Internet DRAFT - draft-wood-privacypass-auth-scheme-extensions
draft-wood-privacypass-auth-scheme-extensions
Privacy Pass S. Hendrickson
Internet-Draft Google
Intended status: Standards Track C. A. Wood
Expires: 26 May 2024 Cloudflare, Inc.
23 November 2023
The PrivateToken HTTP Authentication Scheme Extensions Parameter
draft-wood-privacypass-auth-scheme-extensions-01
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://chris-
wood.github.io/draft-wood-privacypass-extensible-token/draft-wood-
privacypass-extensible-token.html. Status information for this
document may be found at https://datatracker.ietf.org/doc/draft-wood-
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/chris-wood/draft-wood-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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Internet-Drafts are draft documents valid for a maximum of six months
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This Internet-Draft will expire on 26 May 2024.
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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 . . . . . . . . . . . . . . . . . . . 4
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.
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;
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 is a 2-octet integer, in network byte order.
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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 1 to 65535. 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,
specifices 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 by which Clients and Origins 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 Clients and Origins and therefore 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.
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.
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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 about 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>.
[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>.
[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>.
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[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-02, 30 June 2023,
<https://datatracker.ietf.org/doc/html/draft-hendrickson-
privacypass-public-metadata-02>.
Authors' Addresses
Scott Hendrickson
Google
Email: scott@shendrickson.com
Christopher A. Wood
Cloudflare, Inc.
Email: caw@heapingbits.net
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