Digest Headers
draft-ietf-httpbis-digest-headers-05

Document Type Active Internet-Draft (httpbis WG)
Authors Roberto Polli  , Lucas Pardue 
Last updated 2021-04-13
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HTTP                                                            R. Polli
Internet-Draft                         Team Digitale, Italian Government
Obsoletes: 3230 (if approved)                                  L. Pardue
Intended status: Standards Track                              Cloudflare
Expires: 15 October 2021                                   13 April 2021

                             Digest Headers
                  draft-ietf-httpbis-digest-headers-05

Abstract

   This document defines the HTTP Digest and Want-Digest fields, thus
   allowing client and server to negotiate an integrity checksum of the
   exchanged resource representation data.

   This document obsoletes RFC 3230.  It replaces the term "instance"
   with "representation", which makes it consistent with the HTTP
   Semantic and Context defined in draft-ietf-httpbis-semantics.

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/httpwg/http-extensions (https://github.com/httpwg/
   http-extensions).

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."

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   This Internet-Draft will expire on 15 October 2021.

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 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  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  A Brief History of HTTP Integrity Fields  . . . . . . . .   4
     1.2.  This Proposal . . . . . . . . . . . . . . . . . . . . . .   4
     1.3.  Goals . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     1.4.  Notational Conventions  . . . . . . . . . . . . . . . . .   6
   2.  Representation Digest . . . . . . . . . . . . . . . . . . . .   6
   3.  The Digest Field  . . . . . . . . . . . . . . . . . . . . . .   7
   4.  The Want-Digest Field . . . . . . . . . . . . . . . . . . . .   8
   5.  Digest Algorithm Values . . . . . . . . . . . . . . . . . . .   8
   6.  Use of Digest when acting on resources  . . . . . . . . . . .  11
     6.1.  Digest and PATCH  . . . . . . . . . . . . . . . . . . . .  11
   7.  Deprecate Negotiation of Content-MD5  . . . . . . . . . . . .  11
   8.  Obsolete Digest Field Parameters  . . . . . . . . . . . . . .  12
   9.  Relationship to Subresource Integrity (SRI) . . . . . . . . .  12
     9.1.  Supporting Both SRI and Representation Digest . . . . . .  13
   10. Examples of Unsolicited Digest  . . . . . . . . . . . . . . .  13
     10.1.  Server Returns Full Representation Data  . . . . . . . .  13
     10.2.  Server Returns No Representation Data  . . . . . . . . .  14
     10.3.  Server Returns Partial Representation Data . . . . . . .  14
     10.4.  Client and Server Provide Full Representation Data . . .  15
     10.5.  Client Provides Full Representation Data, Server Provides
             No Representation Data  . . . . . . . . . . . . . . . .  15
     10.6.  Client and Server Provide Full Representation Data, Client
             Uses id-sha-256.  . . . . . . . . . . . . . . . . . . .  16
     10.7.  POST Response does not Reference the Request URI . . . .  17
     10.8.  POST Response Describes the Request Status . . . . . . .  18
     10.9.  Digest with PATCH  . . . . . . . . . . . . . . . . . . .  18
     10.10. Error responses  . . . . . . . . . . . . . . . . . . . .  19
     10.11. Use with Trailer Fields and Transfer Coding  . . . . . .  20
   11. Examples of Want-Digest Solicited Digest  . . . . . . . . . .  21

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     11.1.  Server Selects Client's Least Preferred Algorithm  . . .  21
     11.2.  Server Selects Algorithm Unsupported by Client . . . . .  22
     11.3.  Server Does Not Support Client Algorithm and Returns an
            Error  . . . . . . . . . . . . . . . . . . . . . . . . .  22
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  22
     12.1.  Digest Does Not Protect the Full HTTP Message  . . . . .  22
     12.2.  Broken Cryptographic Algorithms  . . . . . . . . . . . .  23
     12.3.  Other Deprecated Algorithms  . . . . . . . . . . . . . .  23
     12.4.  Digest for End-to-End Integrity  . . . . . . . . . . . .  23
     12.5.  Digest and Content-Location in Responses . . . . . . . .  23
     12.6.  Usage in Signatures  . . . . . . . . . . . . . . . . . .  24
     12.7.  Usage in Trailer Fields  . . . . . . . . . . . . . . . .  24
     12.8.  Usage with Encryption  . . . . . . . . . . . . . . . . .  25
     12.9.  Algorithm Agility  . . . . . . . . . . . . . . . . . . .  25
       12.9.1.  Duplicate digest-algorithm in field value  . . . . .  25
     12.10. Resource exhaustion  . . . . . . . . . . . . . . . . . .  26
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  26
     13.1.  Establish the HTTP Digest Algorithm Values Registry  . .  26
     13.2.  The "status" Field in the HTTP Digest Algorithm Values
             Registry  . . . . . . . . . . . . . . . . . . . . . . .  26
     13.3.  Deprecate "MD5" Digest Algorithm . . . . . . . . . . . .  26
     13.4.  Update "UNIXsum" Digest Algorithm  . . . . . . . . . . .  26
     13.5.  Update "UNIXcksum" Digest Algorithm  . . . . . . . . . .  27
     13.6.  Update "CRC32c" Digest Algorithm . . . . . . . . . . . .  27
     13.7.  Deprecate "SHA" Digest Algorithm . . . . . . . . . . . .  27
     13.8.  Obsolete "ADLER32" Digest Algorithm  . . . . . . . . . .  28
     13.9.  Obsolete "contentMD5" token in Digest Algorithm  . . . .  28
     13.10. The "id-sha-256" Digest Algorithm  . . . . . . . . . . .  28
     13.11. The "id-sha-512" Digest Algorithm  . . . . . . . . . . .  29
     13.12. Changes Compared to RFC5843  . . . . . . . . . . . . . .  29
     13.13. Want-Digest Field Registration . . . . . . . . . . . . .  29
     13.14. Digest Field Registration  . . . . . . . . . . . . . . .  29
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  30
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  30
     14.2.  Informative References . . . . . . . . . . . . . . . . .  31
   Appendix A.  Resource Representation and Representation-Data  . .  33
   Appendix B.  FAQ  . . . . . . . . . . . . . . . . . . . . . . . .  35
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  36
   Code Samples  . . . . . . . . . . . . . . . . . . . . . . . . . .  36
   Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  37
     Since draft-ietf-httpbis-digest-headers-04  . . . . . . . . . .  38
     Since draft-ietf-httpbis-digest-headers-03  . . . . . . . . . .  38
     Since draft-ietf-httpbis-digest-headers-02  . . . . . . . . . .  38
     Since draft-ietf-httpbis-digest-headers-01  . . . . . . . . . .  38
     Since draft-ietf-httpbis-digest-headers-00  . . . . . . . . . .  39
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  39

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1.  Introduction

   The core specification of HTTP does not define a means to protect the
   integrity of resources.  When HTTP messages are transferred between
   endpoints, the protocol might choose to make use of features of the
   lower layer in order to provide some integrity protection; for
   instance TCP checksums or TLS records [RFC2818].

   However, there are cases where relying on this alone is insufficient.
   An HTTP-level integrity mechanism that operates independent of
   transfer can be used to detect programming errors and/or corruption
   of data in flight or at rest, be used across multiple hops in order
   to provide end-to-end integrity guarantees, can aid fault diagnosis
   across hops and system boundaries, and can be used to validate
   integrity when reconstructing a resource fetched using different HTTP
   connections.

   This document defines a mechanism that acts on HTTP representation-
   data.  It can be combined with other mechanisms that protect
   representation-metadata, such as digital signatures, in order to
   protect the desired parts of an HTTP exchange in whole or in part.

