The PrivateToken HTTP Authentication Scheme Extensions Parameter

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Table of Contents

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;

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.¶

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.¶

Future documents may specify extensions to be included in this structure. Registration details for these extensions are in Section 5.¶

Each Privacy Pass issuance protocol, identified by a token type, specifices the structure of the PrivateToken value to be used. Issuance protocols that support public metadata would define a way to convey this metadata as a set of extensions in an Extensions structure.¶

4. Security Considerations

Privacy considerations for tokens that include additional information are discussed in Section 6.1 of [ARCHITECTURE].¶

5. 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].¶

Values 0xF000-0xFFFF are reserved for private use, to enable proprietary uses and limited experimentation.¶

Authors' Addresses