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 31 March 2022                                 27 September 2021

                             Digest Headers
                  draft-ietf-httpbis-digest-headers-05 Fields
                  draft-ietf-httpbis-digest-headers-06

Abstract

   This document defines the HTTP fields that support integrity checksums.
   The Digest and field can be used for the integrity of HTTP
   representations.  The Content-Digest field can be used for the
   integrity of HTTP message content.  Want-Digest fields, thus
   allowing client and server Want-Content-
   Digest can be used to negotiate an indicate a sender's desire to receive integrity checksum of the
   exchanged resource representation data.
   fields respectively.

   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
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   This Internet-Draft will expire on 15 October 2021. 31 March 2022.

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   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4   3
     1.1.  A Brief History of HTTP Integrity Fields  Document Structure  . . . . . . . .   4
     1.2.  This Proposal . . . . . . . . . . .   4
     1.2.  Concept Overview  . . . . . . . . . . .   4
     1.3.  Goals . . . . . . . . .   4
     1.3.  Replacing RFC 3230  . . . . . . . . . . . . . . . . . . .   5
     1.4.  Notational Conventions  . . . . . . . . . . . . . . . . .   6
   2.  Representation Digest . . . . . . . . . . . . . . . . . . . .   6
   3.  The Digest Field  . . . . . . . . . . . . . . . . . . . . . .   7
   4.  The Want-Digest Content-Digest Field  . . . . . . . . . . . . . . . . . .   8
   5.  Want-Digest and Want-Content-Digest Fields  . . . . . . . . .   8
   5.
   6.  Digest Algorithm Values . . . . . . . . . . . . . . . . . . .   8
   6.  Use of   9
   7.  Using Digest when acting on resources in State-Changing Requests . . . . . . . . . . .  11
     6.1.  13
     7.1.  Digest and PATCH Content-Location in Responses  . . . . . . . .  13
   8.  Security Considerations . . . . . . . . . . . . .  11
   7.  Deprecate Negotiation of Content-MD5 . . . . . .  13
     8.1.  Digest Does Not Protect the Full HTTP Message . . . . . .  11
   8.  Obsolete  13
     8.2.  Digest Field Parameters for End-to-End Integrity . . . . . . . . . . . . .  14
     8.3.  Usage in Signatures .  12
   9.  Relationship to Subresource Integrity (SRI) . . . . . . . . .  12
     9.1.  Supporting Both SRI and Representation Digest . . . . . .  13
   10. Examples of Unsolicited Digest . . .  14
     8.4.  Usage in Trailer Fields . . . . . . . . . . . .  13
     10.1.  Server Returns Full Representation Data . . . . .  15
     8.5.  Usage with Encryption . . .  13
     10.2.  Server Returns No Representation Data . . . . . . . . .  14
     10.3.  Server Returns Partial Representation Data . . . . . .  15
     8.6.  Algorithm Agility .  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.
     8.7.  Duplicate digest-algorithm in field value . . . . . . . .  16
     8.8.  Resource exhaustion . . . . . . . . . . .  16
     10.7.  POST Response does not Reference the Request URI . . . .  17
     10.8.  POST Response Describes the Request Status . . . . .  16
   9.  IANA Considerations . .  18
     10.9.  Digest with PATCH . . . . . . . . . . . . . . . . . . .  18
     10.10. Error responses  16
     9.1.  Establish the HTTP Digest Algorithm Values Registry . . .  16
     9.2.  Obsolete "contentMD5" token in Digest Algorithm . . . . .  17
     9.3.  Changes Compared to RFC3230 . . . . . . . . . . . .  19
     10.11. Use with Trailer Fields and Transfer Coding . . .  17
     9.4.  Changes Compared to RFC5843 . . .  20
   11. Examples of Want-Digest Solicited Digest . . . . . . . . . .  21
     11.1.  Server Selects Client's Least Preferred Algorithm . .  17
     9.5.  Want-Digest Field Registration  .  21
     11.2.  Server Selects Algorithm Unsupported by Client . . . . .  22
     11.3.  Server Does Not Support Client Algorithm and Returns an
            Error . . . . . . .  17
     9.6.  Digest Field Registration . . . . . . . . . . . . . . . .  18
     9.7.  Want-Content-Digest Field Registration  . .  22
   12. Security Considerations . . . . . . .  18
     9.8.  Content-Digest Field Registration . . . . . . . . . . . .  22
     12.1.  Digest Does Not Protect the Full HTTP Message  18

