Privacy Pass Geolocation Hint Extension

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Table of Contents

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 geolocation hints. These hints can be used by origins that redeem tokens to influence its behavior in various ways, such as determining the content of HTTP responses.¶

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. Geolocation Hint Extension

The geolocation hint extension is an extension used to convey rough geolocation hints to origins that do not already have accurate or authoritative mappings for the IP addresses of clients. This can be particularly useful for cases where IP geolocation mappings have changed recently, or a client is using a VPN or proxy that may not be commonly recognized by servers.¶

The value of this extension is an GeoHintTimestamp, defined as follows.¶

struct {
   opaque geo_hint<1..2^16-1>;
} GeoHint;

The GeoHintTimestamp fields are defined as follows:¶

• "geo_hint" is a string formatted as defined in Section 2.1.1 of [GEOFEED]. It contains a comma-separated list of Alpha2code, Region, and City. The value SHOULD NOT contain a Postal Code.¶

As an example, a GeoHint structure corresponding to the "192.0.2.5,US,US-AL,Alabaster" entry would be:¶

struct {
   opaque geo_hint<1..2^16-1>; // "US,US-AL,Alabaster"
} GeoHint;

4. Security Considerations

Geolocation information can contribute to a client's fingerprint. Exposing precise geolocation information can therefore lead to anonymity set partitioning, as described in [ARCHITECTURE]. More general information regarding the use of extensions and their possible impact on client privacy can be found in [ARCHITECTURE].¶

Servers MUST NOT use IP Geolocation Client Hints for making security or access-control decisions, as the value can be spoofed by a client. The hint is intended only for use in optimizing behavior.¶

5. IANA Considerations

This document registers the following entry into the "Privacy Pass PrivateToken Extensions" registry.¶

• Expiration extension¶

  • Type: 0x0002¶
  • Name: Geolocation hint¶
  • Value: GeoHint value as defined in Section 3¶
  • Reference: This document¶
  • Notes: None¶

Acknowledgments

This document received input and feedback from Jim Laskey.¶

Authors' Addresses