Internet DRAFT - draft-mcfadden-pp-centralization-problem
draft-mcfadden-pp-centralization-problem
Privacy Pass M. McFadden
Internet Draft internet policy advisors, llc
Intended status: Informational March 7, 2022
Expires: September 7, 2022
Privacy Pass: Centralization Problem Statement
draft-mcfadden-pp-centralization-problem-03.txt
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Abstract
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. Key Role Definitions - Terminology.............................3
3. Potential Privacy Concerns.....................................4
4. Centralization in Privacy Pass - Problem Statement.............5
4.1. Architectural Problems....................................5
4.2. Engineering Problems......................................6
4.3. Practical Problems........................................6
5. Problem Statement and Potential for Mitigations................7
5.1. Problem Statement.........................................7
5.2. Potential Mitigations.....................................7
5.3. Redemption Contexts as a Mitigation.......................8
5.4. Implementation Base as a Mitigation.......................8
6. Security Considerations........................................9
7. IANA Considerations............................................9
8. References.....................................................9
8.1. Normative References......................................9
8.2. Informative References....................................9
9. Acknowledgments................................................9
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.ietf-privacypass-protocol-02]
and the architectural framework is in [ID.ietf-privacypass-
architecture-02].
The architecture document provides a concise representation of the
roles in the Privacy Pass ecosystem, which is repeated for reference
here:
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Origin Client Attester Issuer
/-----------------------------------------------------------------
---
| /-----------------------------------------\
| Challenge ----> Attest ---> |
| | TokenRequest ---------------> |
| Redemption | (validate) |
Issuance
| Flow | (evaluate) |
Flow
| | <------------------- TokenResponse |
| <--- Response | |
| \-----------------------------------------/
\-----------------------------------------------------------------
---
Figure 1: Privacy Pass Roles and Flows
An important feature of the Privacy Pass Architecture is the concept
of the anonymity set of each individual client. The Privacy Pass
ecosystem has a set of issuers which issue tokens to clients which
can then be redeemed in the Privacy Pass redemption process for
authentication.
Trust is an important component in Privacy Pass. The issuers 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.
Having a large number of Issuers results in the possibility that an
Origin could learn further information about a Client. The
architecture draft [ID.ietf-privacypass-architecture-02] suggests
that the mitigation for this should be an upper limit on the number
of Issuers that are allowed in the Privacy Pass ecosystem. The
motivation for limiting the number of Issuers is that there is a
correlation between larger numbers of servers and dilution of
privacy.
2. Key Role Definitions - Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
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"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.
A set of words that are in common use have very specific meaning in
the context of Privacy Pass. This document relies entirely on the
draft architecture [ID.ietf-privacypass-architecture-02] for the
definition of the following:
Client - An entity that seeks authorization to an Origin.
Origin - An entity that challenges Clients for tokens.
Issuer - An entity that issues tokens to Clients for properties
attested to by the Attester.
Attester - An entity that attests to properties of Client for the
purposes of token issuance.
3. 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
protocol.
The architecture document, [ID.ietf-privacypass-architecture-02],
provides an example where, if there are 32 Issuers, then Origins
learn 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
Issuers in the ecosystem. [ID.ietf-privacypass-architecture-02]."
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 Issuers.
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4. 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 Issuers 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 Issuers creates architectural,
engineering and practical problems for the deployment of the
protocol. Any successful deployment of Privacy Pass must find
mitigations for these problems.
4.1. Architectural Problems
Centralization is a problem space that has been exhaustively
explored by others; not least of which in the IETF itself. The
current draft on avoiding Internet centralization [I-D.nottingham-
avoiding-internet-centralization-02] provides a useful guide to
understand why centralization is undesirable. 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 of 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
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problem appears when there are machine learning opportunities to
collect and process data about clients requesting tokens.
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.
4.2. 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
how centralization affects a broad community of users and uses DNS
over HTTP as an example. Another draft [I-D.draft-lazanski-
consolidation-03] discusses the technical, economic and security
implications of consolidation.
4.3. Practical Problems
Limits to the number of Issuers also results in practical problems
outside the protocol. In the event that a small number of Issuers
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? The idea of Issuer churn is discussed in the architecture
document, but is limited to discussing ensuring that trusted
registries record which Issuers are active in the ecosystem.
With the requirement for a small number of Issuers, the architecture
also doesn't consider the possibility that an organization or
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government could require Privacy Pass and the use of a particular
set of Issuers. Such a requirement could potentially turn the goals
of Privacy Pass against itself.
5. Problem Statement and Potential for Mitigations
5.1. Problem Statement
An upper bound to available Privacy Pass Issuers creates
architectural, engineering and practical problems for the deployment
of the protocol. Any successful deployment of Privacy Pass must find
mitigations for these problems.
5.2. Potential Mitigations
The motivation for having an upper bound to available Privacy Pass
Issuers is to limit the amount of information that could be
gathered, because a client could be forced to redeem tokens for any
issuing key. A large number of keys, means a greater about of
information exposed.
One alternative to limiting the number of Issuers is to constrain
the Clients so that they only possess redemption tokens for a small
number of Issuers. 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 Issuer ecosystem.
In addition, limiting the number of Issuers increases the impact of
failure in the event that there is insufficient redundancy. It is a
situation that has impacted other protocols such as OAuth: when a
critical provider of services (such as an Issuer) is unavailable or
compromised, the impact of the failure affects services beyond the
provider of services. In the case of Privacy Pass, failure at the
Issuer would require the client to use another trusted Issuer.
However, if the Origin trusted a limited number of Issuers, the
Origin's service could be rendered unavailable.
If the motivation for restricting the number of Issuers is essential
for the success of Privacy Pass - and the mitigations at either the
Issuer or Client are difficult to overcome - it is hard to
understand where the mitigations for the problem statement will
emerge.
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5.3. 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
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 suggested as a possible
remediation 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.
What is not clear is how the consolidation considerations are
affected by the development of a "symmetric mode" for Privacy Pass.
The symmetric mode provides optional metadata but will not enable
the use of public verifiability. The goal of this change is to
remain consistent with work in the W3C. The mode will use the POPRF
algorithm which does not change the architectural characteristics
considered in this paper.
5.4. Implementation Base as a Mitigation
Protocol parameterization in Privacy Pass provides a guide to both
architecture and implementation. The calculation of the maximum
number of Issuers is based on [0.5 * (U/2^(2I)] where U is the user
base.
The architecture document notes that with an implementation base of
5 million users the maximum number of allowed Issuers is about 4.
Using the parameterization choices in the architecture draft,
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increasing the size of the installed base to 50 million users only
increases the maximum number of allowed Issuers to about 6.
6. 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.
7. 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
IETF.
8. References
8.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
8.2. Informative References
[2] Celi, S., Davidson, A., Valdez,S., Wood, C.and A. Faz-
Hernandez, "Privacy Pass Protocol Specification", Work in
Progress, Internet-Draft, draft-ietf-privacypass-protocol-02,
31 January 2022, <http://www.ietf.org/internet-drafts/draft-
ietf-privacypass-protocol-02.txt>.
[3] Davoidson, A., Iyengar, J, and Wood, C., "Privacy Pass
Architectural Framework", [I-D.ietf-privacypass-architecture-
02] Work in Progress, Internet-Draft, 31 January 2022,
<http://www.ietf.org/internet-drafts/draft-ietf-privacypass-
architecture-02.txt>.
9. 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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