   10. References  . . . . .  22
     12.2.  Broken Cryptographic Algorithms . . . . . . . . . . . .  23
     12.3.  Other Deprecated Algorithms . . . . . . . .  18
     10.1.  Normative References . . . . . .  23
     12.4.  Digest for End-to-End Integrity . . . . . . . . . . . .  23
     12.5.  Digest and Content-Location in Responses  18
     10.2.  Informative References . . . . . . . .  23
     12.6.  Usage in Signatures . . . . . . . . .  20
   Appendix A.  Resource Representation and Representation-Data  . .  22
   Appendix B.  Examples of Unsolicited Digest . . . . . . .  24
     12.7.  Usage in Trailer Fields . . . .  24
     B.1.  Server Returns Full Representation Data . . . . . . . . .  24
     B.2.  Server Returns No Representation Data . . .  24
     12.8.  Usage with Encryption . . . . . . .  24
     B.3.  Server Returns Partial Representation Data  . . . . . . .  25
     B.4.  Client and Server Provide Full Representation Data  . . .  25
     12.9.  Algorithm Agility
     B.5.  Client Provides Full Representation Data, Server Provides
            No Representation Data . . . . . . . . . . . . . . . . .  26
     B.6.  Client and Server Provide Full Representation Data, Client
            Uses id-sha-256. . .  25
       12.9.1.  Duplicate digest-algorithm in field value . . . . .  25
     12.10. Resource exhaustion . . . . . . . . . . . . .  27
     B.7.  POST Response does not Reference the Request URI  . . . .  27
     B.8.  POST Response Describes the Request Status  .  26
   13. IANA Considerations . . . . . .  28
     B.9.  Digest with PATCH . . . . . . . . . . . . . . .  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 . . . . .  29
     B.10. Error responses . . . . . . .  26
     13.4.  Update "UNIXsum" Digest Algorithm . . . . . . . . . . .  26
     13.5.  Update "UNIXcksum" Digest Algorithm . . .  30
     B.11. Use with Trailer Fields and Transfer Coding . . . . . . .  27
     13.6.  Update "CRC32c"  30
   Appendix C.  Examples of Want-Digest Solicited Digest Algorithm . . . . . . . . . . . .  27
     13.7.  Deprecate "SHA" Digest  31
     C.1.  Server Selects Client's Least Preferred Algorithm . . . . . . . . . . . .  27
     13.8.  Obsolete "ADLER32" Digest  31
     C.2.  Server Selects Algorithm Unsupported by Client  . . . . . . . . . .  28
     13.9.  Obsolete "contentMD5" token in Digest  32
     C.3.  Server Does Not Support Client Algorithm and Returns an
           Error . . . .  28
     13.10. The "id-sha-256" Digest Algorithm . . . . . . . . . . .  28
     13.11. The "id-sha-512" Digest Algorithm . . . . . . . . . . .  29
     13.12.  32
   Appendix D.  Changes Compared to RFC5843  . from RFC3230 . . . . . . . . . . . . .  29
     13.13. Want-Digest Field Registration . . .  32
     D.1.  Deprecate Negotiation of Content-MD5  . . . . . . . . . .  29
     13.14.  33
     D.2.  Obsolete Digest Field Registration  . . . . . . . . . . . . . . .  29
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  30
     14.1.  Normative References . Parameters  . . . . . . . . . . . .  33
   Acknowledgements  . . . . .  30
     14.2.  Informative References . . . . . . . . . . . . . . . . .  31
   Appendix A.  Resource Representation and Representation-Data . .  33
   Appendix B.
   FAQ . . . . . . . . . . . . . . . . . . . . . . . .  35
   Acknowledgements  . . . . . . . . . . . . . .  33
   Code Samples  . . . . . . . . . .  36
   Code Samples . . . . . . . . . . . . . . . .  35
   Changes . . . . . . . . . .  36
   Changes . . . . . . . . . . . . . . . . . . .  36
     Since draft-ietf-httpbis-digest-headers-05  . . . . . . . . . .  37  36
     Since draft-ietf-httpbis-digest-headers-04  . . . . . . . . . .  38  37
     Since draft-ietf-httpbis-digest-headers-03  . . . . . . . . . .  38  37
     Since draft-ietf-httpbis-digest-headers-02  . . . . . . . . . .  38  37
     Since draft-ietf-httpbis-digest-headers-01  . . . . . . . . . .  38  37
     Since draft-ietf-httpbis-digest-headers-00  . . . . . . . . . .  39  38
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  39  38

1.  Introduction

   The core specification of

   HTTP does not define a means to protect the integrity of resources.
   representations.  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
   instance, TCP checksums or TLS records [RFC2818].

   However, there are cases where relying on this alone is insufficient.
   An HTTP-level

   This document defines two digest integrity mechanism that operates mechanisms for HTTP.
   First, representation data integrity, which acts on representation
   data (Section 3.2 of [SEMANTICS]).  Second, content digest integrity,
   which acts on conveyed content (Section 6.4 of [SEMANTICS]).  Both
   mechanisms operate independent of
   transfer can be used transport integrity, offering the
   potential to detect programming errors and/or and corruption of data in
   flight or at rest, rest.  They can be used across multiple hops in order to
   provide end-to-end integrity guarantees, which can aid fault
   diagnosis when resources are transferred across hops and system boundaries, and
   boundaries.  Finally, they 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 obsoletes [RFC3230].

1.1.  Document Structure

   This document is structured as digital signatures, in order follows:

   *  Section 2 describes concepts related to
   protect representation digests,

   *  Section 3 defines 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 Digest request and response header field has been removed because it was
      inconsistently implemented with respect to partial responses.

   [RFC3230] provided a more flexible solution introducing and
      trailer field,

   *  Section 4 defines the concept
   of "instance", Content-Digest request and response header
      and trailer field,

   *  Section 5 defines the fields "Digest" Want-Digest and "Want-Digest".

1.2.  This Proposal

   The concept of "selected representation" defined in Want-Content-Digest request
      and response header and trailer field,

   *  Section 3.2 6 and Appendix D.1 describe algorithms and their relation
      to Digest,

   *  Section 7 details computing representation digests,

   *  Appendix D.2 obsoletes Digest field parameters, and

   *  Appendix B and Appendix C provide examples of
   [SEMANTICS] makes [RFC3230] definitions inconsistent with current
   HTTP semantics. using Digest and
      Want-Digest.

1.2.  Concept Overview

   This document updates defines the "Digest" Digest request and "Want-Digest"
   field definitions to align with [SEMANTICS] concepts.

   Basing "Digest" on response header and
   trailer field; see Section 3.  At a high level, the value contains a
   checksum, computed over selected representation makes it
   straightforward to apply data (Section 3.2 of
   [SEMANTICS]), that the recipient can use to validate integrity.
   Basing Digest on the selected representation makes it straightforward
   to apply it to use-cases where the transferred data
   does require requires some
   sort of manipulation to be considered a
   representation, representation or conveys a
   partial representation of a resource eg. resource, such as Range Requests (see
   Section 14.2 of [SEMANTICS]).