1.1.  A Brief History of HTTP Integrity Fields

   The Content-MD5 header field was originally introduced to provide
   integrity, but HTTP/1.1 ([RFC7231], Appendix B) obsoleted it:

      The Content-MD5 header field has been removed because it was
      inconsistently implemented with respect to partial responses.

   [RFC3230] provided a more flexible solution introducing the concept
   of "instance", and the fields "Digest" and "Want-Digest".

1.2.  This Proposal

   The concept of "selected representation" defined in Section 3.2 of
   [SEMANTICS] makes [RFC3230] definitions inconsistent with current
   HTTP semantics.  This document updates the "Digest" and "Want-Digest"
   field definitions to align with [SEMANTICS] concepts.

   Basing "Digest" on the selected representation makes it
   straightforward to apply it to use-cases where the transferred data
   does require some sort of manipulation to be considered a
   representation, or conveys a partial representation of a resource eg.
   Range Requests (see Section 14.2 of [SEMANTICS]).

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   This document replaces [RFC3230] to better align with [SEMANTICS] and
   to provide more detailed description of "Digest" usage in request and
   response cases.  Changes are intended to be semantically compatible
   with existing implementations but note that negotiation of "Content-
   MD5" is deprecated Section 7, "Digest" field parameters are obsoleted
   Section 8, "md5" and "sha" digest-algorithms are obsoleted
   Section 12.2 and the "adler32" algorithm is deprecated Section 12.3.

   The value of "Digest" is calculated on selected representation, which
   is tied to the value contained in any "Content-Encoding" or "Content-
   Type" header fields.  Therefore, a given resource may have multiple
   different digest values.

   To allow both parties to exchange a Digest of a representation with
   no content codings (see Section 8.4.1 of [SEMANTICS]) two more
   digest-algorithms are added ("id-sha-256" and "id-sha-512").

1.3.  Goals

   The goals of this proposal are:

   1.  Digest coverage for either the resource's "representation data"
       or "selected representation data" communicated via HTTP.

   2.  Support for multiple digest-algorithms.

   3.  Negotiation of the use of digests.

   The goals do not include:

   HTTP message integrity:  Digest mechanisms do not cover the full HTTP
      message nor its semantic, as representation metadata is not
      included in the checksum.

   HTTP field integrity:  Digest mechanisms cover only representation
      and selected representation data, and do not protect the integrity
      of associated representation metadata or other message fields.

   Authentication:  Digest mechanisms do not support authentication of
      the source of a digest, message or anything else.  These
      mechanisms, therefore, are not a sufficient defense against many
      kinds of malicious attacks.

   Privacy:  Digest mechanisms do not provide message privacy.

   Authorization:  Digest mechanisms do not support authorization or
      other kinds of access controls.

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1.4.  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.

   This document uses the Augmented BNF defined in [RFC5234] and updated
   by [RFC7405] along with the "#rule" extension defined in
   Section 5.6.1 of [SEMANTICS].

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

   Algorithm names respect the casing used in their definition document
   (eg.  SHA-1, CRC32c) whereas digest-algorithm tokens are quoted (eg.
   "sha", "crc32c").

2.  Representation Digest

   The representation digest is an integrity mechanism for HTTP
   resources which uses a checksum that is calculated independently of
   the content (see Section 6.4 of [SEMANTICS]).  It uses the
   representation data (see Section 8.1 of [SEMANTICS]), that can be
   fully or partially contained in the content, or not contained at all:

      representation-data := Content-Encoding( Content-Type( bits ) )

   This takes into account the effect of the HTTP semantics on the
   messages; for example, the content can be affected by Range Requests
   or methods such as HEAD, while the way the content is transferred "on
   the wire" is dependent on other transformations (e.g. transfer
   codings for HTTP/1.1 - see Section 6.1 of [HTTP11]).  To help
   illustrate how such things affect "Digest", several examples are
   provided in Appendix A.

   A representation digest consists of the value of a checksum computed
   on the entire selected "representation data" (see Section 8.1 of
   [SEMANTICS]) of a resource identified according to Section 6.4.2 of
   [SEMANTICS] together with an indication of the algorithm used:

      representation-data-digest = digest-algorithm "="
                                   <encoded digest output>

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   When a message has no representation data it is still possible to
   assert that no representation data was sent computing the
   representation digest on an empty string (see Section 12.6).

   The checksum is computed using one of the digest-algorithms listed in
   Section 5 and then encoded in the associated format.

   The example below shows the "sha-256" digest-algorithm that uses
   base64 encoding.

      sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=

3.  The Digest Field

   The "Digest" field contains a list of one or more representation
   digest values as defined in Section 2.  It can be used in both
   requests and responses.

      Digest = 1#representation-data-digest

   For example:

   Digest: id-sha-512=WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+TaPm
                      AbwAgBWnrIiYllu7BNNyealdVLvRwE\nmTHWXvJwew==

   The relationship between "Content-Location" (see Section 8.7 of
   [SEMANTICS]) and "Digest" is demonstrated in Section 10.7.  A
   comprehensive set of examples showing the impacts of representation
   metadata, payload transformations and HTTP methods on Digest is
   provided in Section 10 and Section 11.

   A "Digest" field MAY contain multiple representation-data-digest
   values.  For example, a server may provide representation-data-digest
   values using different algorithms, allowing it to support a
   population of clients with different evolving capabilities; this is
   particularly useful in support of transitioning away from weaker
   algorithms should the need arise (see Section 12.9).

   Digest: sha-256=4REjxQ4yrqUVicfSKYNO/cF9zNj5ANbzgDZt3/h3Qxo=,
           id-sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=

   A recipient MAY ignore any or all of the representation-data-digests
   in a Digest field.  This allows the recipient to choose which digest-
   algorithm(s) to use for validation instead of verifying every
   received representation-data-digest.

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   A sender MAY send a representation-data-digest using a digest-
   algorithm without knowing whether the recipient supports the digest-
   algorithm, or even knowing that the recipient will ignore it.

   "Digest" can be sent in a trailer section.  When an incremental
   digest-algorithms is used, the sender and the receiver can
   dynamically compute the digest value while streaming the content.

4.  The Want-Digest Field

   The "Want-Digest" field indicates the sender's desire to receive a
   representation digest on messages associated with the request URI and
   representation metadata.

      Want-Digest = 1#want-digest-value
      want-digest-value = digest-algorithm [ ";" "q" "=" qvalue]
      qvalue = ( "0"  [ "."  0*1DIGIT ] ) /
               ( "1"  [ "."  0*1( "0" ) ] )

   If a digest-algorithm is not accompanied by a "qvalue", it is treated
   as if its associated "qvalue" were 1.0.

   The sender is willing to accept a digest-algorithm if and only if it
   is listed in a "Want-Digest" field of a message, and its "qvalue" is
   non-zero.

   If multiple acceptable digest-algorithm values are given, the
   sender's preferred digest-algorithm is the one (or ones) with the
   highest "qvalue".

   Two examples of its use are:

   Want-Digest: sha-256
   Want-Digest: sha-512;q=0.3, sha-256;q=1, unixsum;q=0

5.  Digest Algorithm Values

   Digest-algorithm values are used to indicate a specific digest
   computation.

      digest-algorithm = token

   All digest-algorithm values are case-insensitive but lower case is
   preferred.

   The Internet Assigned Numbers Authority (IANA) acts as a registry for
   digest-algorithm values.  The registry contains the tokens listed
   below.

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   Some digest-algorithms, although registered, rely on vulnerable
   algorithms and MUST not be used:

   *  "md5", see [CMU-836068] and [NO-MD5];

   *  "sha", see [IACR-2020-014] and [NO-SHA1].

