Internet DRAFT - draft-hendrickson-privacypass-expiration-extension
draft-hendrickson-privacypass-expiration-extension
Privacy Pass S. Hendrickson
Internet-Draft Google
Intended status: Standards Track C. A. Wood
Expires: 8 February 2024 Cloudflare, Inc.
7 August 2023
Privacy Pass Token Expiration Extension
draft-hendrickson-privacypass-expiration-extension-01
Abstract
This document describes an extension for Privacy Pass that allows
tokens to encode expiration information.
About This Document
This note is to be removed before publishing as an RFC.
Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-hendrickson-privacypass-
expiration-extension/.
Discussion of this document takes place on the Privacy Pass Working
Group mailing list (mailto:privacy-pass@ietf.org), which is archived
at https://mailarchive.ietf.org/arch/browse/privacy-pass/. Subscribe
at https://www.ietf.org/mailman/listinfo/privacy-pass/.
Source for this draft and an issue tracker can be found at
https://github.com/chris-wood/draft-hendrickson-privacypass-
expiration-extension.
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
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Internet-Drafts are draft documents valid for a maximum of six months
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on 8 February 2024.
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Copyright Notice
Copyright (c) 2023 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 (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3
3. Expiration Extension . . . . . . . . . . . . . . . . . . . . 3
4. Privacy Considerations . . . . . . . . . . . . . . . . . . . 4
5. Security Considerations . . . . . . . . . . . . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
7.1. Normative References . . . . . . . . . . . . . . . . . . 5
7.2. Informative References . . . . . . . . . . . . . . . . . 6
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
Some Privacy Pass token types support binding additional information
to the tokens, often referred to as public metadata.
[AUTH-EXTENSIONS] describes an extension parameter to the basic
PrivateToken HTTP authentication scheme [AUTH-SCHEME] for supplying
this metadata alongside a token. [EXTENDED-ISSUANCE] describes
variants of the basic Privacy Pass issuance protocols
[BASIC-ISSUANCE] that support issuing tokens with public metadata.
However, there are no existing extensions defined to make use of
these protocol extensions.
This document describes an extension for Privacy Pass that allows
tokens to encode expiration information. The use case and deployment
considerations, especially with respect to the resulting privacy
impact, are also discussed.
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2. Conventions and Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"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.
3. Expiration Extension
The expiration extension is an extension used to convey the
expiration for an issued token. It is useful for Privacy Pass
deployments that make use of cached tokens, i.e., those that are not
bound to a specific TokenChallenge redemption context, without having
to frequently rotate issuing public keys.
For example, consider a Privacy Pass deployment wherein Clients use
cached tokens that are valid for one hour. Clients could pre-fetch
these tokens each hour and the Issuer and Origin could rotate the
verification key every hour to force expiration. Alternatively,
Clients could pre-fetch tokens for the entire day all at once,
including an expiration timestamp in each token to indicate the time
window for which the token is valid.
The value of this extension is an ExpirationTimestamp, defined as
follows.
struct {
uint64 timestamp_precision;
uint64 timestamp;
} ExpirationTimestmap;
The ExpirationTimestmap fields are defined as follows:
* "timestamp_precision" is an 8-octet integer, in network byte
order, representing the granularity of the timestamp, i.e., the
target to which the timestamp is rounded for loss of precision.
* "timestamp" is an 8-octet integer, in network byte order,
representing the expiration timestamp. The expiration timestamp
is the UNIX time in seconds at which a token expires.
As an example, an ExpirationTimestamp structure with the following
value would be interpreted as an expiration timestamp of 1688583600,
i.e., July 05, 2023 at 19:00:00 GMT+0000, which is the timestamp
rounded to the nearest hour (timestamp_precision = 3600).
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struct {
uint64 timestamp_precision = 3600;
uint64 timestamp = 1688583600;
} ExpirationTimestmap;
4. Privacy Considerations
This extension intentionally adds more information to a token that
might not otherwise be visibile to Attester, Issuer, or Origin. As
such, how this information is chosen can have an impact on Origin-
Client, Issuer-Client, Attester-Origin, or redemption context
unlinkability as defined in Section 3.2 of [ARCHITECTURE].
Mitigating risk of privacy violation requires that the extension be
constructed in a way that does not induce anonymity set partitioning,
as described in Section 6.1 of [ARCHITECTURE].
The best way to achieve this in practice is for Clients to use the
same limited sets of information in the extension. Consistency can
be achieved in a variety of ways. For example, Client
implementations might insist that all Clients use the same
deterministic function for computing the expiration timestamp, e.g.,
some function F(current time). This function would round the current
timestamp, resulting in a loss of precision but overall less unique
value. One way to implement this function would by rounding the
timestamp to the nearest hour, day, or week. Of course, this does
not account for clock skew, which occurs with some non-neglgiible
probability in practice [CLOCK-SKEW].
