



Privacy Pass                                                  T. Meunier
Internet-Draft                                           Cloudflare Inc.
Intended status: Informational                              21 June 2025
Expires: 23 December 2025


                       Privacy Pass Reverse Flow
               draft-meunier-privacypass-reverse-flow-01

Abstract

   This document specifies an instantiation of Privacy Pass Architecture
   [RFC9576] that allows for a "reverse" flow from the Origin to the
   Client.  It describes a method for an Origin to issue new tokens in
   response to a request in which a token is redeemed.

About This Document

   This note is to be removed before publishing as an RFC.

   The latest revision of this draft can be found at
   https://thibmeu.github.io/draft-meunier-privacypass-reverse-flow-
   informational/draft-meunier-privacypass-reverse-flow.html.  Status
   information for this document may be found at
   https://datatracker.ietf.org/doc/draft-meunier-privacypass-reverse-
   flow/.

   Discussion of this document takes place on the Privacy Pass Working
   Group mailing list (mailto:privacy-pass@ietf.org), which is archived
   at https://mailarchive.ietf.org/arch/browse/privacy-pass/.  Subscribe
   at https://www.ietf.org/mailman/listinfo/privacy-pass/.

   Source for this draft and an issue tracker can be found at
   https://github.com/thibmeu/draft-meunier-privacypass-reverse-flow-
   informational.

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/.






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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on 23 December 2025.

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   document authors.  All rights reserved.

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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Motivation  . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Refunding tokens  . . . . . . . . . . . . . . . . . . . .   3
     2.2.  Bootstraping issuer . . . . . . . . . . . . . . . . . . .   3
     2.3.  Attester feedback loop  . . . . . . . . . . . . . . . . .   4
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Architecture overview . . . . . . . . . . . . . . . . . . . .   5
   5.  Reverse flow with an HTTP header  . . . . . . . . . . . . . .   5
     5.1.  Client behaviour  . . . . . . . . . . . . . . . . . . . .   6
     5.2.  Origin/Issuer/Attester deployment . . . . . . . . . . . .   6
     5.3.  Split Origin-Attester deployment  . . . . . . . . . . . .   7
   6.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .   8
     6.1.  Issuer face values  . . . . . . . . . . . . . . . . . . .   8
     6.2.  Token for specific Clients  . . . . . . . . . . . . . . .   9
     6.3.  Sending more than one token . . . . . . . . . . . . . . .   9
     6.4.  Swap endpoint and its privacy implication . . . . . . . .  10
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  11
   Changelog . . . . . . . . . . . . . . . . . . . . . . . . . . . .  11
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  12





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1.  Introduction

   This document specifies an instantiation of Privacy Pass Architecture
   [RFC9576] that allows for a reverse flow from the Origin to the
   Client.

   In other words, it specifies a way for an Origin to act as an
   Attester + Issuer.

2.  Motivation

   With Privacy Pass issuance as described in [RFC9576], once a token is
   sent by a Client, it is considered spent and cannot be reused in
   order to guarantee unlinkability.  If a token were to be spent twice,
   the two requests would be linkable by the Origin.

   However, requiring that all tokens are spent only once means that
   Clients might need to request more tokens for new requests, even if
   the request that included the original token doesn't need to "spend"
   that token from the Origin's perspective (due to the request ending
   up being insignificant to the Origin).  This draft provides a
   mechanism for an Origin to provide tokens, allowing reuse without
   reaching out to the initial Attester and Issuer.

2.1.  Refunding tokens

   Certain Origin use Privacy Pass tokens to rate-limit requests they
   receive over a certain time window because of resource constraints.
   If a Client sends a request that can be served without utilising that
   resource, the Origin would like to authorise them to do a second
   request.  This is the case for request requiring compute and the
   compute is low, or when the request leads to a redirection instead of
   content generation for instance.

   With a reverse flow, a Client that has already been authorised by an
   Origin can maintain that authorization, without losing the
   unlinkability property provided by Privacy Pass.

