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Versions: 00 01 02 03                                                   
Privacy Pass                                                M. McFadden
Internet Draft                            internet policy advisors, llc
Intended status: Informational                              May 4, 2021
Expires: November 4, 2021

              Privacy Pass: Centralization Problem Statement

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
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   This Internet-Draft will expire on November 4, 2021.

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   This document discusses the problems associated with strict upper
   bounds on the number of Privacy Pass servers in the proposed Privacy
   Pass ecosystem. It documents a proposed problem statement.

Table of Contents

   1. Introduction...................................................2
   2. Potential Privacy Concerns.....................................3
   3. Centralization in Privacy Pass - Problem Statement.............4
      3.1. Architectural Problems....................................4
      3.2. Engineering Problems......................................5
      3.3. Practical Problems........................................5
   4. Problem Statement and Potential for Mitigations................6
      4.1. Problem Statement.........................................6
      4.2. Potential Mitigations.....................................6
   5. Security Considerations........................................7
   6. IANA Considerations............................................7
   7. References.....................................................7
      7.1. Normative References......................................7
      7.2. Informative References....................................7
   8. Acknowledgments................................................8

1. Introduction

   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
   attestation of a previously authenticated server action, without
   having to reauthenticate manually.  The tokens retain anonymity in
   the sense that the act of revealing them cannot be linked back to
   the session where they were initially issued.

   The protocol itself in defined in [ID.davidson-pp-protocol-01] and
   the architectural framework is in [ID.davidson-pp-architecture-01].

   The architecture document leaves for a later time  the issue of
   server centralization.  This document is a discussion of the
   problems related to server centralization in Privacy Pass, the
   impact of centralization on the protocol's privacy goals, and some
   potential mitigations for the problem.

   An important feature of the Privacy Pass Architecture is the concept
   of the anonymity set of each individual client. The Privacy Pass

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   ecosystem has a set of servers which issue tokens to clients which
   can then be redeemed at the application layer for authentication.

   Trust is an important component in Privacy Pass. The servers have to
   publish their public keys and details of the ciphersuite they are
   using. It is necessary to publish these in a globally consistent,
   tamper-proof data structure. Clients that use the same registry of
   server information need to coordinate in some way to validate that
   they have the same view of the registry and its data.

   Four server running modes are discussed in [ID.davidson-pp-
   architecture-01]. Common to all four is a discussion of the need to
   set an upper limit on the number of servers that are allowed. The
   motivation for limiting the number of servers is that the is a
   correlation between larger numbers of servers and dilution of

2. Potential Privacy Concerns

   When a client redeems a token in Privacy Pass, there is very little
   information in the token itself other than the key that was used to
   sign the token. A key feature of the protocol is that any client can
   only remain private relative to the entire space of users using the

   In three of the four server running modes, a Privacy Pass verifier
   is able to trigger redemption for any of the available servers. The
   greater the number of servers, the greater the loss in anonymity.

   The architecture document, [ID.davidson-pp-architecture-01],
   provides an example where, if there are 32 servers, then the
   verifier learns 32 bits of information about the client. In certain
   circumstances, having that much information about the client can
   lead to the client being uniquely identified and the goals of
   Privacy Pass thwarted. As a result, the architecture document
   supplies the following mitigation:

   "In cases where clients can hold tokens for all servers at any given
   time, a strict bound SHOULD be applied to the active number of
   servers in the ecosystem. [ID.davidson-pp-architecture-01]."

   Putting restrictions on the number of redemption tokens at the
   client is considered. However, establishing control of the client,
   and the number of tokens it has, is far more difficult than
   restricting the number of active servers.

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3. Centralization in Privacy Pass - Problem Statement

   For Privacy Pass to succeed clients must be able to acquire tokens
   that they can later redeem with greater privacy and anonymity. This
   document does not discuss the goals of privacy or anonymity.
   Instead, it identifies a problem related to the upper bound in
   number of servers that affects the Privacy Pass ecosystem.

   For the purposes of this draft, "server centralization" is the
   strict limit or upper bound in the number of servers available from
   which a client can acquire a token for later redemption.

   The architecture draft specifies an upper limit of four for this
   upper bound.

   The problem statement for Privacy pass can be summarized: an upper
   bound to available Privacy Pass servers creates architectural,
   engineering and practical problems for the deployment of the
   protocol. Any successful deployment of Privacy Pass must find
   mitigations for these problems.

3.1. Architectural Problems

   Centralization is a problem space that has been exhaustively
   explored by others; not least of which in the IETF itself. The now
   expired IAB draft, [I-D.arkko-arch-infrastructure-centralisation-
   00], discussed six separate issues related to centralization and
   several of them appear to apply to Privacy Pass.

   Having a very limited number of servers available creates an
   architectural strain on avoiding single points of failure.  While
   the Privacy Pass architecture document does specify up to four
   servers, this is a very small number for, potentially, billions of
   possible users. And this assumes that the protocol is only used in
   "human-to-server" applications and not in situations where the
   client is not a human but some other device - either acting on
   behalf oa human or autonomously. Strict limitations on the number of
   servers poses the question of how the Privacy Pass architecture can
   scale in the presence of a large user base.

