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

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

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

   token_key_id:  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.

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

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

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

   3.  TokenChallenge with an empty Origin and redemption context.

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

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

   6.  TokenChallenge for greasing.

   These test vectors are below.

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

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

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

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

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

   // Test vector 6:
   //   token_type(0000), structure(random_bytes)
   token_type: 0000
   token_authenticator_input: 000058405ad31e286e874cb42d0ef9d50461ae
   703bb71a21178beb429c43c0effe587456d856f0f2bdfc216ef93d5c225e2a93e
   84cb686e63919788087f7ab1054aa817f09dcb919a0ed6f90fe887e8b08cd1eee
   44d5be8d813eda9f2656db61c932db8d73f8690604ded0120157923bbd19d5549
   e639e4de07530aee1d370f5187b678685715bd878dde24346751eb532a87b71ea
   40bbe5a13218658e303c648eb03817453690bfcbe8255081bf27ff0891cd02ee2
   483e48a2c494bdef696f943fa992a65303292c25d0d3f62da86a70d0b020f0ff5
   b90d0ff0f6abdb097d321fde04f3a1994e63bcd35a88c21236c7dc67600482223
   f54b25e39a250439f27ecb5ae9eb8ed548a3ec1f1d6f510d08281929c8fe08834
   2959e35ea9b3b6f6a96fc1a8edba4ed297f4cf02d0e4482b79a11f671745d7b7d
   b120eddd8a4c2b6501bbc895b2160b8071615d9c1b18f32e056bfee29deac6a7d
   6cf7b522a5badd63b9cb

A.2.  HTTP Header Test Vectors

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

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

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

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

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

   WWW-Authenticate: PrivateToken challenge="AAIADmlzc3Vlci5leGFtcGxlII
   o-g6M9mABdLzC-9Bn6a_TNXGAF42sShbu0zNQPpLODAA5vcmlnaW4uZXhhbXBsZQ==",
    token-key="MIIBUjA9BgkqhkiG9w0BAQowMKANMAsGCWCGSAFlAwQCAqEaMBgGCSqG
   SIb3DQEBCDALBglghkgBZQMEAgKiAwIBMAOCAQ8AMIIBCgKCAQEAyxrta2qV9bHOATpM
   _KsluUsuZKIwNOQlCn6rQ8DfOowSmTrxKxEZCNS0cb7DHUtsmtnN2pBhKi7pA1I-beWi
   JNawLwnlw3TQz-Adj1KcUAp4ovZ5CPpoK1orQwyB6vGvcte155T8mKMTknaHl1fORTtS
   bvm_bOuZl5uEI7kPRGGiKvN6qwz1cz91l6vkTTHHMttooYHGy75gfYwOUuBlX9mZbcWE
   7KC-h6-814ozfRex26noKLvYHikTFxROf_ifVWGXCbCWy7nqR0zq0mTCBz_kl0DAHwDh
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   token-key-0: 856de3c710b892e7cca1ae5eb121af42ca8e779137a11224228c9b9
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   max-age-1: 10
   token-challenge-1: 0001000e6973737565722e6578616d706c65208a3e83a33d9
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   WWW-Authenticate: Basic realm="grease", PrivateToken challenge="AACs
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   PrivateToken challenge="AAEADmlzc3Vlci5leGFtcGxlIIo-g6M9mABdLzC-9Bn6
   a_TNXGAF42sShbu0zNQPpLODAA5vcmlnaW4uZXhhbXBsZQ==", token-key="67H-0z
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Authors' Addresses

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

   Steven Valdez
   Google LLC
   Email: svaldez@chromium.org

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