   This document replaces [RFC3230] to better align with [SEMANTICS] and
   to provide more detailed description

   To support use-cases where a simple checksum of "Digest" usage in the content bytes is
   required, this document introduces the Content-Digest 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" header and "sha" digest-algorithms are obsoleted trailer field; see Section 12.2 4.

   Digest and the "adler32" Content-Digest support algorithm is deprecated Section 12.3. agility.  The value Want-Digest
   and Want-Content-Digest fields allows endpoints to express interest
   in Digest and Content-Digest respectively, and preference of "Digest" is calculated on selected representation, which
   is
   algorithms in either.

   Digest field calculations are tied to the value contained in any "Content-Encoding" or "Content-
   Type" Content-Encoding and
   Content-Type header fields.  Therefore, a given resource may have
   multiple different digest values. checksum values when transferred with HTTP.  To
   allow both parties to exchange a Digest of a representation simple checksum with no content
   codings (see Section 8.4.1 of [SEMANTICS]) [SEMANTICS]), two more
   digest-algorithms 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 fields do not include: provide integrity for HTTP message integrity:  Digest messages or fields.
   However, they can be combined with other mechanisms do not cover the full HTTP
      message nor its semantic, that protect
   metadata, such as representation metadata is not
      included digital signatures, in the checksum.

   HTTP field integrity:  Digest mechanisms cover only representation
      and selected representation data, and do not order to protect the integrity phases
   of associated representation metadata an HTTP exchange in whole or other message fields.

   Authentication:  Digest mechanisms do in part.

   This specification does not support authentication of
      the source of a digest, message define means for authentication,
   authorization or anything else.  These
      mechanisms, therefore, are not privacy.

1.3.  Replacing RFC 3230

   Historically, the Content-MD5 header field provided an HTTP integrity
   mechanism but HTTP/1.1 ([RFC7231], Appendix B) obsoleted it due to
   inconsistent handling of partial responses.  [RFC3230] defined the
   concept of "instance" digests and a sufficient defense against many
      kinds more flexible integrity scheme to
   help address issues with Content-MD5.  It first introduced the Digest
   and Want-Digest fields.  HTTP terminology has evolved since [RFC3230]
   was published.  The concept of malicious attacks.

   Privacy: "instance" has been superseded by
   selected representation.

   This document replaces [RFC3230].  The Digest mechanisms do not provide message privacy.

   Authorization: and Want-Digest field
   definitions are updated to align with the terms and notational
   conventions in [SEMANTICS].  Changes are intended to be semantically
   compatible with existing implementations but note that negotiation of
   Content-MD5 is deprecated Appendix D.1 and has been replaced by
   Content-Digest negotiation via Want-Content-Digest.  Digest mechanisms do not support authorization or
      other kinds field
   parameters are obsoleted Appendix D.2 and the algorithm table has
   been updated to reflect the current state of access controls. the art.

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] and the "qvalue" rule defined in
   Section 12.4.2 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.
   (e.g.  SHA-1, CRC32c) whereas digest-algorithm tokens are quoted (eg.
   (e.g. "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 ) ) all.

   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", 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" 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>

   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). 8.3).

   The checksum is computed using one of the digest-algorithms listed in
   the HTTP Digest Algorithm Values Registry (see Section 5 6) 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" Digest field contains a comma-separated 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). 8.6).

   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.

   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"

   Digest can be sent in a trailer section.  In this case, Digest MAY be
   merged into the header section; see Section 6.5.1 of [SEMANTICS].

   When an incremental
   digest-algorithms digest-algorithm is used, the sender and the
   receiver can dynamically compute the digest value while streaming the
   content.

   A non-comprehensive set of examples showing the impacts of
   representation metadata, payload transformations and HTTP methods on
   Digest is provided in Appendix B and Appendix C.

4.  The Want-Digest Content-Digest Field

   The "Want-Digest" Content-Digest field indicates the sender's desire to contains a comma-separated list of one or
   more content digest values.  A content digest value is computed by
   applying a digest-algorithm to the actual message content (see
   Section 6.4 of [SEMANTICS]).  It can be used in both requests and
   responses.

      Content-Digest = 1#content-digest
      content-digest = digest-algorithm "="
                       <encoded digest output>

   For example:

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

   A Content-Digest field MAY contain multiple content-digest values,
   similarly to Digest (see Section 3)

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

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

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

   Content-Digest can be sent in a trailer section.  In this case,
   Content-Digest MAY be merged into the header section; see
   Section 6.5.1 of [SEMANTICS].

   When an incremental digest-algorithm is used, the sender and the
   receiver can dynamically compute the digest value while streaming the
   content.

5.  Want-Digest and Want-Content-Digest Fields

   Senders can indicate their integrity checksum preferences using the
   Want-Digest or Want-Content-Digest fields.  These can be used in both
   requests and responses.

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

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

      Want-Digest = 1#want-digest-value
      Want-Content-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 indicates the sender's digest-algorithm if and only if it
   is listed in a "Want-Digest" field preferences.
   Section 12.4.2 of a message, [SEMANTICS]) describes qvalue usage and its "qvalue" is
   non-zero.

   If semantics.

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

   Two examples of its use are: same
   qvalue.

   Examples:

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

5.
   Want-Content-Digest: sha-256
   Want-Content-Digest: sha-512;q=0.3, sha-256;q=1, unixsum;q=0

6.  Digest Algorithm Values

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

      digest-algorithm = token

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

   The Internet Assigned Numbers Authority (IANA) acts as preferred; digest-algorithm token values MUST be compared in a registry for
   case-insensitive fashion.

