Privacy Pass Reverse Flow
draft-meunier-privacypass-reverse-flow-02
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| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Author | Thibault Meunier | ||
| Last updated | 2025-10-20 | ||
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draft-meunier-privacypass-reverse-flow-02
Privacy Pass T. Meunier
Internet-Draft Cloudflare Inc.
Intended status: Informational 20 October 2025
Expires: 23 April 2026
Privacy Pass Reverse Flow
draft-meunier-privacypass-reverse-flow-02
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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This Internet-Draft will expire on 23 April 2026.
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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. TokenRequest, TokenResponse, and Finalisation . . . . . . . . 6
6. Reverse flow with an HTTP header . . . . . . . . . . . . . . 6
6.1. Client behaviour . . . . . . . . . . . . . . . . . . . . 6
6.2. Origin/Issuer/Attester deployment . . . . . . . . . . . . 7
6.3. Split Origin-Attester deployment . . . . . . . . . . . . 8
7. Privacy Considerations . . . . . . . . . . . . . . . . . . . 9
7.1. Issuer face values . . . . . . . . . . . . . . . . . . . 9
7.2. Token for specific Clients . . . . . . . . . . . . . . . 10
7.3. Sending more than one token . . . . . . . . . . . . . . . 10
7.4. Swap endpoint and its privacy implication . . . . . . . . 11
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
9.1. Normative References . . . . . . . . . . . . . . . . . . 11
9.2. Informative References . . . . . . . . . . . . . . . . . 12
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 12
Changelog . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 13
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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 -------+
| | | | |
| | Finalisation | |
| | | | |
| |<-- Request+Token+TokenRequest(Origin) ---+ | |
|<-- TokenRequest ---+ | | |
+--- TokenResponse ->| | | |
| |---- Response+TokenResponse(Origin) ----->| | |
| | | | |
| | Finalisation | |
| | | | |
Figure 1: Architec ture of Privacy Pass with a Reverse Flow
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.
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5. TokenRequest, TokenResponse, and Finalisation
In Figure 1, the Client sends an TokenRequest and receives an
TokenResponse. These are meant to abstract request from different
protocol to the Issuer.
As specified in Section 3.5 of [RFC9576],
The structure and semantics of the TokenRequest and TokenResponse
messages depend on the issuance protocol and token type being
used.
The introducion of Privacy Pass issuance protocol based on Anonymous
credential, such as [PRIVACYPASS-ARC] or [PRIVACYPASS-ACT], modifies
TokenRequest to use CredentialRequest instead.
Upon receiving an TokenResponse, the Client has to finalise the Token
so it can be presented to an origin. This may be a Finalisation for
type 0x0002 as defined in Section 7 of [RFC9578], a presentation for
Section 7 of [PRIVACYPASS-ARC], or even a refund for
[PRIVACYPASS-ACT].
6. Reverse flow with an HTTP header
This section defines a Reverse Flow, as presented in Section 4,
leveraging a new HTTP headers.
TokenRequest(Origin) and TokenResponse(Origin) happen through a new
HTTP Header PrivacyPass-Reverse. PrivacyPass-Reverse is a base64url
([RFC4648]) encoded GenericBatchTokenRequest as defined in Section 6
of [BATCHED-TOKENS].
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
6.1. Client behaviour
Along with sending PrivateToken from the Initial Issuer to the
Origin, the Client sends a TokenRequest as defined in [RFC9578],
[BATCHED-TOKENS], or [PRIVACYPASS-ARC]. In all these definitions,
TokenRequest MUST prepended by a uint16_t representing the token
type. 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.
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6.2. Origin/Issuer/Attester deployment
In this model, the Origin, Attester, and Issuer are all operated by
the same entity, as shown in Figure 2. 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) ------->|
|<--------- TokenResponse(Origin) ----------+
| |
Figure 2: 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.
This model allows for 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. [PRIVACYPASS-ACT]
These scheme allow for optimisation of Token finalisation by the
Client.
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6.3. 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) --+ | |
|<-- TokenRequest ---+ | | |
+-- TokenResponse -->| | | |
| +--- Response+TokenResponse(Origin) ----->| | |
| | | | |
Figure 3: 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 4
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+--------------------------.
+---------------+ +--------+ +--------+ | +----------+ +--------+ |
| Origin Issuer | | Origin | | Client | | | Attester | | Issuer | |
+---+-----------+ +---+----+ +---+----+ | +-----+----+ +----+---+ |
| | | `-------|-----------|-----'
| +-- TokenChallenge (Issuer) ------------->| | |
| | |<== Attestation ==>| |
| | | | |
| | +--------- TokenRequest ------->|
| | |<-------- TokenResponse -------+
| |<-- Request+Token+TokenRequest(Origin) --+ | |
|<-- TokenRequest ---+ | | |
+--- TokenResponse ->| | | |
| +--- Response+TokenResponse(Origin) ----->| | |
| | +------------ Token ----------->|
| | | | |
Figure 4: 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.
7. 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.
7.1. 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
7.2. 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.
7.3. 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.
7.4. 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.
8. IANA Considerations
This document has no IANA actions.
9. References
9.1. 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-05, 3 July 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-
privacypass-batched-tokens-05>.
[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>.
9.2. Informative References
[PRIVACYPASS-ACT]
"Anonymous Credit Tokens", n.d.,
<https://samuelschlesinger.github.io/draft-act/draft-
schlesinger-privacypass-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-01, 6 August
2025, <https://datatracker.ietf.org/doc/html/draft-yun-
privacypass-arc-01>.
[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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