2.2.  Bootstraping issuer

   An Origin wants to grant 30 access for Clients that solved a CAPTCHA.
   To do so, it consumes a type 0x0002 public veriable token from an
   initial issuer that guarantees a CAPTCHA has been solved, and use it
   to issue 30 type 0x0001 private tokens.  Without a reverse flow, the
   Origin would have to require 30 0x0002 issuer tokens, which have
   lower performance and a higher number of requests going to the
   issuer.




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2.3.  Attester feedback loop

   In [RFC9576], a Client gets a token from an Issuer and redeems it at
   an Origin.  However, if the Client's request is deemed unwanted by
   the Origin at redemption time, there is no mechanism that prevents
   the Client from going back to the initial Issuer to get a new token
   and be authorized again.

   With a reverse flow, the initial Issuer may require Clients to
   present an Origin-issued token before providing them with a second
   token.  This allows for a feedback loop between the Origin and the
   initial Issuer, without breaking Client unlinkability.

3.  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.

   We reuse terminology from [RFC9576].

   The following terms are used throughout this document:

   *Flow:*  Direction from PrivateToken issuance to its redemption.  The
      entity starting the flow acts as an Issuer, while the end of the
      flow acts as an Origin.  The Client is always included, as it
      finalises the TokenResponse, and coordinate interactions.

   *Initial Flow:*  Issuer -> Attester -> Client -> Origin.  This flow
      produces a PrivateToken that is used by the Origin to kickstart a
      Reverse Flow.

   *Reverse Flow:*  Issuer <- Attester <- Client <- Origin.  This flow
      allows Origin to issues PrivateToken.  In the reverse flow, the
      Origin operates one or more Issuer, and the Client MAY provide
      these tokens either to the Initial Attester/Issuer, or use them
      against the Origin

   *Initial Attester/Issuer:*  Attester/Issuer part of the Initial Flow

   *Origin Issuer:*  Issuer operated by the Origin

   *Origin PrivateToken:*  PrivateToken issued by the Origin

   *Reverse Origin:*  An entity that consumes the Origin PrivateToken.
      It can be the Origin, or the Initial Attester/Issuer



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4.  Architecture overview

   Along with sending their PrivateToken for authentication (as
   specified in [RFC9576]), Client sends TokenRequest

+---------------+    +--------+                                       +--------+         +----------+ +--------+
| Origin Issuer |    | Origin |                                       | Client |         | Attester | | Issuer |
+---+-----------+    +---+----+                                       +---+----+         +----+-----+ +---+----+
    |                    |                                                |                   |           |
    |                    |<----- Request ---------------------------------+                   |           |
    |                    +-- TokenChallenge (Issuer) -------------------->|                   |           |
    |                    |                                                |<== Attestation ==>|           |
    |                    |                                                |                   |           |
    |                    |                                                +--------- TokenRequest ------->|
    |                    |                                                |<-------- TokenResponse -------+
    |                    |<-- Request+Token+TokenRequest(Origin Issuer) --+                   |           |
    |<-- TokenRequest ---+                                                |                   |           |
    +-- TokenResponse -->|                                                |                   |           |
    |                    |--- Response+TokenResponse(Origin Issuer) ----->|                   |           |
    |                    |                                                |                   |           |

   The initial flow matches the one defined by [RFC9576].  A Client gets
   challenged when accessing a resource on an Origin.  The Client goes
   to the Attester to get issue a Token.

   Through configuration mechanism not defined in this document, the
   Client is aware the Origin acts as a Reverse Flow issuer.

   This is an extension of [RFC9576].  The redemption flow of a Privacy
   Pass token is defined in Section 3.6.4 of [RFC9576].  Reverse flow
   extends this so that redemption flow is interleaved with the issuance
   flow described in Section 3.6.3 of [RFC9576].  This is denoted in the
   diagram above by the Client sending Request+Token+TokenRequest(Origin
   Issuer).  The Origin runs the issuance protocol, and returns
   Response+TokenResponse(Origin Issuer).

