Compression Dictionary Transport

Internet-Draft	compression-dictionary	June 2023
Meenan & Weiss	Expires 1 January 2024	[Page]

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1. Introduction

This specification defines a mechanism for using designated [HTTP] responses as an external dictionary for future HTTP responses for compression schemes that support using external dictionaries (e.g. [Brotli] and [Zstandard]).¶

This document describes the HTTP headers used for negotiating dictionary usage and registers media types for content encoding Brotli and Zstandard using a negotiated dictionary.¶

2. Dictionary Negotiation

2.1. Use-As-Dictionary

When responding to a HTTP Request, a server can advertise that the response can be used as a dictionary for future requests for URLs that match the pattern specified in the Use-As-Dictionary response header.¶

The Use-As-Dictionary response header is a Structured Field [RFC8941] sf-dictionary with values for "match", "ttl" and "hashes".¶

2.1.1. match

The "match" value of the Use-As-Dictionary header is a sf-string value that provides an URL-matching pattern for requests where the dictionary can be used.¶

The sf-string is parsed as a URL [RFC3986], and supports absolute URLs as well as relative URLs. When stored, any relative URLs MUST be expanded so that only absolute URL patterns are used for matching against requests.¶

The match URL supports using * as a wildcard within the match string for pattern-matching multiple URLs. URLs with a natural * in them are not directly supported unless they can rely on the behavior of * matching an arbitrary string.¶

The "match" value is required and MUST be included in the Use-As-Dictionary sf-dictionary for the dictionary to be considered valid.¶

2.1.2. ttl

The "ttl" value of the Use-As-Dictionary header is a sf-integer value that provides the time in seconds that the dictionary is valid for (time to live).¶

This is independent of the cache lifetime of the resource being used for the dictionary. If the underlying resource is evicted from cache then it is also removed but this allows for setting an explicit time to live for use as a dictionary independent of the underlying resource in cache. Expired resources can still be useful as dictionaries while they are in cache and can be used for fetching updates of the expired resource. It can also be useful to artificially limit the life of a dictionary in cases where the dictionary is updated frequently, to limit the number of possible incoming dictionary values.¶

The "ttl" value is optional and defaults to 31536000 (1 year).¶

2.1.3. hashes

The "hashes" value of the Use-As-Dictionary header is a inner-list value that provides a list of supported hash algorithms in order of server preference.¶

The dictionaries are identified by the hash of their contents and this value allows for negotiation of the algorithm to use.¶

The "hashes" value is optional and defaults to (sha-256).¶

2.1.4. Examples

2.1.4.1. Path Prefix

A response that contained a response header:¶

Use-As-Dictionary: match="/product/*", ttl=604800, hashes=(sha-256 sha-512)¶

Would specify matching any URL with a path prefix of /product/ on the same [Origin] as the original request, expiring as a dictionary in 7 days independent of the cache lifetime of the resource, and advertise support for both sha-256 and sha-512 hash algorithms.¶

2.1.4.2. Versioned Directories

A response that contained a response header:¶

Use-As-Dictionary: match="/app/*/main.js"¶

Would match main.js in any directory under /app/, expiring as a dictionary in one year and support using the sha-256 hash algorithm.¶

2.2. Sec-Available-Dictionary

When a HTTP client makes a request for a resource for which it has an appropriate dictionary, it can add a "Sec-Available-Dictionary" request header to the request to indicate to the server that it has a dictionary available to use for compression.¶

The "Sec-Available-Dictionary" request header is a Structured Field [RFC8941] sf-string value that contains the hash of the contents of a single available dictionary calculated using one of the algorithms advertised as being supported by the server.¶

The client MUST only send a single "Sec-Available-Dictionary" request header with a single hash value for the best available match that it has available.¶

2.2.1. Dictionary freshness requirement

To be considered as a match, the dictionary must not yet be expired as a dictionary. When iterating through dictionaries looking for a match, the expiration time of the dictionary is calculated by taking the last time the dictionary was written and adding the "ttl" seconds from the "Use-As-Dictionary" response. If the current time is beyond the expiration time of the dictionary, it MUST be ignored.¶

2.2.2. Dictionary URL matching

When a dictionary is stored as a result of a "Use-As-Dictionary" directive, it includes a "match" string with the URL pattern of request URLs that the dictionary can be used for.¶

When comparing request URLs to the available dictionary match patterns, the comparison should account for the * wildcard when matching against request URLs. This can be accomplished with the following algorithm which returns TRUE for a successful match and FALSE for no-match:¶