   Every digest-algorithm defines its computation procedure and encoding
   output.  Unless specified otherwise, comparison of encoded output is
   case-sensitive.

   The "HTTP Digest Algorithm Values Registry", maintained by IANA at
   https://www.iana.org/assignments/http-dig-alg/
   (https://www.iana.org/assignments/http-dig-alg/) registers digest-
   algorithm values.  Registrations MUST include the following fields:

   *  Digest algorithm: the token value.  The registry contains can be used to
      reserve token values

   *  Status: the tokens listed
   below.

   Some digest-algorithms, although registered, rely on vulnerable status of the algorithm.  Use "standard" for
      standardized algorithms without known problems; "experimental" or
      some other appropriate value

      -  e.g. according to the type and MUST not be used: status of the primary document
         in which the algorithm is defined; "deprecated" when the
         algorithm is insecure or otherwise undesirable; "reserved" when
         Digest algorithm references a reserved token value

   *  "md5", see [CMU-836068]  Description: the description of the digest-algorithm and [NO-MD5]; its
      encoding

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

   See  Reference: a set of pointers to the references above primary documents defining the
      digest-algorithm

   The associated encoding for further information.

   sha-256 new digest-algorithms MUST either be
   represented as a quoted string or MUST NOT include ";" or "," in the
   character sets used for the encoding.

   Deprecated digest algorithms MUST NOT be used.

   The registry is initialized with the tokens listed below.

   sha-512
      *  Digest Algorithm: sha-512

      *  Description: The SHA-256 SHA-512 algorithm [RFC6234].  The output of
         this algorithm is encoded using the base64 encoding [RFC4648].

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

      *  Status: standard

   sha-512

   sha-256
      *  Digest Algorithm: sha-256

      *  Description: The SHA-512 SHA-256 algorithm [RFC6234].  The output of
         this algorithm is encoded using the base64 encoding [RFC4648].

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

      *  Status: standard

   md5
      *  Digest Algorithm: 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 is now vulnerable to
         collision attacks.  See [NO-MD5] and [CMU-836068].

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

      *  Status: deprecated

   sha
      *  Digest Algorithm: 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 is now vulnerable to collision attacks.  See
         [NO-SHA1] and [IACR-2020-014].

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

      *  Status: deprecated

   unixsum
      *  Digest Algorithm: 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 deprecated

   unixcksum
      *  Digest Algorithm: 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 deprecated

   adler32
      *  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.

      *  Reference: [RFC1950], this document.

      *  Status: deprecated

   crc32c
      *  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: deprecated.

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

   id-sha-512
      *  Description: The sha-512 digest of the representation-data 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 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 ","

7.  Using Digest in State-Changing Requests

   When the character sets used for the encoding.

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 representation enclosed
   representation, including its metadata, refers to that action.

   In these requests in a state-changing request does not
   describe the target resource, the representation digest MUST be
   computed on the
   representation-data of that action. representation-data.  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" Digest
      MUST be computed on the enclosed representation (see
      Section 10.8 Appendix B.8
      );

   *  if there is a referenced resource "Digest" 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" Digest on the enclosed representation.

   The latter case might be is done according to the HTTP semantics of the given
   method, for example using the "Content-Location" Content-Location header field. field (see
   Section 8.7 of [SEMANTICS]).  In contrast, the "Location" Location header field
   does not affect "Digest" Digest because it is not representation metadata.

6.1.  Digest and PATCH

   In

   For example, in PATCH requests, the representation digest MUST will 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, instead, the representation
   digest MUST will 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

7.1.  Digest and by Content-Location in Responses

   When a state-changing method returns the patch
   document.

7.  Deprecate Negotiation of Content-MD5

   This RFC deprecates Content-Location header
   field, the negotiation of Content-MD5 as it has been
   obsoleted enclosed representation refers to the resource identified
   by [RFC7231].  The "contentMD5" token defined its value and Digest is computed accordingly.  An example is given
   in Section 5
   of [RFC3230] MUST NOT be used as a digest-algorithm. Appendix B.7.

8.  Obsolete  Security Considerations

8.1.  Digest Field Parameters

   Section 4.1.1 and 4.2 of [RFC3230] defined field parameters. Does Not Protect the Full HTTP Message

   This document obsoletes the usage of parameters with "Digest" because this
   feature has specifies a data integrity mechanism that protects HTTP
   representation data or content, but not been widely deployed HTTP header and complicates field-value
   processing.

   [RFC3230] trailer
   fields, from certain kinds of accidental corruption.

   Digest fields are not intended field parameters to provide a common way to attach
   additional information to be 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 general protection against
   malicious tampering with HTTP messages.  This can still be parameterized achieved by defining its own way
   to encode parameters into the representation-data-digest, in
   combining it with other approaches such a
   way as to mitigate transport-layer security risks related to its computation.

9.  Relationship to Subresource Integrity (SRI)

   Subresource
   or digital signatures.

8.2.  Digest for End-to-End Integrity [SRI] is an integrity mechanism that shares
   some similarities

   Digest fields can help detect representation data or content
   modification due to the present document's mechanism.  However,
   there are differences in motivating factors, threat model and
   specification implementation errors, undesired "transforming
   proxies" (see Section 7.7 of integrity digest generation, signalling and
   validation.

   SRI allows [SEMANTICS]) or other actions as the
   data passes across multiple hops or system boundaries.  Even a first-party authority to declare an simple
   mechanism for end-to-end representation data integrity assertion
   on a resource served by a first or third party authority.  This is
   done via the "integrity" attribute that valuable
   because user-agent can be added validate that resource retrieval succeeded
   before handing off to "script" or
   "link" a HTML elements.  Therefore, the 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 assertion is always
   made out-of-band to the because they allow
   piecing together a resource fetch.  In contrast, the "Digest"
   field is supplied in-band alongside the selected representation,
   meaning from different sources with different
   HTTP messaging characteristics.  For example, different servers that an authority can only declare an
   apply different content codings.