   See the references above for further information.

   sha-256
      *  Description: The SHA-256 algorithm [RFC6234].  The output of
         this algorithm is encoded using the base64 encoding [RFC4648].

      *  Reference: [RFC6234], [RFC4648], this document.

      *  Status: standard

   sha-512
      *  Description: The SHA-512 algorithm [RFC6234].  The output of
         this algorithm is encoded using the base64 encoding [RFC4648].

      *  Reference: [RFC6234], [RFC4648], this document.

      *  Status: standard

   md5
      *  Description: The MD5 algorithm, as specified in [RFC1321].  The
         output of this algorithm is encoded using the base64 encoding
         [RFC4648].  This digest-algorithm MUST NOT be used as it's now
         vulnerable to collision attacks.  See [NO-MD5] and
         [CMU-836068].

      *  Reference: [RFC1321], [RFC4648], this document.

      *  Status: deprecated

   sha
      *  Description: The SHA-1 algorithm [RFC3174].  The output of this
         algorithm is encoded using the base64 encoding [RFC4648].  This
         digest-algorithm MUST NOT be used as it's now vulnerable to
         collision attacks.  See [NO-SHA1] and [IACR-2020-014].

      *  Reference: [RFC3174], [RFC6234], [RFC4648], this document.

      *  Status: deprecated

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   unixsum
      *  Description: The algorithm computed by the UNIX "sum" command,
         as defined by the Single UNIX Specification, Version 2 [UNIX].
         The output of this algorithm is an ASCII decimal-digit string
         representing the 16-bit checksum, which is the first word of
         the output of the UNIX "sum" command.

      *  Reference: [UNIX], this document.

      *  Status: standard

   unixcksum
      *  Description: The algorithm computed by the UNIX "cksum"
         command, as defined by the Single UNIX Specification, Version 2
         [UNIX].  The output of this algorithm is an ASCII digit string
         representing the 32-bit CRC, which is the first word of the
         output of the UNIX "cksum" command.

      *  Reference: [UNIX], this document.

      *  Status: standard

   To allow sender and recipient to provide a checksum which is
   independent from "Content-Encoding", the following additional digest-
   algorithms are defined:

   id-sha-512
      *  Description: The sha-512 digest of the representation-data of
         the resource when no content coding is applied

      *  Reference: [RFC6234], [RFC4648], this document.

      *  Status: standard

   id-sha-256
      *  Description: The sha-256 digest of the representation-data of
         the resource when no content coding is applied

      *  Reference: [RFC6234], [RFC4648], this document.

      *  Status: standard

   If other digest-algorithm values are defined, the associated encoding
   MUST either be represented as a quoted string or MUST NOT include ";"
   or "," in the character sets used for the encoding.

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6.  Use of Digest when acting on resources

   POST and PATCH requests can appear to convey partial representations
   but are semantically acting on resources.  The enclosed
   representation, including its metadata, refers to that action.

   In these requests the representation digest MUST be computed on the
   representation-data of that action.  This is the only possible choice
   because representation digest requires complete representation
   metadata (see Section 2).

   In responses,

   *  if the representation describes the status of the request,
      "Digest" MUST be computed on the enclosed representation (see
      Section 10.8 );

   *  if there is a referenced resource "Digest" MUST be computed on the
      selected representation of the referenced resource even if that is
      different from the target resource.  That might or might not
      result in computing "Digest" on the enclosed representation.

   The latter case might be done according to the HTTP semantics of the
   given method, for example using the "Content-Location" header field.
   In contrast, the "Location" header field does not affect "Digest"
   because it is not representation metadata.

6.1.  Digest and PATCH

   In PATCH requests, the representation digest MUST be computed on the
   patch document because the representation metadata refers to the
   patch document and not to the target resource (see Section 2 of
   [PATCH]).

   In PATCH responses, the representation digest MUST be computed on the
   selected representation of the patched resource.

   "Digest" usage with PATCH is thus very similar to POST, but with the
   resource's own semantic partly implied by the method and by the patch
   document.

7.  Deprecate Negotiation of Content-MD5

   This RFC deprecates the negotiation of Content-MD5 as it has been
   obsoleted by [RFC7231].  The "contentMD5" token defined in Section 5
   of [RFC3230] MUST NOT be used as a digest-algorithm.

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8.  Obsolete Digest Field Parameters

   Section 4.1.1 and 4.2 of [RFC3230] defined field parameters.  This
   document obsoletes the usage of parameters with "Digest" because this
   feature has not been widely deployed and complicates field-value
   processing.

   [RFC3230] intended field parameters to provide a common way to attach
   additional information to a representation-data-digest.  However, if
   parameters are used as an input to validate the checksum, an attacker
   could alter them to steer the validation behavior.

   A digest-algorithm can still be parameterized by defining its own way
   to encode parameters into the representation-data-digest, in such a
   way as to mitigate security risks related to its computation.

9.  Relationship to Subresource Integrity (SRI)

   Subresource Integrity [SRI] is an integrity mechanism that shares
   some similarities to the present document's mechanism.  However,
   there are differences in motivating factors, threat model and
   specification of integrity digest generation, signalling and
   validation.

   SRI allows a first-party authority to declare an integrity assertion
   on a resource served by a first or third party authority.  This is
   done via the "integrity" attribute that can be added to "script" or
   "link" HTML elements.  Therefore, the integrity assertion is always
   made out-of-band to the resource fetch.  In contrast, the "Digest"
   field is supplied in-band alongside the selected representation,
   meaning that an authority can only declare an integrity assertion for
   itself.  Methods to improve the security properties of representation
   digests are presented in Section 12.  This contrast is interesting
   because on one hand self-assertion is less likely to be affected by
   coordination problems such as the first-party holding stale
   information about the third party, but on the other hand the self-
   assertion is only as trustworthy as the authority that provided it.

   The SRI "integrity" attribute contains a cryptographic hash algorithm
   and digest value which is similar to "representation-data-digest"
   (see Section 2).  The major differences are in serialization format.

   SRI does not specify handling of partial representation data (e.g.
   Range requests).  In contrast, this document specifies handling in
   terms that are fully compatible with core HTTP concepts (an example
   is provided in Section 10.3).

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   SRI specifies strong requirements on the selection of algorithm for
   generation and validation of digests.  In contrast, the requirements
   in this document are weaker.

   SRI defines no method for a client to declare an integrity assertion
   on resources it transfers to a server.  In contrast, the "Digest"
   field can appear on requests.

9.1.  Supporting Both SRI and Representation Digest

   The SRI and Representation Digest mechanisms are different and
   complementary but one is not capable of replacing the other because
   they have different threat, security and implementation properties.

   A user agent that supports both mechanisms is expected to apply the
   rules specified for each but since the two mechanisms are
   independent, the ordering is not important.  However, a user agent
   supporting both could benefit from performing representation digest
   validation first because it does not always require a conversion into
   identity encoding.

   There is a chance that a user agent supporting both mechanisms may
   find one validates successfully while the other fails.  This document
   specifies no requirements or guidance for user agents that experience
   such cases.

10.  Examples of Unsolicited Digest

   The following examples demonstrate interactions where a server
   responds with a "Digest" field even though the client did not solicit
   one using "Want-Digest".

   Some examples include JSON objects in the content.  For presentation
   purposes, objects that fit completely within the line-length limits
   are presented on a single line using compact notation with no leading
   space.  Objects that would exceed line-length limits are presented
   across multiple lines (one line per key-value pair) with 2 spaced of
   leading indentation.

   "Digest" is media-type agnostic and does not provide canonicalization
   algorithms for specific formats.  Examples of "Digest" are calculated
   inclusive of any space.

