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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   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
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   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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