   Note that using digest fields alone does not provide end-to-end
   integrity assertion for
   itself. of HTTP messages over multiple hops, since metadata could
   be manipulated at any stage.  Methods to improve the security properties of representation
   digests protect metadata are presented
   discussed in Section 12.  This contrast is interesting
   because on one hand self-assertion is less likely 8.3.

8.3.  Usage in Signatures

   Digital signatures are widely used together with checksums to be affected by
   coordination problems such as the first-party holding stale
   information about the third party, but on the other hand provide
   the self-
   assertion is only as trustworthy as certain identification of the authority that provided it.

   The SRI "integrity" attribute contains origin of a cryptographic hash algorithm message [NIST800-32].
   Such signatures can protect one or more HTTP fields and digest value which is similar to "representation-data-digest"
   (see Section 2).  The major differences there are
   additional considerations when Digest is included 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 set.

   Since digest fields are fully compatible with core HTTP concepts (an example
   is provided in Section 10.3).

   SRI specifies strong requirements hashes of resource representations, they
   explicitly depend on the selection of algorithm for
   generation and validation representation metadata (e.g. the values of digests.  In contrast,
   Content-Type, Content-Encoding etc).  A signature that protects
   Digest but not other representation metadata can expose the requirements
   in this document are weaker.

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

9.1.  Supporting Both SRI and Representation Digest

   The SRI and Representation Digest mechanisms are different Content-Type field-value and
   complementary but one is not capable of replacing cause a digest validation failure at
   the other because
   they have different threat, security and implementation properties.

   A user agent that supports both mechanisms is expected to apply recipient, preventing the
   rules specified for each but since application from accessing the two mechanisms are
   independent,
   representation.  Such an attack consumes the ordering is not important.  However, a user agent
   supporting resources of both could benefit from performing representation digest
   validation first because it does not
   endpoints.  See also Section 7.1.

   Digest fields SHOULD always require a conversion into
   identity encoding.

   There is be used over a chance connection that a user agent supporting both mechanisms may
   find one validates successfully while provides
   integrity at the other fails.  This document
   specifies no requirements or guidance for user agents transport layer that experience
   such cases.

10.  Examples of Unsolicited protects HTTP fields.

   A 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 NOT RECOMMENDED digest-algorithms SHOULD NOT be
   used in signatures.

   Using signatures to protect the content.  For presentation
   purposes, objects that fit completely within the line-length limits
   are presented on checksum of an empty representation
   allows receiving endpoints to detect if an eventual payload has been
   stripped or added.

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

8.4.  Usage in Trailer Fields

   Before sending digest fields in a single line using compact notation with no leading
   space.  Objects trailer section, the sender should
   consider that would exceed line-length limits intermediaries 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 explicitly allowed to drop any
   trailer (see Section 6.5.2 of "Digest" [SEMANTICS]).

   When digest fields are calculated
   inclusive of any space.

10.1.  Server Returns Full Representation Data

   Request:

   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, used in a HEAD request trailer section, the field-values
   are received after the content.  Eager processing of content before
   the trailer section prevents digest validation, possibly leading to
   processing of invalid data.

   Not every digest-algorithm is used suitable for use in the trailer
   section, some may require to retrieve pre-process the checksum of whole payload before
   sending a
   resource.

   The response "Digest" field-value is calculated over message (e.g. see [I-D.thomson-http-mice]).

8.5.  Usage with Encryption

   Digest fields may expose details of encrypted payload when the JSON object
   "{"hello": "world"}", which is not shown because there
   checksum is no payload computed on the unencrypted 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  For example, the client makes a range request and use
   of the server
   responds "id-sha-256" digest-algorithm in conjunction 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:

   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
   encrypted content-coding [RFC8188].

   The request contains a "Digest" field-value calculated on the
   enclosed representation.  It also includes checksum of an "Accept-Encoding: br"
   header field that advertises encrypted payload can change between different
   messages depending on the client supports brotli encoding.

   The response includes encryption algorithm used; in those cases
   its value could not be used to provide a "Content-Encoding: br" that indicates proof of integrity "at rest"
   unless the
   selected representation whole (e.g. encoded) content is brotli encoded. persisted.

8.6.  Algorithm Agility

   The "Digest" field-value security properties of digest-algorithms are not fixed.
   Algorithm Agility (see [RFC7696]) is therefore different compared to achieved by providing
   implementations with flexibility choose digest-algorithms from the request.

   For presentation purposes,
   IANA Digest Algorithm Values registry in Section 9.1.

   To help endpoints distinguish weaker algorithms from stronger ones,
   this document adds to the response body is displayed as IANA Digest Algorithm Values registry 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.

   The response "Digest" field-value depends on new
   "Status" field containing the representation
   metadata header fields, including "Content-Encoding: br" even when most recent appraisal of 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 digest-
   algorithm.

   An endpoint might have 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:

   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 preference for algorithms, such as
   preferring "standard" algorithms over "deprecated" ones.  Transition
   from weak algorithms is computed on the enclosed
   representation (see Section 6).

   The representation enclosed in the response refers to the resource
   identified supported by "Content-Location" negotiation of digest-algorithm
   using Want-Digest or Want-Content-Digest (see [SEMANTICS], Section 6.4.2).
   "Digest" is thus computed on 5) or by
   sending multiple representation-data-digest values from which 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
   receiver chooses.  Endpoints are advised that a "204 No Content" response without content but with the
   same "Digest" field-value would have been legitimate too.

10.8.  POST Response Describes the Request Status

   The request "Digest" field-value is computed on sending multiple values
   consumes resources, which may be wasted if the enclosed
   representation receiver ignores them
   (see Section 6).