Let MATCH represent the absolute URL pattern from the "match" value for the given dictionary.¶
LET URL represent the request URL being checked.¶
If there are no * characters in MATCH: a. If the MATCH and URL strings are identical, return TRUE. b. Else, return FALSE.¶
If there is a single * character in MATCH and it is at the end of the string: a. If the MATCH string is identical to the start of the URL string, return TRUE. b. Else, return FALSE.¶
Split the MATCH string by the * character into an array of MATCHES (excluding the * deliminator from the individual entries).¶
Pop the first entry in MATCHES from the front of the array into PATTERN. a. If PATTERN is identical to the start of the URL string, remove the beginning of the URL string until the end of the match to PATTERN. b. Else, return FALSE.¶
If there is not a * character at the end of MATCH: a. Pop the last entry in MATCHES from the end of the array into PATTERN. b. If PATTERN is identical to the end of the URL string, remove the end of the URL string to the beginning of the match to PATTERN. c. Else, return FALSE.¶
Pop each entry off of the front of the MATCHES array into PATTERN. For each PATTERN, in order: a. Search for PATTERN in URL from the beginning of URL and stop at the first match. b. If no match is found, return FALSE. c. Remove the beginning of the URL string until the end of the match to the first occurrence of PATTERN.¶
Return TRUE.¶

2.2.3. Multiple matching dictionaries

When there are multiple dictionaries that match a given request URL, the client MUST pick the dictionary with the longest match pattern string length.¶

3. Negotiating the compression algorithm

When a compression dictionary is available for use for a given request, the algorithm to be used is negotiated through the regular mechanism for negotiating content encoding in HTTP.¶

This document introduces two new content encoding algorithms:¶

"br-d" - Brotli using an external compression dictionary. "zstd-d" - Zstandard using an external compression dictionary.¶

The dictionary to use is negotiated separately and advertised in the "Sec-Available-Dictionary" request header.¶

3.1. Accept-Encoding

The client adds the algorithms that it supports to the "Accept-Encoding" request header. e.g.:¶

Accept-Encoding: gzip, deflate, br, zstd, br-d, zstd-d¶

3.2. Content-Encoding

If a server supports one of the dictionary algorithms advertised by the client and chooses to compress the content of the response using the dictionary that the client has advertised then it sets the "Content-Encoding" response header to the appropriate value for the algorithm selected. e.g.:¶

Content-Encoding: br-d¶

4. IANA Considerations

4.1. Content Encoding

IANA will add the following entries to the "HTTP Content Coding Registry" within the "Hypertext Transfer Protocol (HTTP) Parameters" registry:¶

Name: sbr Description: A stream of bytes compressed using the Brotli protocol with an external dictionary¶

Name: szstd Description: A stream of bytes compressed using the Zstandard protocol with an external dictionary¶

4.2. Header Field Registration

5. Compatibility Considerations

To minimize the risk of middle-boxes incorrectly processing dictionary-compressed responses, compression dictionary transport MUST only be used in secure contexts (HTTPS).¶

6. Security Considerations

The security considerations for [Brotli] and [Zstandard] apply to the dictionary-based versions of the respective algorithms.¶

6.1. Changing content

The dictionary must be treated with the same security precautions as the content, because a change to the dictionary can result in a change to the decompressed content.¶

6.2. Reading content

The CRIME attack shows that it's a bad idea to compress data from mixed (e.g. public and private) sources -- the data sources include not only the compressed data but also the dictionaries. For example, if you compress secret cookies using a public-data-only dictionary, you still leak information about the cookies.¶

Not only can the dictionary reveal information about the compressed data, but vice versa, data compressed with the dictionary can reveal the contents of the dictionary when an adversary can control parts of data to compress and see the compressed size. On the other hand, if the adversary can control the dictionary, the adversary can learn information about the compressed data.¶

6.3. Security Mitigations

If any of the mitigations do not pass, the client MUST drop the response and return an error.¶

6.3.1. Cross-origin protection

To make sure that a dictionary can only impact content from the same origin where the dictionary was served, the "match" pattern used for matching a dictionary to requests MUST be for the same origin that the dictionary is served from.¶

6.3.2. Response readability

For clients, like web browsers, that provide additional protection against the readability of the payload of a response and against user tracking, additional protections MUST be taken to make sure that the use of dictionary-based compression does not reveal information that would not otherwise be available.¶

In these cases, dictionary compression MUST only be used when both the dictionary and the compressed response are fully readable by the client.¶

In browser terms, that means that both are either same-origin to the context they are being fetched from or that both include an "Access-Control-Allow-Origin" response header that matches the "Origin" request header they are fetched from.¶

7. Privacy Considerations

Since dictionaries are advertised in future requests using the hash of the content of the dictionary, it is possible to abuse the dictionary to turn it into a tracking cookie.¶

To mitigate any additional tracking concerns, clients MUST treat dictionaries in the same way that they treat cookies. This includes partitioning the storage as cookies are partitioned as well as clearing the dictionaries whenever cookies are cleared.¶

Authors' Addresses

Patrick Meenan

Google LLC

Email: pmeenan@google.com

Yoav Weiss