10.1.  Server Returns Full Representation Data

   Request:

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   GET /items/123 HTTP/1.1
   Host: foo.example

   Response:

   HTTP/1.1 200 OK
   Content-Type: application/json
   Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=

   {"hello": "world"}

10.2.  Server Returns No Representation Data

   In this example, a HEAD request is used to retrieve the checksum of a
   resource.

   The response "Digest" field-value is calculated over the JSON object
   "{"hello": "world"}", which is not shown because there is no payload
   data.

   Request:

   HEAD /items/123 HTTP/1.1
   Host: foo.example

   Response:

   HTTP/1.1 200 OK
   Content-Type: application/json
   Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=

10.3.  Server Returns Partial Representation Data

   In this example, the client makes a range request and the server
   responds with partial content.  The "Digest" field-value represents
   the entire JSON object "{"hello": "world"}".

   Request:

   GET /items/123 HTTP/1.1
   Host: foo.example
   Range: bytes=1-7

   Response:

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   HTTP/1.1 206 Partial Content
   Content-Type: application/json
   Content-Range: bytes 1-7/18
   Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=

   "hello"

10.4.  Client and Server Provide Full Representation Data

   The request contains a "Digest" field-value calculated on the
   enclosed representation.  It also includes an "Accept-Encoding: br"
   header field that advertises the client supports brotli encoding.

   The response includes a "Content-Encoding: br" that indicates the
   selected representation is brotli encoded.  The "Digest" field-value
   is therefore different compared to the request.

   For presentation purposes, the response body is displayed as a
   base64-encoded string because it contains non-printable characters.

   Request:

   PUT /items/123 HTTP/1.1
   Host: foo.example
   Content-Type: application/json
   Accept-Encoding: br
   Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=

   {"hello": "world"}

   Response:

   HTTP/1.1 200 Ok
   Content-Type: application/json
   Content-Location: /items/123
   Content-Encoding: br
   Content-Length: 22
   Digest: sha-256=4REjxQ4yrqUVicfSKYNO/cF9zNj5ANbzgDZt3/h3Qxo=

   iwiAeyJoZWxsbyI6ICJ3b3JsZCJ9Aw==

10.5.  Client Provides Full Representation Data, Server Provides No
       Representation Data

   The request "Digest" field-value is calculated on the enclosed
   payload.

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   The response "Digest" field-value depends on the representation
   metadata header fields, including "Content-Encoding: br" even when
   the response does not contain content.

   Request:

   PUT /items/123 HTTP/1.1
   Host: foo.example
   Content-Type: application/json
   Content-Length: 18
   Accept-Encoding: br
   Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=

   {"hello": "world"}

   Response:

   HTTP/1.1 204 No Content
   Content-Type: application/json
   Content-Encoding: br
   Digest: sha-256=4REjxQ4yrqUVicfSKYNO/cF9zNj5ANbzgDZt3/h3Qxo=

10.6.  Client and Server Provide Full Representation Data, Client Uses
       id-sha-256.

   The response contains two digest values:

   *  one with no content coding applied, which in this case
      accidentally matches the unencoded digest-value sent in the
      request;

   *  one taking into account the "Content-Encoding".

   As the response body contains non-printable characters, it is
   displayed as a base64-encoded string.

   Request:

   PUT /items/123 HTTP/1.1
   Host: foo.example
   Content-Type: application/json
   Accept-Encoding: br
   Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=

   {"hello": "world"}

   Response:

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   HTTP/1.1 200 OK
   Content-Type: application/json
   Content-Encoding: br
   Content-Location: /items/123
   Digest: sha-256=4REjxQ4yrqUVicfSKYNO/cF9zNj5ANbzgDZt3/h3Qxo=,
           id-sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=

   iwiAeyJoZWxsbyI6ICJ3b3JsZCJ9Aw==

10.7.  POST Response does not Reference the Request URI

   The request "Digest" field-value is computed on the enclosed
   representation (see Section 6).

   The representation enclosed in the response refers to the resource
   identified by "Content-Location" (see [SEMANTICS], Section 6.4.2).
   "Digest" is thus computed on the enclosed representation.

   Request:

   POST /books HTTP/1.1
   Host: foo.example
   Content-Type: application/json
   Accept: application/json
   Accept-Encoding: identity
   Digest: sha-256=bWopGGNiZtbVgHsG+I4knzfEJpmmmQHf7RHDXA3o1hQ=

   {"title": "New Title"}

   Response:

   HTTP/1.1 201 Created
   Content-Type: application/json
   Content-Location: /books/123
   Location: /books/123
   Digest: id-sha-256=yxOAqEeoj+reqygSIsLpT0LhumrNkIds5uLKtmdLyYE=

   {
     "id": "123",
     "title": "New Title"
   }

   Note that a "204 No Content" response without content but with the
   same "Digest" field-value would have been legitimate too.

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10.8.  POST Response Describes the Request Status

   The request "Digest" field-value is computed on the enclosed
   representation (see Section 6).

   The representation enclosed in the response describes the status of
   the request, so "Digest" is computed on that enclosed representation.

   Response "Digest" has no explicit relation with the resource
   referenced by "Location".

   Request:

   POST /books HTTP/1.1
   Host: foo.example
   Content-Type: application/json
   Accept: application/json
   Accept-Encoding: identity
   Digest: sha-256=bWopGGNiZtbVgHsG+I4knzfEJpmmmQHf7RHDXA3o1hQ=
   Location: /books/123

   {"title": "New Title"}

   Response:

   HTTP/1.1 201 Created
   Content-Type: application/json
   Digest: id-sha-256=2LBp5RKZGpsSNf8BPXlXrX4Td4Tf5R5bZ9z7kdi5VvY=
   Location: /books/123

   {
     "status": "created",
     "id": "123",
     "ts": 1569327729,
     "instance": "/books/123"
   }

10.9.  Digest with PATCH

   This case is analogous to a POST request where the target resource
   reflects the effective request URI.

   The PATCH request uses the "application/merge-patch+json" media type
   defined in [RFC7396].

   "Digest" is calculated on the enclosed payload, which corresponds to
   the patch document.

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   The response "Digest" field-value is computed on the complete
   representation of the patched resource.

   Request:

   PATCH /books/123 HTTP/1.1
   Host: foo.example
   Content-Type: application/merge-patch+json
   Accept: application/json
   Accept-Encoding: identity
   Digest: sha-256=bWopGGNiZtbVgHsG+I4knzfEJpmmmQHf7RHDXA3o1hQ=

   {"title": "New Title"}

   Response:

   HTTP/1.1 200 OK
   Content-Type: application/json
   Digest: id-sha-256=yxOAqEeoj+reqygSIsLpT0LhumrNkIds5uLKtmdLyYE=

   {
     "id": "123",
     "title": "New Title"
   }

   Note that a "204 No Content" response without content but with the
   same "Digest" field-value would have been legitimate too.

10.10.  Error responses

   In error responses, the representation-data does not necessarily
   refer to the target resource.  Instead, it refers to the
   representation of the error.

   In the following example a client attempts to patch the resource
   located at /books/123.  However, the resource does not exist and the
   server generates a 404 response with a body that describes the error
   in accordance with [RFC7807].

   The response "Digest" field-value is computed on this enclosed
   representation.

