Privacy Pass Protocol Specification

Document Type Active Internet-Draft (privacypass WG)
Authors Sofia Celi  , Alex Davidson  , Armando Faz-Hern├índez 
Last updated 2021-01-11 (latest revision 2021-01-05)
Replaces draft-davidson-pp-protocol
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Network Working Group                                            S. Celi
Internet-Draft                                                Cloudflare
Intended status: Informational                               A. Davidson
Expires: 9 July 2021                                                 LIP
                                                        A. Faz-Hernandez
                                                          5 January 2021

                  Privacy Pass Protocol Specification


   This document specifies the Privacy Pass protocol.  This protocol
   provides anonymity-preserving authorization of clients to servers.
   In particular, client re-authorization events cannot be linked to any
   previous initial authorization.  Privacy Pass is intended to be used
   as a performant protocol in the application-layer.

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
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   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 9 July 2021.

Copyright Notice

   Copyright (c) 2021 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 (
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
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   extracted from this document must include Simplified BSD License text
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   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction
   2.  Terminology
   3.  Background
     3.1.  Motivating use-cases
     3.2.  Anonymity and security guarantees
     3.3.  Basic assumptions
   4.  Protocol description
     4.1.  Server setup
     4.2.  Client setup
     4.3.  Issuance phase
     4.4.  Redemption phase
       4.4.1.  Client info
       4.4.2.  Double-spend protection
     4.5.  Handling errors
   5.  Functionality
     5.1.  Data structures
       5.1.1.  Ciphersuite
       5.1.2.  Keys
       5.1.3.  IssuanceInput
       5.1.4.  IssuanceResponse
       5.1.5.  RedemptionToken
       5.1.6.  RedemptionRequest
       5.1.7.  RedemptionResponse
     5.2.  API functions
       5.2.1.  Generate
       5.2.2.  Issue
       5.2.3.  Process
       5.2.4.  Redeem
       5.2.5.  Verify
     5.3.  Error types
   6.  Security considerations
     6.1.  Unlinkability
     6.2.  One-more unforgeability
     6.3.  Double-spend protection
     6.4.  Additional token metadata
     6.5.  Maximum number of tokens issued
   7.  VOPRF instantiation
     7.1.  Recommended ciphersuites
     7.2.  Protocol contexts
     7.3.  Functionality
       7.3.1.  Generate
       7.3.2.  Issue
       7.3.3.  Process
       7.3.4.  Redeem
       7.3.5.  Verify
     7.4.  Security justification
   8.  Protocol ciphersuites
     8.1.  PP(OPRF2)
     8.2.  PP(OPRF4)
     8.3.  PP(OPRF5)
   9.  Extensions framework policy
   10. References
     10.1.  Normative References
     10.2.  Informative References
   Appendix A.  Document contributors
   Authors' Addresses

1.  Introduction

   A common problem on the Internet is providing an effective mechanism
   for servers to derive trust from clients that they interact with.
   Typically, this can be done by providing some sort of authorization
   challenge to the client.  But this also negatively impacts the
   experience of clients that regularly have to solve such challenges.

   To mitigate accessibility issues, a client that correctly solves the
   challenge can be provided with a cookie.  This cookie can be
   presented the next time the client interacts with the server, instead
   of performing the challenge.  However, this does not solve the
   problem of reauthorization of clients across multiple domains.  Using
   current tools, providing some multi-domain authorization token would
   allow linking client browsing patterns across those domains, and
   severely reduces their online privacy.

   The Privacy Pass protocol provides a set of cross-domain
   authorization tokens that protect the client's anonymity in message
   exchanges with a server.  This allows clients to communicate an
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