Privacy Pass Token Expiration 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 expiration information. The use case and deployment considerations, especially with respect to the resulting privacy impact, are also discussed.¶

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. Expiration Extension

The expiration extension is an extension used to convey the expiration for an issued token. It is useful for Privacy Pass deployments that make use of cached tokens, i.e., those that are not bound to a specific TokenChallenge redemption context, without having to frequently rotate issuing public keys.¶

For example, consider a Privacy Pass deployment wherein clients use cached tokens that are valid for one hour. Clients could pre-fetch these tokens each hour and the issuer and origin could rotate the verification key every hour to force expiration. Alternatively, clients could pre-fetch tokens for the entire day all at once, including an expiration timestamp in each token to indicate the time window for which the token is valid.¶

Including a specific expiration timestamp could reveal specific information about a client, if, say, a client has a uniquely skewed clock. For that reason, clients can round the timestamp, resulting in a loss of precision but overall less unique value. Clients do this by choosing an expiration timestamp and the precision they would like, e.g., the UNIX time of expiration rounded to the nearest hour, day, or week.¶

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

struct {
   uint64 timestamp_precision;
   uint64 timestamp;
} ExpirationTimestmap;

The ExpirationTimestmap fields are defined as follows:¶

• "timestamp_precision" is an 8-octet integer, in network byte order, representing the granularity of the timestamp, i.e., the target to which the timestamp is rounded for loss of precision.¶
• "timestamp" is an 8-octet integer, in network byte order, representing the expiration timestamp. The expiration timestamp is the UNIX time in seconds at which a token expires.¶

As an example, an ExpirationTimestamp structure with the following value would be interpreted as an expiration timestamp of 1688583600, i.e., July 05, 2023 at 19:00:00 GMT+0000, which is the timestamp rounded to the nearest hour (timestamp_precision = 3600).¶

struct {
   uint64 timestamp_precision = 3600;
   uint64 timestamp = 1688583600;
} ExpirationTimestmap;

4. Security Considerations

Use of extensions risks revealing additional information to parties that see these extensions, including the Attester, Issuer, and Origin. Each extension in this document specifies security and privacy considerations relevant to its use. More general information regarding the use of extensions and their possible impact on client privacy can be found in [ARCHITECTURE].¶

5. IANA Considerations

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

• Expiration extension¶

  • Type: 0x0001¶
  • Name: Expiration¶
  • Value: ExpirationTimestamp value as defined in Section 3¶
  • Reference: This document¶
  • Notes: None¶

Acknowledgments

This document received input and feedback from Jim Laskey.¶

Authors' Addresses