   Such flow can be performed through various means.  This document
   introduces one to serve as example and first basis.

5.  Reverse flow with an HTTP header

   This section defines a Reverse Flow, as presented in Section 4,
   leveraging a new HTTP headers.

   TokenRequest(Origin Issuer) and TokenResponse(Origin Issuer) happen
   through a new HTTP Header PrivacyPass-Reverse.  PrivacyPass-Reverse
   is a base64url ([RFC4648]) encoded GenericBatchTokenRequest as
   defined in Section 6 of [BATCHED-TOKENS].



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      The use of generic batch tokens as defined in Section 6 of
      [BATCHED-TOKENS] is because this already provides encoding for
      request and response, error wrapping, and a concise format.  One
      could use binary http or a new format

5.1.  Client behaviour

   Along with sending PrivateToken from the Initial Issuer to the
   Origin, the Client sends a TokenRequest as defined in [RFC9578] or
   [BATCHED-TOKENS], and wraps them as a generic batch token request.
   The Client SHOULD consider Privacy Pass Reverse Flow like the initial
   flow.  The Client is responsible to coordinate between the different
   entities.  Specifically, if the Reverse Origin is the Initial
   Attester/Issuer, the Client SHOULD account for possible privacy
   leakage.

5.2.  Origin/Issuer/Attester deployment

   In this model, the Origin, Attester, and Issuer are all operated by
   the same entity, as shown in Figure 1.  The Reverse Flow is the same
   as the Initial Flow, except for the request/response encapsulation.
   The Origin is the Reverse Origin.

                    +---------------------------------------------.
   +--------+       |  +----------+     +--------+     +--------+  |
   | Client |       |  | Attester |     | Issuer |     | Origin |  |
   +---+----+       |  +-----+----+     +----+---+     +---+----+  |
       |             `-------|---------------|-------------|------'
       |<----------------------- TokenChallenge (Issuer) --+
       |                     |               |             |
       |<=== Attestation ===>|               |             |
       |                     |               |             |
       +----------- TokenRequest ----------->|             |
       |<---------- TokenResponse -----------+             |
       |                                                   |
       +------- Token+TokenRequest(Origin Issuer) -------->+
       |<--------- TokenResponse(Origin Issuer) -----------|
       |                                                   |

                     Figure 1: Shared Deployment Model

   Similar to the original Shared Deployment Model, the Attester,
   Issuer, and Origin share the attestation, issuance, and redemption
   contexts.  Even if this context changes between the Initial and
   Reverse Flow, attestation mechanism that can uniquely identify a
   Client are not appropriate as they could lead to unlinkability
   violations.




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      These models allow for fully private verifiability.  Even though
      no optimised scheme is available at the time of writting, the
      author recommends to follow advances of anonymous credential
      within the Privacy Pass group.

      Specifically

      1.  [PRIVACYPASS-ARC]

      2.  [PRIVACYPASS-BBS]

      3.  [ANONYMOUS-CREDIT-TOKENS]

      These scheme allow to mimic a reverse flow to some extent.

5.3.  Split Origin-Attester deployment

   In this model, the Attester and Issuer are operated by the same
   entity that is separate from the Origin.  The Origin trusts the joint
   Attester and Issuer to perform attestation and issue Tokens.  Origin
   Tokens can then be sent by Client on new requests, as long as the
   Reverse Origin trusts the Origin to perform attestation and issue
   Tokens.