   The representation enclosed 3).

8.7.  Duplicate digest-algorithm in the response describes the status of
   the request, 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 "Digest" validation of duplicates is computed left as an implementation
   decision.  Endpoints might select all, some, or none of the values
   for checksum comparison and, based on that enclosed representation.

   Response "Digest" has no explicit relation with the intersection of those
   results, conditionally pass or fail digest validation.

8.8.  Resource exhaustion

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

9.  IANA Considerations

9.1.  Establish the HTTP Digest with PATCH Algorithm Values Registry

   This case is analogous memo sets this specification to a POST request where the target resource
   reflects be the effective request URI.

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

   "Digest" HTTP Digest Algorithm Values (https://www.iana.org/assignments/
   http-dig-alg/) registry.

   IANA is calculated on asked to update the enclosed payload, which corresponds "Reference" for this registry to refer
   this document and to inizialize the patch document.

   The response "Digest" field-value is computed on registry with the complete
   representation tokens defined
   in Section 6.

   This registry uses the Specification Required policy (Section 4.6 of
   [RFC8126]).

9.2.  Obsolete "contentMD5" token in Digest Algorithm

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

   *  Digest Algorithm: contentMD5

   *  Description: Section 5 of [RFC3230] defined the error.

   In the following example a client attempts "contentMD5" token
      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 be used only in accordance with [RFC7807].

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

   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/1.1 404 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 obsoleted and thus mitigate resource
   consumption. MUST
      NOT be used.

   *  Reference: Section 9.2 of this document, Section 5 of [RFC3230].

   *  Status: obsoleted

9.3.  Changes Compared to RFC3230

   The "Digest" field-value is the same as contentMD5 digest-algorithm token defined in Section 10.1
   because "Digest" is designed 5 of
   [RFC3230] has been added 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:

   HTTP/1.1 200 OK
   Content-Type: application/json
   Transfer-Encoding: chunked
   Trailer: HTTP 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) Algorithm Values Registry
   with 2 spaced of
   leading indentation.

   "Digest" is media-type agnostic and does not provide canonicalization
   algorithms for specific formats.  Examples of "Digest" the "obsoleted" status.

   All digest-algorithms defined in [RFC3230] 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 now "deprecated".

9.4.  Changes Compared 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"}

11.2.  Server Selects Algorithm Unsupported by Client RFC5843

   The client requests a "sha" digest only. digest-algorithm tokens for "MD5", "SHA", "SHA-256", "SHA-512"
   have been updated to lowercase.

   The server is currently
   free status of "MD5" and "SHA" has been updated 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 "deprecated", and Returns an Error
   their description has been modified accordingly.

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

12.2.  Broken Cryptographic Algorithms

   Cryptographic "id-sha-512" 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 have been added to collisions attacks, so they MUST NOT be used with "Digest".

12.3.  Other Deprecated Algorithms

   The ADLER32 algorithm defined the
   registry.

9.5.  Want-Digest Field Registration

   This section registers the Want-Digest field in [RFC1950] has been deprecated by
   [RFC3309] because, under certain conditions, it provides weak
   detection the "Hypertext
   Transfer Protocol (HTTP) Field Name Registry" [SEMANTICS].

   Field name: Want-Digest

   Status: permanent

   Specification document(s): Section 5 of errors.  It is now NOT RECOMMENDED for use with
   "Digest".

12.4. this document

9.6.  Digest for End-to-End Integrity

   "Digest" only covers Field Registration

   This section registers the "representation data" and not Digest field in the
   "representation metadata".  "Digest" could help protect "Hypertext Transfer
   Protocol (HTTP) Field Name Registry" [SEMANTICS].

   Field name: Digest

   Status: permanent

   Specification document(s): Section 3 of this document

9.7.  Want-Content-Digest Field Registration

   This section registers the
   "representation data" from buggy manipulation, undesired
   "transforming proxies" (see Want-Content-Digest field in the
   "Hypertext Transfer Protocol (HTTP) Field Name Registry" [SEMANTICS].

   Field name: Want-Content-Digest

   Status: permanent

   Specification document(s): Section 7.7 5 of [SEMANTICS]) or other
   actions as this document

9.8.  Content-Digest Field Registration

   This section registers 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.

12.6.  Usage Content-Digest field in Signatures

   Digital signatures are widely used together with checksums to provide
   the certain identification of the origin "Hypertext
   Transfer Protocol (HTTP) Field Name Registry" [SEMANTICS].

   Field name: Content-Digest

   Status: permanent

   Specification document(s): Section 4 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 document

10.  References

10.1.  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 hash Shambles", 5
              January 2020, <https://eprint.iacr.org/2020/014.pdf>.

   [NIST800-32]
              National Institute of a resource representation, it
   explicitly depends on the "representation metadata" (eg. the values Standards and Technology, U.S.
              Department of "Content-Type", "Content-Encoding" etc).  A signature that
   protects "Digest" but not other "representation metadata" can expose
   the communication Commerce, "Introduction to tampering.  For example, an actor could
   manipulate the "Content-Type" field-value Public Key
              Technology 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 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 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 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 the trailer
   section, some may require RFCs to pre-process the whole payload before
   sending a message (eg. see [I-D.thomson-http-mice]).

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.

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 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 Values (https://www.iana.org/assignments/
   http-dig-alg/http-dig-alg.xhtml) registry.

13.2.  The "status" Field 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 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 HTTP",
              RFC 3230, DOI 10.17487/RFC3230, January 2002,
              <https://www.rfc-editor.org/rfc/rfc3230>.

   [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 the "Status" fields are described
   below.

   Status
      *  "standard" 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 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 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>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations 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:

   *  Digest Algorithm: As specified in Section 5.

