The PrivateToken HTTP Authentication Scheme Extensions Parameter
draft-ietf-privacypass-auth-scheme-extensions-01
| Document | Type | Active Internet-Draft (privacypass WG) | |
|---|---|---|---|
| Authors | Scott Hendrickson , Christopher A. Wood | ||
| Last updated | 2024-11-06 | ||
| Replaces | draft-wood-privacypass-auth-scheme-extensions | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | (None) | ||
| Formats | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | WG Document | |
| Document shepherd | (None) | ||
| IESG | IESG state | I-D Exists | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-ietf-privacypass-auth-scheme-extensions-01
Privacy Pass S. Hendrickson
Internet-Draft Google
Intended status: Standards Track C. A. Wood
Expires: 11 May 2025 Cloudflare, Inc.
7 November 2024
The PrivateToken HTTP Authentication Scheme Extensions Parameter
draft-ietf-privacypass-auth-scheme-extensions-01
Abstract
This document specifies new parameters for the "PrivateToken" HTTP
authentication scheme. This purpose of these parameters is to
negotiate and 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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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 11 May 2025.
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Copyright (c) 2024 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
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3
3. PrivateToken Extensions Parameter . . . . . . . . . . . . . . 3
4. PrivateToken Challenge Extension Parameter . . . . . . . . . 5
5. Extensions Negotiation . . . . . . . . . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . 6
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
8.1. Normative References . . . . . . . . . . . . . . . . . . 7
8.2. Informative References . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
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.
This document additionally defines an optional extension negotiation
scheme which allows origins to indicate what extension types they
expect, and allows clients to respond with the extensions appropriate
for their context.
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.
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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 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 7.
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].
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4. PrivateToken Challenge Extension Parameter
As defined in Section 2.1 of [AUTHSCHEME], the "PrivateToken"
authentication scheme defines two parameters, "challenge" which
contains the base64url-encoded TokenChallenge struct, and a "token-
key" which contains the base64url-encoded public key used for this
challenge. This document defines two OPTIONAL new parameters,
"extension-set," which contains the base64url-encoded representation
of the following ExtensionSet structure, and "extensions" which
contain the base64url-encoded representation of the Extensions
strucuture defined in {#extensions}. This document follows the
default padding behavior described in Section 3.2 of [RFC4648], so
the base64url value MUST include padding.
struct {
enum { false(0), true(1) } Bool;
Bool is_required;
ExtensionType extension_type;
} ExtensionEntry;
enum {
reserved(0),
(65535)
} ExtensionType;
struct {
ExtensionEntry extension_types<0..2^16-1>;
} ExtensionSet;
The contents of ExtensionSet is a list of ExtensionEntry structs
containing extensions (defined in #extensions), each of which is a
type-length-value structure whose semantics are determined by the
type, and a bit marking whether the extension is required or
optional. T he type and length of each ExtensionType is a 2-octet
integer, in network byte order. The length of the extension_types
list is also a 2-octet integer, in network byte order.
ExtensionTypes are to be defined outside of this document.
The extensions parameter is to be used for pre-populated extension
structs the origin suggests to the client.
When presented with an ExtensionSet, a client should expect to be
rejected if not providing required extensions. A client MAY provide
optional extensions. A client MAY use the pre-populated extension
provided by the origin, or craft its own.
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WWW-Authenticate:
PrivateToken challenge="abc...", token-key="123...", extension-set="0x0001,0x0002..." extensions="def..."
5. Extensions Negotiation
In some Privacy Pass deployments, the set of extensions may be well
known to Clients and Origins and thus do not require negotiation. In
this case, no extension-set or extensions are provided by the origin
in the PrivateToken. In other settings, negotiation may be required.
However, negotiation can raise privacy risks, by partitioning Clients
by their chosen provided extensions 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.
6. 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 5.
7. 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
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implications of using the extension, framed in the context of
partitioning the client anonymity set as described in Section 6.1 of
[ARCHITECTURE].
8. References
8.1. 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>.
[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>.
8.2. 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>.
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Authors' Addresses
Scott Hendrickson
Google
Email: scott@shendrickson.com
Christopher A. Wood
Cloudflare, Inc.
Email: caw@heapingbits.net
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