   Request:

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   PATCH /books/123 HTTP/1.1
   Host: foo.example
   Content-Type: application/merge-patch+json
   Accept: application/json
   Accept-Encoding: identity
   Digest: sha-256=bWopGGNiZtbVgHsG+I4knzfEJpmmmQHf7RHDXA3o1hQ=

   {"title": "New Title"}

   Response:

   HTTP/1.1 404 Not Found
   Content-Type: application/problem+json
   Digest: sha-256=KPqhVXAT25LLitV1w0O167unHmVQusu+fpxm65zAsvk=

   {
     "title": "Not Found",
     "detail": "Cannot PATCH a non-existent resource",
     "status": 404
   }

10.11.  Use with Trailer Fields and Transfer Coding

   An origin server sends "Digest" as trailer field, so it can calculate
   digest-value while streaming content and thus mitigate resource
   consumption.  The "Digest" field-value is the same as in Section 10.1
   because "Digest" is designed to be independent from the use of one or
   more transfer codings (see Section 2).

   Request:

   GET /items/123 HTTP/1.1
   Host: foo.example

   Response:

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   HTTP/1.1 200 OK
   Content-Type: application/json
   Transfer-Encoding: chunked
   Trailer: Digest

   8\r\n
   {"hello"\r\n
   8
   : "world\r\n
   2\r\n
   "}\r\n
   0\r\n
   Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=

11.  Examples of Want-Digest Solicited Digest

   The following examples demonstrate interactions where a client
   solicits a "Digest" using "Want-Digest".

   Some examples include JSON objects in the content.  For presentation
   purposes, objects that fit completely within the line-length limits
   are presented on a single line using compact notation with no leading
   space.  Objects that would exceed line-length limits are presented
   across multiple lines (one line per key-value pair) with 2 spaced of
   leading indentation.

   "Digest" is media-type agnostic and does not provide canonicalization
   algorithms for specific formats.  Examples of "Digest" are calculated
   inclusive of any space.

11.1.  Server Selects Client's Least Preferred Algorithm

   The client requests a digest, preferring "sha".  The server is free
   to reply with "sha-256" anyway.

   Request:

   GET /items/123 HTTP/1.1
   Host: foo.example
   Want-Digest: sha-256;q=0.3, sha;q=1

   Response:

   HTTP/1.1 200 OK
   Content-Type: application/json
   Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=

   {"hello": "world"}

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11.2.  Server Selects Algorithm Unsupported by Client

   The client requests a "sha" digest only.  The server is currently
   free to reply with a Digest containing an unsupported algorithm.

   Request:

   GET /items/123 HTTP/1.1
   Host: foo.example
   Want-Digest: sha;q=1

   Response:

   HTTP/1.1 200 OK
   Content-Type: application/json
   Digest: id-sha-512=WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+TaPm
                      +AbwAgBWnrIiYllu7BNNyealdVLvRwE\nmTHWXvJwew==

   {"hello": "world"}

11.3.  Server Does Not Support Client Algorithm and Returns an Error

   The client requests a "sha" Digest, the server advises "sha-256" and
   "sha-512".

   Request:

   GET /items/123 HTTP/1.1
   Host: foo.example
   Want-Digest: sha;q=1

   Response:

   HTTP/1.1 400 Bad Request
   Want-Digest: sha-256, sha-512

12.  Security Considerations

12.1.  Digest Does Not Protect the Full HTTP Message

   This document specifies a data integrity mechanism that protects HTTP
   "representation data", but not HTTP "representation metadata" fields,
   from certain kinds of accidental corruption.

   "Digest" is not intended to be a general protection against malicious
   tampering with HTTP messages.  This can be achieved by combining it
   with other approaches such as transport-layer security or digital
   signatures.

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12.2.  Broken Cryptographic Algorithms

   Cryptographic algorithms are intended to provide a proof of integrity
   suited towards cryptographic constructions such as signatures.

   However, these rely on collision-resistance for their security proofs
   [CMU-836068].  The "md5" and "sha" digest-algorithms are vulnerable
   to collisions attacks, so they MUST NOT be used with "Digest".

12.3.  Other Deprecated Algorithms

   The ADLER32 algorithm defined in [RFC1950] has been deprecated by
   [RFC3309] because, under certain conditions, it provides weak
   detection of errors.  It is now NOT RECOMMENDED for use with
   "Digest".

12.4.  Digest for End-to-End Integrity

   "Digest" only covers the "representation data" and not the
   "representation metadata".  "Digest" could help protect the
   "representation data" from buggy manipulation, undesired
   "transforming proxies" (see Section 7.7 of [SEMANTICS]) or other
   actions as the data passes across multiple hops or system boundaries.
   Even a simple mechanism for end-to-end "representation data"
   integrity is valuable because user-agent can validate that resource
   retrieval succeeded before handing off to a HTML parser, video player
   etc. for parsing.

   Identity digest-algorithms (e.g. "id-sha-256" and "id-sha-512") are
   particularly useful for end-to-end integrity because they allow
   piecing together a resource from different sources with different
   HTTP messaging characteristics.  For example, different servers that
   apply different content codings.

   Note that using "Digest" alone does not provide end-to-end integrity
   of HTTP messages over multiple hops, since metadata could be
   manipulated at any stage.  Methods to protect metadata are discussed
   in Section 12.6.

12.5.  Digest and Content-Location in Responses

   When a state-changing method returns the "Content-Location" header
   field, the enclosed representation refers to the resource identified
   by its value and "Digest" is computed accordingly.

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12.6.  Usage in Signatures

   Digital signatures are widely used together with checksums to provide
   the certain identification of the origin of a message [NIST800-32].
   Such signatures can protect one or more HTTP fields and there are
   additional considerations when "Digest" is included in this set.

   Since the "Digest" field is a hash of a resource representation, it
   explicitly depends on the "representation metadata" (eg. the values
   of "Content-Type", "Content-Encoding" etc).  A signature that
   protects "Digest" but not other "representation metadata" can expose
   the communication to tampering.  For example, an actor could
   manipulate the "Content-Type" field-value and cause a digest
   validation failure at the recipient, preventing the application from
   accessing the representation.  Such an attack consumes the resources
   of both endpoints.  See also Section 12.5.

   "Digest" SHOULD always be used over a connection that provides
   integrity at the transport layer that protects HTTP fields.

   A "Digest" field using NOT RECOMMENDED digest-algorithms SHOULD NOT
   be used in signatures.

   Using signatures to protect the "Digest" of an empty representation
   allows receiving endpoints to detect if an eventual payload has been
   stripped or added.

   Any mangling of "Digest", including de-duplication of representation-
   data-digest values or combining different field values (see
   Section 5.2 of [SEMANTICS]) might affect signature validation.

12.7.  Usage in Trailer Fields

   When "Digest" is used in trailer fields, the receiver gets the digest
   value after the content and may thus be tempted to process the data
   before validating the digest value.  It is prefereable that data is
   only be processed after validating the Digest.

   If received in trailers, "Digest" MUST NOT be discarded; instead, it
   MAY be merged in the header section (See Section 6.5.1 of
   [SEMANTICS]).

   Not every digest-algorithm is suitable for use in the trailer
   section, some may require to pre-process the whole payload before
   sending a message (eg. see [I-D.thomson-http-mice]).

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12.8.  Usage with Encryption

   "Digest" may expose details of encrypted payload when the checksum is
   computed on the unencrypted data.  For example, the use of the "id-
   sha-256" digest-algorithm in conjunction with the encrypted content-
   coding [RFC8188].

   The representation-data-digest of an encrypted payload can change
   between different messages depending on the encryption algorithm
   used; in those cases its value could not be used to provide a proof
   of integrity "at rest" unless the whole (e.g. encoded) content is
   persisted.

12.9.  Algorithm Agility

   The security properties of digest-algorithms are not fixed.
   Algorithm Agility (see [RFC7696]) is achieved by providing
   implementations with flexibility choose digest-algorithms from the
   IANA Digest Algorithm Values registry in Section 13.1.