   *  Description: As specified RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/rfc/rfc8126>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in Section 5.

   *  Status: As specified 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 Section 5.

13.5.  Update "UNIXcksum" Digest Algorithm

   This memo updates the "UNIXcksum" digest-algorithm Progress, Internet-Draft, draft-ietf-
              httpbis-semantics-19, 12 September 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
              semantics-19>.

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

10.2.  Informative References

   [HTTP11]   Fielding, R. T., Nottingham, M., and J. Reschke,
              "HTTP/1.1", Work in the HTTP Digest
   Algorithm Progress, Internet-Draft, draft-ietf-
              httpbis-messaging-19, 12 September 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
              messaging-19>.

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

   [I-D.thomson-http-mice]
              Thomson, M. and SCTP.  The 32-bit output
      is encoded J. Yasskin, "Merkle Integrity Content
              Encoding", Work 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 Progress, Internet-Draft, draft-
              thomson-http-mice-03, 13 August 2018,
              <https://datatracker.ietf.org/doc/html/draft-thomson-http-
              mice-03>.

   [NO-MD5]   Turner, S. and crc32c=A72A4DF are both valid checksums L. Chen, "Updated Security Considerations
              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.

   *  Status: As specified in Section 5.

13.8.  Obsolete "ADLER32" Digest Algorithm

   This memo updates MD5 Message-Digest and the "ADLER32" digest-algorithm in 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 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 SHA-0 and 8 ASCII characters
      from 0-9, A-F, 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>.

   [RFC7696]  Housley, R., "Guidelines for Cryptographic Algorithm
              Agility and a-f; leading 0's are allowed).  For example,
      adler32=03da0195 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 adler32=3DA0195 are both valid checksums 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>.

Appendix A.  Resource Representation and Representation-Data

   The following examples show how representation metadata, payload
   transformations and method impacts on 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 content.  When
   the "contentMD5" token
      to be used only in Want-Digest.  This token content contains non-printable characters (e.g. when it 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
   compressed) it is shown as a Base64-encoded string.

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

   {"hello": "world"}

   Figure 1: Request containing a JSON object without any content coding

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

   H4sIAItWyFwC/6tWSlSyUlAypANQqgUAREcqfG0AAAA=

          Figure 2: Request containing a gzip-encoded JSON object

   Now the "id-sha-256" digest-algorithm in same content conveys a malformed JSON object, because 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.

13.11.
   request does not indicate a content coding.

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

   H4sIAItWyFwC/6tWSlSyUlAypANQqgUAREcqfG0AAAA=

                Figure 3: Request containing malformed JSON

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

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

                   Figure 4: Request for partial content

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

   iwgAla3RXA==

       Figure 5: Partial response from a gzip-encoded representation

   The "id-sha-512" Digest Algorithm

   This memo registers method can also alter the "id-sha-512" digest-algorithm in content.  For example, 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 response to
   lowercase.

   The status of "MD5" has been updated
   a HEAD request does not carry content.

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

                           Figure 6: HEAD request

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

             Figure 7: Response to "deprecated", and its
   description states that this algorithm MUST NOT be used.

   The status HEAD request (empty content)

   Finally, the semantics 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 an HTTP response might decouple the
   registry.

13.13.  Want-Digest Field Registration

   This section registers
   effective request URI from the enclosed representation.  In the
   example response below, the "Want-Digest" Content-Location header field in indicates
   that the "Hypertext
   Transfer Protocol (HTTP) Field Name Registry" [SEMANTICS].

   Field name: "Want-Digest"

   Status: permanent

   Specification document(s): Section 4 enclosed representation refers to the resource available at
   /authors/123, even though the request is directed to /authors/.

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

   {"author": "Camilleri"}

                           Figure 8: POST request

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

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

              Figure 9: Response with Content-Location header

Appendix B.  Examples of this document

13.14. Unsolicited Digest Field Registration

   This section registers

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

   Some examples include JSON objects in the "Hypertext Transfer
   Protocol (HTTP) Field Name Registry" [SEMANTICS].

   Field name: "Digest"
   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 content.  For presentation
   purposes, objects that fit completely within 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 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.

   Checksum mechanisms defined in HTTP",
              RFC 3230, DOI 10.17487/RFC3230, January 2002,
              <https://www.rfc-editor.org/rfc/rfc3230>.

   [RFC3309]  Stone, J., Stewart, R., this document are media-type agnostic
   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, do not provide canonicalization algorithms for specific formats.
   Examples are calculated inclusive of any space.  While examples can
   include both fields, Digest and Base64 Content-Digest can be returned
   independently.

B.1.  Server Returns Full Representation 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.

   In this example, the message content conveys complete representation
   data, so Digest and P. Overell, "Augmented BNF Content-Digest have the same value.

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

                     Figure 10: GET request 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 an item

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

   {"hello": "world"}

                  Figure 11: Response with Content-Digest

B.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.  Content-Digest is computed on empty content.

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

                    Figure 12: HEAD request for an item

   HTTP/1.1 200 OK
   Content-Type: application/json
   Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
   Content-Digest: sha-256=47DEQpj8HBSa+/TImW+5JCeuQeRkm5NMpJWZG3hSuFU=

       Figure 13: Response with both Content-Digest and Digest; empty
                                  content

B.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"}, while the Content-Digest
   field-value is computed on the message content "hello".

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

                   Figure 14: Request for HTTP Instance
              Digests", RFC 5843, DOI 10.17487/RFC5843, April 2010,
              <https://www.rfc-editor.org/rfc/rfc5843>.