                                                                                     +--------------------------.
+---------------+    +--------+                                       +--------+     |  +----------+ +--------+  |
| Origin Issuer |    | Origin |                                       | Client |     |  | Attester | | Issuer |  |
+---+-----------+    +---+----+                                       +---+----+     |  +-----+----+ +----+---+  |
    |                    |                                                |           `-------|-----------|-----'
    |                    +-- TokenChallenge (Issuer) -------------------->|                   |           |
    |                    |                                                |<== Attestation ==>|           |
    |                    |                                                |                   |           |
    |                    |                                                +--------- TokenRequest ------->|
    |                    |                                                |<-------- TokenResponse -------+
    |                    |<-- Request+Token+TokenRequest(Origin Issuer) --+                   |           |
    |<-- TokenRequest ---+                                                |                   |           |
    +-- TokenResponse -->|                                                |                   |           |
    |                    |--- Response+TokenResponse(Origin Issuer) ----->|                   |           |
    |                    |                                                |                   |           |

         Figure 2: Joint Attester and Issuer Deployment Model

   The Origin Issuer MUST NOT issue privately verifiable tokens, as this
   would lead to secret material being shared between the Origin and the
   Reverse Origin.

   A particular deployment model is when the Reverse Origin is the
   Attester/Issuer.  This model is described in Figure 3



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                                                                                     +--------------------------.
+---------------+    +--------+                                       +--------+     |  +----------+ +--------+  |
| Origin Issuer |    | Origin |                                       | Client |     |  | Attester | | Issuer |  |
+---+-----------+    +---+----+                                       +---+----+     |  +-----+----+ +----+---+  |
    |                    |                                                |           `-------|-----------|-----'
    |                    +-- TokenChallenge (Issuer) -------------------->|                   |           |
    |                    |                                                |<== Attestation ==>|           |
    |                    |                                                |                   |           |
    |                    |                                                +--------- TokenRequest ------->|
    |                    |                                                |<-------- TokenResponse -------+
    |                    |<-- Request+Token+TokenRequest(Origin Issuer) --+                   |           |
    |<-- TokenRequest ---+                                                |                   |           |
    +-- TokenResponse -->|                                                |                   |           |
    |                    |--- Response+TokenResponse(Origin Issuer) ----->|                   |           |
    |                    |                                                +------------ Token ----------->|
    |                    |                                                |                   |           |

  Figure 3: Joint Attester and Issuer Deployment Model with reverse

   This deployment SHOULD not allow the Reverse Origin to infer the
   request made to the Origin, as it would break unlinkability.

6.  Privacy Considerations

   Privacy Pass [RFC9576] states

      In general, limiting the amount of metadata permitted helps limit
      the extent to which metadata can uniquely identify individual
      Clients.  Failure to bound the number of possible metadata values
      can therefore lead to a reduction in Client privacy.  Most token
      types do not admit any metadata, so this bound is implicitly
      enforced.

   In Privacy Pass with a reverse flow, Clients are provided with new
   PrivateTokens depending on their request.  They can spend these
   tokens to continue making further requests.

   While the token are still unlinkable, the token_key_id associated to
   them represent metadata.  It leaks some information about the Client.
   The following subsections discuss the issues that influence the
   anonymity set, and possible mitigations/safeguards to protect against
   this underlying problem.

6.1.  Issuer face values

   When setting up a reverse flow deployment, an Origin MAY operate
   multiple Issuers, and assign them some metadata to them.  The amount
   of possible metadata grows as 2^(origin_issuers).



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   We RECOMMEND that:

   1.  Origin defines their anonimity sets, and deploy no more than
       log2(#anonimity_sets).  This bounds the possible anonimity sets
       by design.

   2.  Client to only send 1 PrivateToken per request.  This is inline
       with RFC9577 and RFC (Web Authentication) which only allows one
       challenge response to be provided as part of Authorization HTTP
       header.

   3.  Issuers metadata to be publicly disclosed via an origin endpoint,
       and externally monitored

6.2.  Token for specific Clients

   In Privacy Pass with a reverse flow, an Origin MAY operate multiple
   Issuers, with arbitrary metadata associated to them.  A malicious
   Origin MAY uses this opportunity to associate certain token values to
   a specific set of Clients.