   To help endpoints understand weaker algorithms from stronger ones,
   this document adds to the IANA Digest Algorithm Values registry a new
   "Status" field containing the most-recent appraisal of the digest-
   algorithm; the allowed values are specified in Section 13.2.

   An endpoint might have a preference for algorithms, such as
   preferring "standard" algorithms over "deprecated" ones.  Transition
   from weak algorithms is supported by negotiation of digest-algorithm
   using "Want-Digest" (see Section 4) or by sending multiple
   representation-data-digest values from which the receiver chooses.
   Endpoints are advised that sending multiple values consumes
   resources, which may be wasted if the receiver ignores them (see
   Section 3).

12.9.1.  Duplicate digest-algorithm in field value

   An endpoint might receive multiple representation-data-digest values
   (see Section 3) that use the same digest-algorithm with different or
   identical digest-values.  For example:

   Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=,
           sha-256=47DEQpj8HBSa+/TImW+5JCeuQeRkm5NMpJWZG3hSuFU=

   A receiver is permitted to ignore any representation-data-digest
   value, so validation of duplicates is left as an implementation
   decision.  Endpoints might select all, some or none of the values for
   checksum comparison and, based on the intersection of those results,
   conditionally pass or fail digest validation.

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12.10.  Resource exhaustion

   "Digest" validation consumes computational resources.  In order to
   avoid resource exhaustion, implementations can restrict validation of
   the algorithm types, number of validations, or the size of content.

13.  IANA Considerations

13.1.  Establish the HTTP Digest Algorithm Values Registry

   This memo sets this specification to be the establishing document for
   the HTTP Digest Algorithm Values (https://www.iana.org/assignments/
   http-dig-alg/http-dig-alg.xhtml) registry.

13.2.  The "status" Field in the HTTP Digest Algorithm Values Registry

   This memo adds the field "Status" to the HTTP Digest Algorithm Values
   (https://www.iana.org/assignments/http-dig-alg/http-dig-alg.xhtml)
   registry.  The allowed values for the "Status" fields are described
   below.

   Status
      *  "standard" for standardized algorithms without known problems;

      *  "experimental", "obsoleted" or some other appropriate value -
         e.g. according to the type and status of the primary document
         in which the algorithm is defined;

      *  "deprecated" when the algorithm is insecure or otherwise
         undesirable.

13.3.  Deprecate "MD5" Digest Algorithm

   This memo updates the "MD5" digest-algorithm in the HTTP Digest
   Algorithm Values (https://www.iana.org/assignments/http-dig-alg/http-
   dig-alg.xhtml) registry:

   *  Digest Algorithm: md5

   *  Description: As specified in Section 5.

   *  Status: As specified in Section 5.

13.4.  Update "UNIXsum" Digest Algorithm

   This memo updates the "UNIXsum" digest-algorithm in the HTTP Digest
   Algorithm Values (https://www.iana.org/assignments/http-dig-alg/http-
   dig-alg.xhtml) registry:

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   *  Digest Algorithm: As specified in Section 5.

   *  Description: As specified in Section 5.

   *  Status: As specified in Section 5.

13.5.  Update "UNIXcksum" Digest Algorithm

   This memo updates the "UNIXcksum" digest-algorithm in the HTTP Digest
   Algorithm Values (https://www.iana.org/assignments/http-dig-alg/http-
   dig-alg.xhtml) registry:

   *  Digest Algorithm: As specified in Section 5.

   *  Description: As specified in Section 5.

   *  Status: As specified in Section 5.

13.6.  Update "CRC32c" Digest Algorithm

   This memo updates the "CRC32c" digest-algorithm in the HTTP Digest
   Algorithm Values (https://www.iana.org/assignments/http-dig-alg/http-
   dig-alg.xhtml) registry:

   *  Digest Algorithm: crc32c

   *  Description: The CRC32c algorithm is a 32-bit cyclic redundancy
      check.  It achieves a better hamming distance (for better error-
      detection performance) than many other 32-bit CRC functions.
      Other places it is used include iSCSI and SCTP.  The 32-bit output
      is encoded in hexadecimal (using between 1 and 8 ASCII characters
      from 0-9, A-F, and a-f; leading 0's are allowed).  For example,
      crc32c=0a72a4df and crc32c=A72A4DF are both valid checksums for
      the 3-byte message "dog".

   *  Reference: [RFC4960] appendix B, this document.

   *  Status: standard.

13.7.  Deprecate "SHA" Digest Algorithm

   This memo updates the "SHA" digest-algorithm in the HTTP Digest
   Algorithm Values (https://www.iana.org/assignments/http-dig-alg/http-
   dig-alg.xhtml) registry:

   *  Digest Algorithm: sha

   *  Description: As specified in Section 5.

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   *  Status: As specified in Section 5.

13.8.  Obsolete "ADLER32" Digest Algorithm

   This memo updates the "ADLER32" digest-algorithm in the HTTP Digest
   Algorithm Values (https://www.iana.org/assignments/http-dig-alg/http-
   dig-alg.xhtml) registry:

   *  Digest Algorithm: adler32

   *  Description: The ADLER32 algorithm is a checksum specified in
      [RFC1950] "ZLIB Compressed Data Format".  The 32-bit output is
      encoded in hexadecimal (using between 1 and 8 ASCII characters
      from 0-9, A-F, and a-f; leading 0's are allowed).  For example,
      adler32=03da0195 and adler32=3DA0195 are both valid checksums for
      the 4-byte message "Wiki".  This algorithm is obsoleted and SHOULD
      NOT be used.

   *  Status: obsoleted

13.9.  Obsolete "contentMD5" token in Digest Algorithm

   This memo adds the "contentMD5" token in the HTTP Digest Algorithm
   Values (https://www.iana.org/assignments/http-dig-alg/http-dig-
   alg.xhtml) registry:

   *  Digest Algorithm: contentMD5

   *  Description: Section 5 of [RFC3230] defined the "contentMD5" token
      to be used only in Want-Digest.  This token is obsoleted and MUST
      NOT be used.

   *  Reference: Section 13.9 of this document, Section 5 of [RFC3230].

   *  Status: obsoleted

13.10.  The "id-sha-256" Digest Algorithm

   This memo registers the "id-sha-256" digest-algorithm in the HTTP
   Digest Algorithm Values (https://www.iana.org/assignments/http-dig-
   alg/http-dig-alg.xhtml) registry:

   *  Digest Algorithm: id-sha-256

   *  Description: As specified in Section 5.

   *  Status: As specified in Section 5.

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13.11.  The "id-sha-512" Digest Algorithm

   This memo registers the "id-sha-512" digest-algorithm in the HTTP
   Digest Algorithm Values (https://www.iana.org/assignments/http-dig-
   alg/http-dig-alg.xhtml) registry:

   *  Digest Algorithm: id-sha-512

   *  Description: As specified in Section 5.

   *  Status: As specified in Section 5.

13.12.  Changes Compared to RFC5843

   The digest-algorithm values for "MD5", "SHA", "SHA-256", "SHA-512",
   "UNIXcksum", "UNIXsum", "ADLER32" and "CRC32c" have been updated to
   lowercase.

   The status of "MD5" has been updated to "deprecated", and its
   description states that this algorithm MUST NOT be used.

   The status of "SHA" has been updated to "deprecated", and its
   description states that this algorithm MUST NOT be used.

   The status for "CRC2c", "UNIXsum" and "UNIXcksum" has been updated to
   "standard".

   The "id-sha-256" and "id-sha-512" algorithms have been added to the
   registry.

13.13.  Want-Digest Field Registration

   This section registers the "Want-Digest" field in the "Hypertext
   Transfer Protocol (HTTP) Field Name Registry" [SEMANTICS].

