The "id-" prefix for Digest Algorithms
draft-polli-id-digest-algorithms-02

Document Type Active Internet-Draft (individual)
Author Roberto Polli 
Last updated 2021-12-02
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Network Working Group                                           R. Polli
Internet-Draft     Digital Transformation Department, Italian Government
Intended status: Experimental                            2 December 2021
Expires: 5 June 2022

                 The "id-" prefix for Digest Algorithms
                  draft-polli-id-digest-algorithms-02

Abstract

   This document defines the "id-" prefix for digest-algorithms used in
   the Digest Fields.  This prefix explicits that the computed checksum
   value is independent from Content-Encoding.

Note to Readers

   _RFC EDITOR: please remove this section before publication_

   Discussion of this draft takes place on the HTTP working group
   mailing list (ietf-http-wg@w3.org), which is archived at
   https://lists.w3.org/Archives/Public/ietf-http-wg/
   (https://lists.w3.org/Archives/Public/ietf-http-wg/).

   The source code and issues list for this draft can be found at
   https://github.com/ioggstream/draft-polli-id-digest-algorithms
   (https://github.com/ioggstream/draft-polli-id-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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 5 June 2022.

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Copyright Notice

   Copyright (c) 2021 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 Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   3
   2.  The "id-" prefix for digest-algorithms  . . . . . . . . . . .   3
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   3
     3.1.  Disclosure of encrypted content . . . . . . . . . . . . .   4
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   5.  Normative References  . . . . . . . . . . . . . . . . . . . .   4
   Appendix A.  Examples . . . . . . . . . . . . . . . . . . . . . .   5
     A.1.  The id-sha-256 digest-algorithm . . . . . . . . . . . . .   5
   FAQ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   Code Samples  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   6
   Change Log  . . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     Since draft-polli-id-digest-algorithms-01 . . . . . . . . . . .   6
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   6

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.

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1.1.  Notational Conventions

   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.  These words may also appear in this
   document in lower case as plain English words, absent their normative
   meanings.

   This document uses the Augmented BNF defined in [RFC5234] and updated
   by [RFC7405].

   The definitions "representation", "selected representation",
   "representation data", "representation metadata", and "content" in
   this document are to be interpreted as described in [SEMANTICS].

   The definitions "digest-algorithm" and "representation-data-digest"
   in this document are to be interpreted as described in [DIGEST].

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

   A new digest-algorithm to be registered within the HTTP Digest
   Algorithm Values Registry (https://www.iana.org/assignments/http-dig-
   alg/) 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 (https://www.iana.org/assignments/http-dig-alg/) 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

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3.1.  Disclosure of encrypted content

   If the content coding provides encryption features, sending the
   checksum of unencoded representation can disclose information about
   the original content.

4.  IANA Considerations

   Please, add the following text to the "Note" section of the HTTP
   Digest Algorithm Values (https://www.iana.org/assignments/http-dig-
   alg/).

   " 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. "

5.  Normative References

   [DIGEST]   Polli, R. and L. Pardue, "Digest Fields", Work in
              Progress, Internet-Draft, draft-ietf-httpbis-digest-
              headers-07, 16 November 2021,
              <https://www.ietf.org/archive/id/draft-ietf-httpbis-
              digest-headers-07.txt>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.

   [RFC7405]  Kyzivat, P., "Case-Sensitive String Support in ABNF",
              RFC 7405, DOI 10.17487/RFC7405, December 2014,
              <https://www.rfc-editor.org/info/rfc7405>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

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   [SEMANTICS]
              Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
              Semantics", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-semantics-19, 12 September 2021,
              <https://www.ietf.org/archive/id/draft-ietf-httpbis-
              semantics-19.txt>.

Appendix A.  Examples

A.1.  The id-sha-256 digest-algorithm

   The following request conveys a brotli encoded json object

   {"hello": "world"}

   The Digest computed using the "sha-256" digest-algorithm present in
   HTTP Digest Algorithm Values (https://www.iana.org/assignments/http-
   dig-alg/) is content coding aware, while its associated "id-" digest-
   algorithm is not.

   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?

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   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._

Since draft-polli-id-digest-algorithms-01

   *  Include id-sha-256 and id-sha-512.

Author's Address

   Roberto Polli
   Digital Transformation Department, Italian Government
   Italy

   Email: robipolli@gmail.com

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