The "id-" prefix for Digest Algorithms

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Table of Contents

1. Introduction

The [DIGEST] defines a way to convey a checksum of a representation-data as specified in [SEMANTICS].¶

As the representation data depends on the value of Content-Encoding, it is useful to convey the checksum value of a representation without any content coding applied.¶

This proposal introduces the id- prefix to specify that the provided digest-algorithm value is computed on the representation-data without any content coding applied.¶

2. The "id-" prefix for digest-algorithms

A new digest-algorithm to be registered within the HTTP Digest Algorithm Values Registry MUST NOT start with the string id-.¶

The following two examples show two digest-algorithm names that cannot be registered¶

   id-sha-256
   id-sha-512

For every digest-algorithm registered in the HTTP Digest Algorithm Values the associate id- digest-algorithm has the following properties:¶

• the checksum is computed on the representation-data of the resource when no content coding is applied;¶
• the checksum is computed according to the original digest-algorithm "Description" field, and uses the same encoding of the original digest-algorithm.¶

3. Security Considerations

4. IANA Considerations

Please, add the following text to the "Note" section of the HTTP Digest Algorithm Values.¶

" For each registered Digest Algorithm, an associated id- algorithm is defined.¶

The associated representation-data-digest is computed according to Section 2 of this document. "¶

Appendix A. Examples

{"hello": "world"}

POST /data HTTP/1.1
Content-Type: application/json
Content-Encoding: br
Digest: sha-256=4REjxQ4yrqUVicfSKYNO/cF9zNj5ANbzgDZt3/h3Qxo=,
        id-sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=

CwGAZiwiAeyJoZWxsbyI6ICJ3b3JsZCJ9Aw==

FAQ

RFC Editor: Please remove this section before publication.¶

Q: Why to convey the checksum independent from the content codings?

This is useful to identify and validate a resource downloaded from different sources using different content codings, or to compare a resource with its stored or signed counterpart.¶

Q: How does it improve the life of checksum providers?

If providers use reverse proxies to eg. compress responses, this could invalidate content coding aware checksums. Providing an id- algorithm, allows the digest checksum to be validated.¶

Code Samples

RFC Editor: Please remove this section before publication.¶

How can I generate an identity digest value?¶

The following python3 code can be used to generate digests for json objects using crc32c algorithm. Note that these are formatted as base64. This function could be adapted to other algorithms and should take into account their specific formatting rules.¶

import base64, json, brotli, hashlib

identity = lambda x: x

def digest(item, content_coding=identity, algorithm=hashlib.sha256) -> bytes:
    json_bytes = json.dumps(item).encode()
    content_encoded = content_coding(json_bytes)
    checksum = algorithm(content_encoded)
    return base64.encodebytes(checksum.digest())


item = {"hello": "world"}

print("sha-256 digest value for a br-coded representation: ",
    digest(item, content_coding=brotli.compress)
)

print("id-sha-256 digest value for a br-coded representation: ",
    digest(item, content_coding=identity)
)

Acknowledgements

This specification was born from a thread created by James Manger and the subsequent discussion here https://github.com/httpwg/http-extensions/issues/885.¶

Change Log

RFC Editor: Please remove this section before publication.¶

Author's Address