An alternative implementation strategy for consistency is to run some
sort of consistency check to ensure that the Client uses a value that
is consistent with other Clients. Several consistency mechanisms
exist; see [CONSISTENCY] for more information. Such an explicit
consistency check would depend less upon the Client's current clock
and thus be more robust at the cost of additional work.
Orthogonal to the mechanism used to ensure consistency, it is also
important that Clients choose expiration timestamps that are shared
by other Clients. Consider, for example, a scenario where two
Clients consistently choose expiration timestamps per the
recommendation above, but only one Client ever requests a token
within a given expiration window. Despite the consistency check in
place, the actual value of the timestamp is still unique to one of
the Clients.
The means by which implementations ensure that some minimum number of
Clients share the same expiration timestamp is a deployment-specific
challenge. For example, in the Split Origin, Attester, and Issuer
deployments as described in Section 4.4 of [ARCHITECTURE], the
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Attester is positioned to ensure that Clients do not choose
consistent yet unique values. General purpose approaches to ensure
that some minimum number of Clients share the same expiration
timestamp are outside the scope of this document; indeed, this
problem is not unique to Privacy Pass and is common to other privacy-
related protocols such as Oblivious HTTP [OHTTP].
5. Security Considerations
Use of the expiration extension risks revealing additional
information to parties that see the extension, including the
Attester, Issuer, and Origin. Section 4 discusses specific privacy
implications for use of this extension that aim to mitigate exposure
of information that can unintentionally partition the Client
anonymity set and lead to Origin-Client, Issuer-Client, Attester-
Origin, or redemption context unlinkability as defined in Section 3.2
of [ARCHITECTURE]. General information regarding the use of
extensions and their possible impact on Client privacy can be found
in Section 3.4.3 of [ARCHITECTURE] and Section 6.1 of [ARCHITECTURE].
6. IANA Considerations
This document registers the following entry into the "Privacy Pass
PrivateToken Extensions" registry.
* Expiration extension
- Type: 0x0001
- Name: Expiration
- Value: ExpirationTimestamp value as defined in Section 3
- Reference: This document
- Notes: None
7. References
7.1. Normative References
[ARCHITECTURE]
Davidson, A., Iyengar, J., and C. A. Wood, "The Privacy
Pass Architecture", Work in Progress, Internet-Draft,
draft-ietf-privacypass-architecture-13, 15 June 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-
privacypass-architecture-13>.
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[AUTH-EXTENSIONS]
Hendrickson, S. and C. A. Wood, "The PrivateToken HTTP
Authentication Scheme Extensions Parameter", Work in
Progress, Internet-Draft, draft-wood-privacypass-auth-
scheme-extensions-00, 10 July 2023,
<https://datatracker.ietf.org/doc/html/draft-wood-
privacypass-auth-scheme-extensions-00>.
[AUTH-SCHEME]
Pauly, T., Valdez, S., and C. A. Wood, "The Privacy Pass
HTTP Authentication Scheme", Work in Progress, Internet-
Draft, draft-ietf-privacypass-auth-scheme-11, 23 June
2023, <https://datatracker.ietf.org/doc/html/draft-ietf-
privacypass-auth-scheme-11>.
[BASIC-ISSUANCE]
Celi, S., Davidson, A., Valdez, S., and C. A. Wood,
"Privacy Pass Issuance Protocol", Work in Progress,
Internet-Draft, draft-ietf-privacypass-protocol-11, 26
June 2023, <https://datatracker.ietf.org/doc/html/draft-
ietf-privacypass-protocol-11>.
[CONSISTENCY]
Davidson, A., Finkel, M., Thomson, M., and C. A. Wood,
"Key Consistency and Discovery", Work in Progress,
Internet-Draft, draft-ietf-privacypass-key-consistency-01,
10 July 2023, <https://datatracker.ietf.org/doc/html/
draft-ietf-privacypass-key-consistency-01>.
[EXTENDED-ISSUANCE]
"*** BROKEN REFERENCE ***".
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
7.2. Informative References
[CLOCK-SKEW]
Acer, M., Stark, E., Felt, A., Fahl, S., Bhargava, R.,
Dev, B., Braithwaite, M., Sleevi, R., and P. Tabriz,
"Where the Wild Warnings Are: Root Causes of Chrome HTTPS
Certificate Errors", ACM, Proceedings of the 2017 ACM
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SIGSAC Conference on Computer and Communications Security,
DOI 10.1145/3133956.3134007, October 2017,
<https://doi.org/10.1145/3133956.3134007>.
[OHTTP] "*** BROKEN REFERENCE ***".
Acknowledgments
This document received input and feedback from Jim Laskey.
Authors' Addresses
Scott Hendrickson
Google
Email: scott@shendrickson.com
Christopher A. Wood
Cloudflare, Inc.
Email: caw@heapingbits.net
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