The "id-" prefix for Digest Algorithms

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.¶

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.¶

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."¶

This Internet-Draft will expire on 21 June 2021.¶

Copyright Notice

Copyright (c) 2020 IETF Trust and the persons identified as the document authors. All rights reserved.¶

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.¶

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

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

   id-crc32c
   id-adler32

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.¶

This definition is compatible, and thus extends, the definition of the "id-sha-256" and "id-sha-512" digest-algorithms contained in Section X of [DIGEST].¶

3. Security Considerations

4. IANA Considerations

5. Examples

{"hello": "world"}

POST /data HTTP/1.1
Content-Type: application/json
Content-Encoding: br
Digest: id-crc32c=43794720, crc32c=DB329237

CwGAZG9nAw==

Appendix A. 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.¶

FAQ

Q: Question 1

Answer 1¶

Code Samples

RFC Editor: Please remove this section before publication.¶

How can I generate and validate the Digest values shown in the examples throughout this document?¶

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, crc32c

identity = lambda x: x

def digest(item, content_coding=identity, algorithm=crc32c.crc32c):
    json_bytes = json.dumps(item).encode()
    content_encoded = content_coding(json_bytes)
    checksum = algorithm(content_encoded)
    # encode result has uppercase hex
    return hex(checksum)[2:].upper()


item = {"hello": "world"}

print("crc32c digest value for a br-coded representation: ",
    digest(item, content_coding=brotli.compress)
)

print("id-crc32c digest value for a br-coded representation: ",
    digest(item, content_coding=identity)
)

Change Log

RFC EDITOR PLEASE DELETE THIS SECTION.¶

Author's Address