   Field name: "Want-Digest"

   Status: permanent

   Specification document(s): Section 4 of this document

13.14.  Digest Field Registration

   This section registers the "Digest" field in the "Hypertext Transfer
   Protocol (HTTP) Field Name Registry" [SEMANTICS].

   Field name: "Digest"

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   Status: permanent

   Specification document(s): Section 3 of this document

14.  References

14.1.  Normative References

   [CMU-836068]
              Carnagie Mellon University, Software Engineering
              Institute, "MD5 Vulnerable to collision attacks", 31
              December 2008, <https://www.kb.cert.org/vuls/id/836068/>.

   [IACR-2020-014]
              Leurent, G. and T. Peyrin, "SHA-1 is a Shambles", 5
              January 2020, <https://eprint.iacr.org/2020/014.pdf>.

   [NIST800-32]
              National Institute of Standards and Technology, U.S.
              Department of Commerce, "Introduction to Public Key
              Technology and the Federal PKI Infrastructure", February
              2001, <https://nvlpubs.nist.gov/nistpubs/Legacy/SP/
              nistspecialpublication800-32.pdf>.

   [RFC1321]  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
              DOI 10.17487/RFC1321, April 1992,
              <https://www.rfc-editor.org/rfc/rfc1321>.

   [RFC1950]  Deutsch, P. and J-L. Gailly, "ZLIB Compressed Data Format
              Specification version 3.3", RFC 1950,
              DOI 10.17487/RFC1950, May 1996,
              <https://www.rfc-editor.org/rfc/rfc1950>.

   [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/rfc/rfc2119>.

   [RFC3174]  Eastlake 3rd, D. and P. Jones, "US Secure Hash Algorithm 1
              (SHA1)", RFC 3174, DOI 10.17487/RFC3174, September 2001,
              <https://www.rfc-editor.org/rfc/rfc3174>.

   [RFC3230]  Mogul, J. and A. Van Hoff, "Instance Digests in HTTP",
              RFC 3230, DOI 10.17487/RFC3230, January 2002,
              <https://www.rfc-editor.org/rfc/rfc3230>.

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   [RFC3309]  Stone, J., Stewart, R., and D. Otis, "Stream Control
              Transmission Protocol (SCTP) Checksum Change", RFC 3309,
              DOI 10.17487/RFC3309, September 2002,
              <https://www.rfc-editor.org/rfc/rfc3309>.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <https://www.rfc-editor.org/rfc/rfc4648>.

   [RFC4960]  Stewart, R., Ed., "Stream Control Transmission Protocol",
              RFC 4960, DOI 10.17487/RFC4960, September 2007,
              <https://www.rfc-editor.org/rfc/rfc4960>.

   [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/rfc/rfc5234>.

   [RFC5843]  Bryan, A., "Additional Hash Algorithms for HTTP Instance
              Digests", RFC 5843, DOI 10.17487/RFC5843, April 2010,
              <https://www.rfc-editor.org/rfc/rfc5843>.

   [RFC6234]  Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
              (SHA and SHA-based HMAC and HKDF)", RFC 6234,
              DOI 10.17487/RFC6234, May 2011,
              <https://www.rfc-editor.org/rfc/rfc6234>.

   [RFC7405]  Kyzivat, P., "Case-Sensitive String Support in ABNF",
              RFC 7405, DOI 10.17487/RFC7405, December 2014,
              <https://www.rfc-editor.org/rfc/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/rfc/rfc8174>.

   [SEMANTICS]
              Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
              Semantics", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-semantics-15, 30 March 2021,
              <https://tools.ietf.org/html/draft-ietf-httpbis-semantics-
              15>.

   [UNIX]     The Open Group, "The Single UNIX Specification, Version 2
              - 6 Vol Set for UNIX 98", February 1997.

14.2.  Informative References

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   [HTTP11]   Fielding, R. T., Nottingham, M., and J. Reschke,
              "HTTP/1.1", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-messaging-15, 30 March 2021,
              <https://tools.ietf.org/html/draft-ietf-httpbis-messaging-
              15>.

   [I-D.ietf-httpbis-header-structure]
              Nottingham, M. and P. Kamp, "Structured Field Values for
              HTTP", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-header-structure-19, 3 June 2020,
              <https://tools.ietf.org/html/draft-ietf-httpbis-header-
              structure-19>.

   [I-D.thomson-http-mice]
              Thomson, M. and J. Yasskin, "Merkle Integrity Content
              Encoding", Work in Progress, Internet-Draft, draft-
              thomson-http-mice-03, 13 August 2018,
              <https://tools.ietf.org/html/draft-thomson-http-mice-03>.

   [NO-MD5]   Turner, S. and L. Chen, "Updated Security Considerations
              for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
              RFC 6151, DOI 10.17487/RFC6151, March 2011,
              <https://www.rfc-editor.org/rfc/rfc6151>.

   [NO-SHA1]  Polk, T., Chen, L., Turner, S., and P. Hoffman, "Security
              Considerations for the SHA-0 and SHA-1 Message-Digest
              Algorithms", RFC 6194, DOI 10.17487/RFC6194, March 2011,
              <https://www.rfc-editor.org/rfc/rfc6194>.

   [PATCH]    Dusseault, L. and J. Snell, "PATCH Method for HTTP",
              RFC 5789, DOI 10.17487/RFC5789, March 2010,
              <https://www.rfc-editor.org/rfc/rfc5789>.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818,
              DOI 10.17487/RFC2818, May 2000,
              <https://www.rfc-editor.org/rfc/rfc2818>.

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,
              <https://www.rfc-editor.org/rfc/rfc7231>.

   [RFC7396]  Hoffman, P. and J. Snell, "JSON Merge Patch", RFC 7396,
              DOI 10.17487/RFC7396, October 2014,
              <https://www.rfc-editor.org/rfc/rfc7396>.

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   [RFC7696]  Housley, R., "Guidelines for Cryptographic Algorithm
              Agility and Selecting Mandatory-to-Implement Algorithms",
              BCP 201, RFC 7696, DOI 10.17487/RFC7696, November 2015,
              <https://www.rfc-editor.org/rfc/rfc7696>.

   [RFC7807]  Nottingham, M. and E. Wilde, "Problem Details for HTTP
              APIs", RFC 7807, DOI 10.17487/RFC7807, March 2016,
              <https://www.rfc-editor.org/rfc/rfc7807>.

   [RFC8188]  Thomson, M., "Encrypted Content-Encoding for HTTP",
              RFC 8188, DOI 10.17487/RFC8188, June 2017,
              <https://www.rfc-editor.org/rfc/rfc8188>.

   [SRI]      Akhawe, D., Braun, F., Marier, F., and J. Weinberger,
              "Subresource Integrity", W3C Recommendation REC-SRI-
              20160623, 23 June 2016,
              <https://www.w3.org/TR/2016/REC-SRI-20160623/>.

Appendix A.  Resource Representation and Representation-Data

   The following examples show how representation metadata, payload
   transformations and method impacts on the message and content.  When
   the content contains non-printable characters (eg. when it is
   compressed) it is shown as base64-encoded string.

   A request with a JSON object without any content coding.

   Request:

   PUT /entries/1234 HTTP/1.1
   Host: foo.example
   Content-Type: application/json

   {"hello": "world"}

   Here is a gzip-compressed JSON object using a content coding.

   Request:

   PUT /entries/1234 HTTP/1.1
   Host: foo.example
   Content-Type: application/json
   Content-Encoding: gzip

   H4sIAItWyFwC/6tWSlSyUlAypANQqgUAREcqfG0AAAA=

   Now the same content conveys a malformed JSON object.

