The PrivateToken HTTP Authentication Scheme Extensions Parameter
draft-ietf-privacypass-auth-scheme-extensions-00

Privacy Pass                                              S. Hendrickson
Internet-Draft                                                    Google
Intended status: Standards Track                              C. A. Wood
Expires: 2 October 2024                                 Cloudflare, Inc.
                                                           31 March 2024

    The PrivateToken HTTP Authentication Scheme Extensions Parameter
            draft-ietf-privacypass-auth-scheme-extensions-00

Abstract

   This document specifies a new parameter for the "PrivateToken" HTTP
   authentication scheme.  This purpose of this parameter is to carry
   extensions for Privacy Pass protocols that support public metadata.

About This Document

   This note is to be removed before publishing as an RFC.

   The latest revision of this draft can be found at https://ietf-wg-
   privacypass.github.io/draft-ietf-privacypass-extensible-token/draft-
   ietf-privacypass-extensible-token.html.  Status information for this
   document may be found at https://datatracker.ietf.org/doc/draft-ietf-
   privacypass-auth-scheme-extensions/.

   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/ietf-wg-privacypass/draft-ietf-privacypass-
   extensible-token.

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/.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   3
   3.  PrivateToken Extensions Parameter . . . . . . . . . . . . . .   3
   4.  Extensions Negotiation  . . . . . . . . . . . . . . . . . . .   4
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   The primary Token structure in the "PrivateToken" HTTP authentication
   scheme [AUTHSCHEME] is composed as follows:

   struct {
       uint16_t token_type;
       uint8_t nonce[32];
       uint8_t challenge_digest[32];
       uint8_t token_key_id[Nid];
       uint8_t authenticator[Nk];
   } Token;

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   Functionally, this structure conveys a single bit of information from
   the issuance protocol: whether or not the token is valid (as
   indicated by a valid authenticator value).  This structure does not
   admit any additional information to flow from the issuance protocol,
   including, for example, public metadata that is incorporated into the
   issuance protocol.

   This document specifies a new parameter for the "PrivateToken" HTTP
   authentication scheme for carrying extensions.  This extensions
   parameter, otherwise referred to as public metadata, is
   cryptographically bound to the Token structure via the Privacy Pass
   issuance protocol.

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.  PrivateToken Extensions Parameter

   As defined in Section 2.2 of [AUTHSCHEME], the "PrivateToken"
   authentication scheme defines one parameter, "token", which contains
   the base64url-encoded Token struct.  This document defines a new
   parameter, "extensions," which contains the base64url-encoded
   representation of the following Extensions structure.  This document
   follows the default padding behavior described in Section 3.2 of
   [RFC4648], so the base64url value MUST include padding.

   struct {
       ExtensionType extension_type;
       opaque extension_data<0..2^16-1>;
   } Extension;

   enum {
       reserved(0),
       (65535)
   } ExtensionType;

   struct {
       Extension extensions<0..2^16-1>;
   } Extensions;

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   The contents of Extensions are a list of Extension values, each of
   which is a type-length-value structure whose semantics are determined
   by the type.  The type and length of each extension are 2-octet
   integers, in network byte order.  The length of the extensions list
   is also a 2-octet integer, in network byte order.

   Clients, Issuers, and Origins all agree on the content and encoding
   of this Extensions structure, i.e., they agree on the same type-
   length-value list.  The list MUST be ordered by ExtensionType value,
   from 0 to 65535.  Extension types MAY be repeated.  The value of the
   Extensions structure is used as-is when verifying the value of the
   corresponding "token" parameter in the "PrivateToken" authentication
   header.  As an example, Clients presenting this extension parameter
   to origins would use an Authorization header field like the
   following:

   Authorization: PrivateToken token="abc...", extensions="def..."

   Future documents may specify extensions to be included in this
   structure.  Registration details for these extensions are in
   Section 6.

   Each Privacy Pass issuance protocol, identified by a token type,
   specifies the structure of the PrivateToken value to be used.
   Extensions are bound to the resulting tokens via the issuance
   protocol.  In particular, the value of an Extensions structure is
   provided as metadata for the issuance protocol.  Candidate issuance
   protocols are specified in [PUBLIC-ISSUANCE].

4.  Extensions Negotiation

   The mechanism Clients and Origins use to determine which set of
   extensions to provide for redemption is out of scope for this
   document.

   In some Privacy Pass deployments, the set of extensions may be well
   known to Clients and Origins and thus do not require negotiation.  In
   other settings, negotiation may be required.  However, negotiation
   can raise privacy risks, especially if negotiation can be abused by
   Origins for partitioning Clients and risking Origin-Client
   unlinkability.  Some of these risks may be mitigated if all Clients
   in a given redemption context respond to negotiation in the same
   manner.  However, if Clients have different observable behavior,
   e.g., if certain extension use is determined by user choice, Origins
   can observe this differential behavior and therefore partition
   Clients in a redemption context.

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5.  Security Considerations

   Privacy considerations for tokens that include additional information
   are discussed in Section 6.1 of [ARCHITECTURE].  Additional
   considerations for use of extensions, including those that arise when
   deciding which extensions to use, are described in Section 4.

6.  IANA Considerations

   IANA is requested to create a new "Privacy Pass PrivateToken
   Extensions" registry in the "Privacy Pass Parameters" page to list
   possible extension values and their meaning.  Each extension has a
   two-byte type, so the maximum possible value is 0xFFFF = 65535.

   Template:

   *  Type: The two-byte extension type

   *  Name: Name of the extension

   *  Value: Syntax and semantics of the extension

   *  Reference: Where this extension and its value are defined

   *  Notes: Any notes associated with the entry

   New entries in this registry are subject to the Specification
   Required registration policy ([RFC8126], Section 4.6).  Designated
   experts need to ensure that the extension is sufficiently clearly
   defined and, importantly, has a clear description of the privacy
   implications of using the extension, framed in the context of
   partitioning the client anonymity set as described in Section 6.1 of
   [ARCHITECTURE].

7.  References

7.1.  Normative References

   [AUTHSCHEME]
              Pauly, T., Valdez, S., and C. A. Wood, "The Privacy Pass
              HTTP Authentication Scheme", Work in Progress, Internet-
              Draft, draft-ietf-privacypass-auth-scheme-15, 23 October
              2023, <https://datatracker.ietf.org/doc/html/draft-ietf-
              privacypass-auth-scheme-15>.

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   [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>.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <https://www.rfc-editor.org/rfc/rfc4648>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/rfc/rfc8126>.

   [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

   [ARCHITECTURE]
              Davidson, A., Iyengar, J., and C. A. Wood, "The Privacy
              Pass Architecture", Work in Progress, Internet-Draft,
              draft-ietf-privacypass-architecture-16, 25 September 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-
              privacypass-architecture-16>.

   [PUBLIC-ISSUANCE]
              Hendrickson, S. and C. A. Wood, "Public Metadata
              Issuance", Work in Progress, Internet-Draft, draft-
              hendrickson-privacypass-public-metadata-03, 25 November
              2023, <https://datatracker.ietf.org/doc/html/draft-
              hendrickson-privacypass-public-metadata-03>.

Authors' Addresses

   Scott Hendrickson
   Google
   Email: scott@shendrickson.com

   Christopher A. Wood
   Cloudflare, Inc.
   Email: caw@heapingbits.net

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