   Let's consider the following deployment: the Origin operates two
   issuers A and B.  The Client sends Token_A, and (TokenRequest_A,
   TokenRequest_B).  Issuer B is associated to croissant aficionados.

   If a Client requests croissant, or sends Token_B, the origin provides
   TokenResponse_B.  If not, it provides TokenResponse_A.

   Over time, this means the Origin is able to track croissants
   aficionados.

   To mitigate this, we RECOMMEND:

   1.  The initial PrivateToken to be provided by an Issuer not in
       control of the Origin.  The joint Origin/Attester/Issuer model
       SHOULD NOT be used.

   2.  Clients to reset their state regularly with the initial Issuer.

6.3.  Sending more than one token

   While that's not part of Privacy Pass with a reverse flow, some
   deployment might consider allowing Clients to send multiple
   PrivateToken, similar to how normal Privacy Pass deployment allow two
   distinct PrivateToken to be sent.






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   In Privacy Pass with a reverse flow deployment, there are as many
   bits as Issuers; each token is one bit.  We RECOMMEND to have a
   maximum of 6 Origin operated Issuers, bounding Client information to
   2^6 = 64.  Accounting for the initial Issuer, this means a total of
   log2(64)+1=7 issuers.

   Origin should have sufficient traffic to not single-out particular
   Client based on timings of requests.

6.4.  Swap endpoint and its privacy implication

   With multiple Issuers, a Client MAY end up with a bunch of tokens,
   for various Issuers.  Origins MAY propose a swap endpoint at which a
   Client can exchange one or more Origin tokens against one or more new
   Origin tokens.

   The Origin SHOULD ensure this endpoint receives enough traffic to not
   reduce the anonymity sets.

7.  IANA Considerations

   This document has no IANA actions.

8.  References

8.1.  Normative References

   [BATCHED-TOKENS]
              Robert, R., Wood, C. A., and T. Meunier, "Batched Token
              Issuance Protocol", Work in Progress, Internet-Draft,
              draft-ietf-privacypass-batched-tokens-04, 2 February 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-
              privacypass-batched-tokens-04>.

   [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>.

   [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>.





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   [RFC9576]  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/rfc/rfc9576>.

   [RFC9578]  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/rfc/rfc9578>.

8.2.  Informative References

   [ANONYMOUS-CREDIT-TOKENS]
              "Anonymous Credit Tokens", n.d.,
              <https://samuelschlesinger.github.io/ietf-anonymous-
              credit-tokens/draft-schlesinger-cfrg-act.html>.

   [PRIVACYPASS-ARC]
              Yun, C. and C. A. Wood, "Privacy Pass Issuance Protocol
              for Anonymous Rate-Limited Credentials", Work in Progress,
              Internet-Draft, draft-yun-privacypass-arc-00, 5 February
              2025, <https://datatracker.ietf.org/doc/html/draft-yun-
              privacypass-arc-00>.

   [PRIVACYPASS-BBS]
              Ladd, W., "BBS for PrivacyPass", Work in Progress,
              Internet-Draft, draft-ladd-privacypass-bbs-01, 26 February
              2024, <https://datatracker.ietf.org/doc/html/draft-ladd-
              privacypass-bbs-01>.

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

Acknowledgments

   The author would like to thank Tommy Pauly, Chris Wood, Raphael
   Robert, and Armando Faz Hernandez for helpful discussion on Privacy
   Pass architecture and its considerations.

Changelog

   v01

   *  Editorial pass on the introduction

   *  Add a motivation section: refunding tokens, bootstraping issuer,
      attester feeback loop



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   *  Split protocol overview via HTTP headers in its own section

   *  Add consideration about anonymous credentials in joint origin/
      issuer deployment

   v00

   *  Initial draft

   *  Possibility of a new HTTP request for inlining request

   *  Privacy considerations about additional metadata

Author's Address

   Thibault Meunier
   Cloudflare Inc.
   Email: ot-ietf@thibault.uk

































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