Privacy Pass Architectural Framework

Document Type Active Internet-Draft (privacypass WG)
Author Alex Davidson 
Last updated 2021-01-11 (latest revision 2021-01-05)
Replaces draft-davidson-pp-architecture
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Network Working Group                                        A. Davidson
Internet-Draft                                                       LIP
Intended status: Informational                            5 January 2021
Expires: 9 July 2021

                  Privacy Pass Architectural Framework


   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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   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.

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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Simplified BSD License.

Table of Contents

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

1.  Introduction

   The Privacy Pass protocol provides an anonymity-preserving mechanism
   for authorization of clients with servers.  The protocol is detailed
   in [draft-davidson-pp-protocol] and is intended for use in the

   The way that the ecosystem around the protocol is set up can have
   significant impacts on the stated privacy and security guarantees of
   the protocol.  For instance, the number of servers issuing Privacy
   Pass tokens, along with the number of registered clients, determines
   the anonymity set of each individual client.  Moreover, this can be
   influenced by other factors, such as: the key rotation policy used by
   each server; and, the number of supported ciphersuites.  There are
   also client behavior patterns that can reduce the effective security
   of the server.

   In this document, we will provide a structural framework for building
   the ecosystem around the Privacy Pass protocol.  The core of the
   document also includes policies for the following considerations.

   *  How server key material should be managed and accessed.

   *  Compatible server issuance and redemption running modes and
      associated expectations.

   *  How clients should evaluate server trust relationships.

   *  Security and privacy properties of the protocol.

   *  A concrete assessment and parametrization of the privacy budget
      associated with different settings of the above policies.
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