   The Privacy Pass architecture, by limiting the number of servers,
   also concentrates information and potentially limits the ability for
   other competing providers of the token generating services. By
   concentrating the information in a small number of servers, a
   problem appears when there are machine learning opportunities to
   collect and process data about clients requesting tokens.

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   A side effect of limiting the number of servers is that a
   significant amount of information ends up being in the control of a
   small number of entities. A client may trust a Privacy Pass server
   as send it information about itself in order to request tokens.
   However, the protocol itself can make no guarantee about the data
   handling practices of the server operator. Situations outside the
   control of the protocol may make it so there are pressures to misuse
   the data concentrated at the small number of servers.

3.2. Engineering Problems

   In the event that a very limited number of servers can be provided
   while still supporting the goals of the protocol, there is clearly a
   global scaling problem that needs to be solved. Each server must
   publish a global, consistent and protected view of its published key
   and the cryptosystem in use. Without access to that view, the system
   appears to have no failure mode.

   With a small number of servers, the ecosystem would likely be
   dominated by a few providers. With a dominant position in the market
   these Privacy Pass server operators would have a significant impact
   on default connectivity parameters in operating systems and
   browsers. As a result, a change to the way the access mechanism
   works for a variety of applications would have broad impacts to a
   wide variety of users. The relationship between engineering and how
   it affects a broad community of users has a recent example in DNS
   over HTTP.

3.3. Practical Problems

   Limits to the number of server operators also results in practical
   problems outside the protocol. In the event that a small number of
   server operators appear in the Privacy Pass ecosystem, and a large
   number of clients enter into trust relationships with those
   operators, what happens when those operators are acquired by other
   organizations that have different data handling and privacy policies
   than the original operator?

   With the requirement for a small number of operators, the
   architecture also doesn't consider the possibility that an
   organization or government could require Privacy Pass and the use of
   a particular set of servers. Such a requirement could potentially
   turn the goals of Privacy Pass against itself.

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4. Problem Statement and Potential for Mitigations

4.1. Problem Statement

   An upper bound to available Privacy Pass servers creates
   architectural, engineering and practical problems for the deployment
   of the protocol. Any successful deployment of Privacy Pass must find
   mitigations for these problems.

4.2. Potential Mitigations

   The motivation for having an upper bound to available Privacy Pass
   servers is to limit the amount of information that could be gather
   because a client could be forced to redeem tokens for any issuing
   key. A large number of keys, means a greater about of information

   One alternative to limiting the number of servers is to constrain
   the clients so that they only possess redemption tokens for a small
   number of servers. This potential mitigation doesn't address how the
   tokens might be cached, but it does discuss how the limitation might
   be implemented. However, there is much engineering experience to
   suggest that making a limitation work in a very large number of
   clients is a much greater engineering and deployment problem than
   placing the restriction in the server.

   If the motivation for restricting the number of servers is essential
   for Privacy Pass - and the mitigations at either the server or
   client are difficult to overcome - it is hard to understand where
   the mitigations for the problem statement will emerge.

4.3. Redemption Contexts as a Mitigation

   Contexts are groupings of resources that have shared anonymity and
   privacy properties. The current architecture statement has a single,
   global context for redemption. It is this feature that causes the
   problem outlined in section 4.1 above: with N issuers in the global
   ecosystem, there are 2^N possible anonymity sets. Adding additional
   metadata bits increases the number of anonymity sets.

   The global redemption context results in a requirement of less than
   ten total issuers in order to maintain anonymity sets of 5,000.

   One possible mitigation is to limit redemptions to a specific,
   shared context. Such an approach could limit the information
   available - and the potential for leakage - to a specific context.
   This type of solution would rely, in part, on strong

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   security/privacy boundaries between contexts. While information
   about redemptions in one context wouldn't affect information in
   another context, this solution depends upon there being no leakage
   of information between those contexts.

   While this potential mitigation is not reflected in the Privacy Pass
   architecture, it is unclear whether it should be a part of the
   protocol design or it should be left to the application layer to
   implement. If left to the application layer, there is potential for
   the anonymity sets to be very small and not meet the privacy goals
   of the protocol.

5. Security Considerations

   This document is all about security considerations for Privacy Pass.
   In particular it addresses the very specific problem associated with
   centralization of Privacy Pass servers.

6. IANA Considerations

   This memo contains no instructions or requests for IANA. The authors
   continue to appreciate the efforts of IANA staff in support of the

7. References

7.1. Normative References

   [1]   Bradner, S., "Key words for use in RFCs to Indicate
         Requirement Levels", BCP 14, RFC 2119, March 1997.

7.2. Informative References

   [2]   Celi, S., Davidson, A., and A. Faz-Hernandez, "Privacy Pass
         Protocol Specification", Work in Progress, Internet-Draft,
         draft-ietf-privacypass-protocol-00, 5 January 2021,

   [3]   [I-D.ietf-privacypass-http-api] Valdez, S., "Privacy Pass HTTP
         API", Work in Progress, Internet-Draft, draft-ietf-
         privacypass-http-api-00, 5 January 2021,

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8. Acknowledgments

   This document was prepared using 2-Word-v2.0.template.dot.

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Authors' Addresses

   Mark McFadden
   Internet policy advisors, ltd
   Chepstow, Wales, United Kingdom

   Email: mark@internetpolicyadvisors.com

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