Privacy Pass T. Meunier Internet-Draft Cloudflare Inc. Intended status: Informational 18 June 2026 Expires: 20 December 2026 Privacy Pass Reverse Flow HTTP Transport draft-meunier-privacypass-reverse-flow-http-00 Abstract This document specifies an instantiation of Privacy Pass Reverse Flow [REVERSE-FLOW] where HTTP is used as a transport mechanism. It describes a novel HTTP header field that Clients and Origins can use to carry reverse flow data. 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-http.html. Status information for this document may be found at https://datatracker.ietf.org/doc/draft-meunier-privacypass-reverse- flow-http/. 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/. Meunier Expires 20 December 2026 [Page 1] Internet-Draft Privacy Pass Reverse Flow HTTP Transport June 2026 Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on 20 December 2026. Copyright Notice Copyright (c) 2026 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 . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. PrivacyPass-Reverse Header Field . . . . . . . . . . . . . . 3 3.1. Example . . . . . . . . . . . . . . . . . . . . . . . . . 3 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4 5. References . . . . . . . . . . . . . . . . . . . . . . . . . 4 5.1. Normative References . . . . . . . . . . . . . . . . . . 4 5.2. Informative References . . . . . . . . . . . . . . . . . 4 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 4 Changelog . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 5 1. Introduction This document specifies an instantiation of Privacy Pass Reverse Flow [REVERSE-FLOW] where HTTP is used as a transport mechanism. [REVERSE-FLOW] specifies an architecture in which a client can both present a token and initiate a new credential issuance flow. As described in Section 4 of [REVERSE-FLOW], this requires in-band encoding of information used by the issuance protocol. Meunier Expires 20 December 2026 [Page 2] Internet-Draft Privacy Pass Reverse Flow HTTP Transport June 2026 This document introduces a new HTTP header field as defined in [RFC9110]. This allows Clients to convey a CredentialRequest, and Origins to transmit a CredentialResponse. 2. 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. 3. PrivacyPass-Reverse Header Field A Client or an Origin following [REVERSE-FLOW] MAY include a PrivacyPass-Reverse header field to communicate Privacy Pass protocol data. This header field contains a base64url (per [BASE64]) encoded CredentialRequest or CredentialResponse. +---------------+ +--------+ +--------+ | Origin Issuer | | Origin | | Client | +--+------------+ +---+----+ +---+----+ | | | | |<-- PrivacyPass-Reverse: CredentialRequest ---+ |<-- CredentialRequest ---+ | +--- CredentialResponse ->| | | +-- PrivacyPass-Reverse: CredentialResponse -->| | | | Figure 1: Privacy Pass with a Reverse Flow through PrivacyPass- Reverse header field 3.1. Example Below is an example request that uses [RFC9577] to pass the request Token, as well as PrivacyPass-Reverse for its reverse flow. GET /foo HTTP/1.1 Host: example.com Authorization: PrivateToken token="abc..." PrivacyPass-Reverse: "def..." HTTP/1.1 200 OK PrivacyPass-Reverse: "001..." [BODY] Meunier Expires 20 December 2026 [Page 3] Internet-Draft Privacy Pass Reverse Flow HTTP Transport June 2026 4. IANA Considerations This document has no IANA actions. 5. References 5.1. Normative References [BASE64] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006, . [REVERSE-FLOW] Meunier, T., "Privacy Pass Reverse Flow", Work in Progress, Internet-Draft, draft-meunier-privacypass- reverse-flow-04, 13 May 2026, . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . 5.2. Informative References [RFC9110] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., "HTTP Semantics", STD 97, RFC 9110, DOI 10.17487/RFC9110, June 2022, . [RFC9577] Pauly, T., Valdez, S., and C. A. Wood, "The Privacy Pass HTTP Authentication Scheme", RFC 9577, DOI 10.17487/RFC9577, June 2024, . Acknowledgments TODO Changelog v00 Meunier Expires 20 December 2026 [Page 4] Internet-Draft Privacy Pass Reverse Flow HTTP Transport June 2026 * Initial draft Author's Address Thibault Meunier Cloudflare Inc. Email: ot-ietf@thibault.uk Meunier Expires 20 December 2026 [Page 5]