Privacy Pass: Architectural Framework
draft-davidson-pp-architecture-01

Document Type Active Internet-Draft (privacypass WG)
Author Alex Davidson 
Last updated 2020-10-07 (latest revision 2020-07-13)
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Network Working Group                                        A. Davidson
Internet-Draft                                       Cloudflare Portugal
Intended status: Informational                              13 July 2020
Expires: 14 January 2021

                 Privacy Pass: Architectural Framework
                   draft-davidson-pp-architecture-01

Abstract

   This document specifies the architectural framework for constructing
   secure and anonymity-preserving instantiations of the Privacy Pass
   protocol.  It provides recommendations on how the protocol ecosystem
   should be constructed to ensure the privacy of clients, and the
   security of all participating entities.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on 14 January 2021.

Copyright Notice

   Copyright (c) 2020 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
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   Please review these documents carefully, as they describe your rights
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Davidson                 Expires 14 January 2021                [Page 1]
Internet-Draft               PP architecture                   July 2020

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Ecosystem participants  . . . . . . . . . . . . . . . . . . .   4
     3.1.  Servers . . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.2.  Clients . . . . . . . . . . . . . . . . . . . . . . . . .   6
       3.2.1.  Client identifying information  . . . . . . . . . . .   6
   4.  Key management framework  . . . . . . . . . . . . . . . . . .   6
     4.1.  Public key registries . . . . . . . . . . . . . . . . . .   7
     4.2.  Key rotation  . . . . . . . . . . . . . . . . . . . . . .   8
     4.3.  Ciphersuites  . . . . . . . . . . . . . . . . . . . . . .   8
   5.  Server running modes  . . . . . . . . . . . . . . . . . . . .   9
     5.1.  Single-Verifier . . . . . . . . . . . . . . . . . . . . .   9
     5.2.  Delegated-Verifier  . . . . . . . . . . . . . . . . . . .   9
     5.3.  Asynchronous-Verifier . . . . . . . . . . . . . . . . . .  10
     5.4.  Public-Verifier . . . . . . . . . . . . . . . . . . . . .  10
     5.5.  Bounded number of servers . . . . . . . . . . . . . . . .  10
   6.  Client-Server trust relationship  . . . . . . . . . . . . . .  11
   7.  Privacy considerations  . . . . . . . . . . . . . . . . . . .  12
     7.1.  Server key rotation . . . . . . . . . . . . . . . . . . .  12
     7.2.  Large numbers of servers  . . . . . . . . . . . . . . . .  13
       7.2.1.  Allowing larger number of servers . . . . . . . . . .  13
     7.3.  Partitioning of server key material . . . . . . . . . . .  14
     7.4.  Additional token metadata . . . . . . . . . . . . . . . .  14
     7.5.  Tracking and identity leakage . . . . . . . . . . . . . .  15
     7.6.  Client incentives for anonymity reduction . . . . . . . .  15
   8.  Security considerations . . . . . . . . . . . . . . . . . . .  15
     8.1.  Double-spend protection . . . . . . . . . . . . . . . . .  16
     8.2.  Token exhaustion  . . . . . . . . . . . . . . . . . . . .  16
     8.3.  Avoiding server centralization  . . . . . . . . . . . . .  16
   9.  Protocol parametrization  . . . . . . . . . . . . . . . . . .  16
     9.1.  Justification . . . . . . . . . . . . . . . . . . . . . .  17
     9.2.  Example parameterization  . . . . . . . . . . . . . . . .  18
     9.3.  Allowing more servers . . . . . . . . . . . . . . . . . .  19
   10. Extension integration policy  . . . . . . . . . . . . . . . .  19
   11. Existing applications . . . . . . . . . . . . . . . . . . . .  19
     11.1.  Cloudflare challenge pages . . . . . . . . . . . . . . .  19
     11.2.  Trust Token API  . . . . . . . . . . . . . . . . . . . .  20
     11.3.  Zero-knowledge Access Passes . . . . . . . . . . . . . .  20
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