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   Request:

   PUT /entries/1234 HTTP/1.1
   Host: foo.example
   Content-Type: application/json

   H4sIAItWyFwC/6tWSlSyUlAypANQqgUAREcqfG0AAAA=

   A Range-Request alters the content, conveying a partial
   representation.

   Request:

   GET /entries/1234 HTTP/1.1
   Host: foo.example
   Range: bytes=1-7

   Response:

   HTTP/1.1 206 Partial Content
   Content-Encoding: gzip
   Content-Type: application/json
   Content-Range: bytes 1-7/18

   iwgAla3RXA==

   Now the method too alters the content.

   Request:

   HEAD /entries/1234 HTTP/1.1
   Host: foo.example
   Accept: application/json
   Accept-Encoding: gzip

   Response:

   HTTP/1.1 200 OK
   Content-Type: application/json
   Content-Encoding: gzip

   Finally the semantics of an HTTP response might decouple the
   effective request URI from the enclosed representation.  In the
   example response below, the "Content-Location" header field indicates
   that the enclosed representation refers to the resource available at
   "/authors/123".

   Request:

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   POST /authors/ HTTP/1.1
   Host: foo.example
   Accept: application/json
   Content-Type: application/json

   {"author": "Camilleri"}

   Response:

   HTTP/1.1 201 Created
   Content-Type: application/json
   Content-Location: /authors/123
   Location: /authors/123

   {"id": "123", "author": "Camilleri"}

Appendix B.  FAQ

   1.  Why remove all references to content-md5?

       Those were unnecessary to understanding and using this
       specification.

   2.  Why remove references to instance manipulation?

       Those were unnecessary for correctly using and applying the
       specification.  An example with Range Request is more than
       enough.  This document uses the term "partial representation"
       which should group all those cases.

   3.  How to use "Digest" with "PATCH" method?

       See Section 6.

   4.  Why remove references to delta-encoding?

       Unnecessary for a correct implementation of this specification.
       The revised specification can be nicely adapted to "delta
       encoding", but all the references here to delta encoding don't
       add anything to this RFC.  Another job would be to refresh delta
       encoding.

   5.  Why remove references to Digest Authentication?

       This specification seems to me completely unrelated to Digest
       Authentication but for the word "Digest".

   6.  What changes in "Want-Digest"?

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       The contentMD5 token defined in Section 5 of [RFC3230] is
       deprecated by Section 7.

       To clarify that "Digest" and "Want-Digest" can be used in both
       requests and responses - [RFC3230] carefully uses "sender" and
       "receiver" in their definition - we added examples on using
       "Want-Digest" in responses to advertise the supported digest-
       algorithms and the inability to accept requests with unsupported
       digest-algorithms.

   7.  Does this specification change supported algorithms?

       Yes. This RFC updates [RFC5843] which is still delegated for all
       algorithms updates, and adds two more algorithms: "id-sha-256"
       and "id-sha-512" which allows to send a checksum of a resource
       representation with no content codings applied.  To simplify a
       future transition to Structured Fields
       [I-D.ietf-httpbis-header-structure] we suggest to use lowercase
       for digest-algorithms.

   8.  What about mid-stream trailer fields?

       While mid-stream trailer fields (https://github.com/httpwg/http-
       core/issues/313#issuecomment-584389706) are interesting, since
       this specification is a rewrite of [RFC3230] we do not think we
       should face that.  As a first thought, nothing in this document
       precludes future work that would find a use for mid-stream
       trailers, for example an incremental digest-algorithm.  A
       document defining such a digest-algorithm is best positioned to
       describe how it is used.

Acknowledgements

   The vast majority of this document is inherited from [RFC3230], so
   thanks to J.  Mogul and A.  Van Hoff for their great work.  The
   original idea of refreshing this document arose from an interesting
   discussion with M.  Nottingham, J.  Yasskin and M.  Thomson when
   reviewing the MICE content coding.

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?

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   The following python3 code can be used to generate digests for JSON
   objects using SHA algorithms for a range of encodings.  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, hashlib, brotli, logging
  log = logging.getLogger()

  def encode_item(item, encoding=lambda x: x):
      indent = 2 if isinstance(item, dict) and len(item) > 1 else None
      json_bytes = json.dumps(item, indent=indent).encode()
      return encoding(json_bytes)

  def digest_bytes(bytes_, algorithm=hashlib.sha256):
      checksum_bytes = algorithm(bytes_).digest()
      log.warning("Log bytes: \n[%r]", bytes_)
      return base64.encodebytes(checksum_bytes).strip()

  def digest(item, encoding=lambda x: x, algorithm=hashlib.sha256):
      content_encoded = encode_item(item, encoding)
      return digest_bytes(content_encoded, algorithm)

  item = {"hello": "world"}

  print("Encoding | digest-algorithm | digest-value")
  print("Identity | sha256 |", digest(item))
  # Encoding | digest-algorithm | digest-value
  # Identity | sha256 | X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=

  print("Encoding | digest-algorithm | digest-value")
  print("Brotli | sha256 |", digest(item, encoding=brotli.compress))
  # Encoding | digest-algorithm | digest-value
  # Brotli | sha256 | 4REjxQ4yrqUVicfSKYNO/cF9zNj5ANbzgDZt3/h3Qxo=

  print("Encoding | digest-algorithm | digest-value")
  print("Identity | sha512 |", digest(item, algorithm=hashlib.sha512))
  # Encoding | digest-algorithm | digest-value
  # Identity | sha512 | b'WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+TaPm'
  #                      '+AbwAgBWnrIiYllu7BNNyealdVLvRwE\nmTHWXvJwew=='

Changes

   _RFC Editor: Please remove this section before publication._

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Since draft-ietf-httpbis-digest-headers-04

   *  Improve SRI section #1354

   *  About duplicate digest-algorithms #1221

   *  Improve security considerations #852

   *  md5 and sha deprecation references #1392

   *  Obsolete 3230 #1395

   *  Editorial #1362

Since draft-ietf-httpbis-digest-headers-03

   *  Reference semantics-12

   *  Detail encryption quirks

   *  Details on Algorithm agility #1250

   *  Obsolete parameters #850

Since draft-ietf-httpbis-digest-headers-02

   *  Deprecate SHA-1 #1154

   *  Avoid id-* with encrypted content

   *  Digest is independent from MESSAGING and HTTP/1.1 is not normative
      #1215

   *  Identity is not a valid field value for content-encoding #1223

   *  Mention trailers #1157

   *  Reference httpbis-semantics #1156

   *  Add contentMD5 as an obsoleted digest-algorithm #1249

   *  Use lowercase digest-algorithms names in the doc and in the
      digest-algorithm IANA table.

Since draft-ietf-httpbis-digest-headers-01

   *  Digest of error responses is computed on the error representation-
      data #1004

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   *  Effect of HTTP semantics on payload and message body moved to
      appendix #1122

   *  Editorial refactoring, moving headers sections up. #1109-#1112,
      #1116, #1117, #1122-#1124

Since draft-ietf-httpbis-digest-headers-00

   *  Align title with document name

   *  Add id-sha-* algorithm examples #880

   *  Reference [RFC6234] and [RFC3174] instead of FIPS-1

   *  Deprecate MD5

   *  Obsolete ADLER-32 but don't forbid it #828

   *  Update CRC32C value in IANA table #828

   *  Use when acting on resources (POST, PATCH) #853

   *  Added Relationship with SRI, draft Use Cases #868, #971

   *  Warn about the implications of "Content-Location"

Authors' Addresses

   Roberto Polli
   Team Digitale, Italian Government
   Italy

   Email: robipolli@gmail.com

   Lucas Pardue
   Cloudflare

   Email: lucaspardue.24.7@gmail.com

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