   [RFC6234]  Eastlake 3rd, D. partial content

   HTTP/1.1 206 Partial Content
   Content-Type: application/json
   Content-Range: bytes 1-7/18
   Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
   Content-Digest: sha-256=Wqdirjg/u3J688ejbUlApbjECpiUUtIwT8lY/z81Tno=

   "hello"

      Figure 15: Partial response with both Content-Digest and T. Hansen, "US Secure Hash Algorithms
              (SHA Digest

B.4.  Client and SHA-based HMAC 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.

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

   {"hello": "world"}

                     Figure 16: PUT Request with Digest

   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==

            Figure 17: Response with Digest of encoded response

B.5.  Client Provides Full Representation Data, Server Provides No
      Representation Data

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

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

   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"}

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

                   Figure 18: Empty response with Digest

B.6.  Client and HKDF)", RFC 6234,
              DOI 10.17487/RFC6234, May 2011,
              <https://www.rfc-editor.org/rfc/rfc6234>.

   [RFC7405]  Kyzivat, P., "Case-Sensitive String Support Server Provide Full Representation Data, Client Uses
      id-sha-256.

   The response contains two digest values:

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

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

   {"hello": "world"}

                     Figure 19: PUT Request with Digest

   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==

             Figure 20: Response with Digest of Encoded Content

B.7.  POST Response does not Reference the Request URI

   The request Digest field-value is computed on the enclosed
   representation (see Section 7).

   The representation enclosed in Progress, Internet-Draft, draft-ietf-
              httpbis-semantics-15, 30 March 2021,
              <https://tools.ietf.org/html/draft-ietf-httpbis-semantics-
              15>.

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

   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"}

                    Figure 21: POST Request with Digest

   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"
   }

                Figure 22: Response with Digest of Resource

   Note that a 204 No Content response without content but with the same
   Digest field-value would have been legitimate too.  In that case,
   Content-Digest would have been computed on an empty content.

B.8.  POST Response Describes the Request Status

   The request Digest field-value is computed on the enclosed
   representation (see Section 7).

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

14.2.  Informative References

   [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 representation enclosed 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 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.

   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"}
                    Figure 23: POST Request with Digest

   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"
   }

             Figure 24: Response with Digest of Representation

B.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 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 [RFC7396].

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

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

   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"}

                    Figure 25: PATCH Request with Digest

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

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

             Figure 26: Response with Digest of Representation

   Note that a 204 No Content response without content but with the MD5 Message-Digest and same
   Digest field-value would have been legitimate too.

B.10.  Error responses

   In error responses, 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 representation-data does not necessarily
   refer to 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>.

   [RFC7696]  Housley, R., "Guidelines for Cryptographic Algorithm
              Agility target resource.  Instead, it refers to the
   representation of the error.

   In the following example, a client sends the same request from
   Figure 25 to patch the resource located at /books/123.  However, the
   resource does not exist 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. 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.

   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
   }

          Figure 27: Response with Digest of Error Representation

B.11.  Use with Trailer Fields 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., Transfer Coding

   An origin server sends Digest as trailer field, so it can calculate
   digest-value while streaming content and J. Weinberger,
              "Subresource Integrity", W3C Recommendation REC-SRI-
              20160623, 23 June 2016,
              <https://www.w3.org/TR/2016/REC-SRI-20160623/>. thus mitigate resource
   consumption.  The Digest field-value is the same as in Appendix A.  Resource Representation and Representation-Data B.1
   because Digest is designed to be independent from the use of one or
   more transfer codings (see Section 2).

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

                           Figure 28: GET Request

   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=

                  Figure 29: Chunked Response with Digest

Appendix C.  Examples of Want-Digest Solicited Digest

   The following examples show how representation metadata, payload
   transformations demonstrate interactions where a client
   solicits a Digest using Want-Digest.  The behavior of Content-Digest
   and method impacts on Want-Content-Digest is identical.

   Some examples include JSON objects in the message and content.  When  For presentation
   purposes, objects that fit completely within the content contains non-printable characters (eg. when it is
   compressed) it is shown as base64-encoded string.

   A request with line-length limits
   are presented on a JSON object without 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.

   Checksum mechanisms described in this document are media-type
   agnostic and do not provide canonicalization algorithms for specific
   formats.  Examples are calculated inclusive of any content coding.

   Request:

   PUT /entries/1234 space.

C.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.

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

                  Figure 30: GET Request with Want-Digest

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

   {"hello": "world"}

   Here is

                Figure 31: Response with Different Algorithm

C.2.  Server Selects Algorithm Unsupported by Client

   The client requests a gzip-compressed JSON object using "sha" digest only.  The server is currently
   free to reply with a content coding.

   Request:

   PUT /entries/1234 Digest containing an unsupported algorithm.

   GET /items/123 HTTP/1.1
   Host: foo.example
   Content-Type: application/json
   Content-Encoding: gzip

   H4sIAItWyFwC/6tWSlSyUlAypANQqgUAREcqfG0AAAA=

   Now the same content conveys a malformed JSON object.

   Request:

   PUT /entries/1234
   Want-Digest: sha;q=1

                  Figure 32: GET Request with Want-Digest

   HTTP/1.1
   Host: foo.example 200 OK
   Content-Type: application/json

   H4sIAItWyFwC/6tWSlSyUlAypANQqgUAREcqfG0AAAA=

   A Range-Request alters the content, conveying
   Digest: id-sha-512=WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+TaPm
                      +AbwAgBWnrIiYllu7BNNyealdVLvRwE\nmTHWXvJwew==

   {"hello": "world"}

               Figure 33: Response with Unsupported Algorithm

C.3.  Server Does Not Support Client Algorithm and Returns an Error

   The client requests a partial
   representation.

   Request: "sha" Digest, the server advises "sha-256" and
   "sha-512".

   GET /entries/1234 /items/123 HTTP/1.1
   Host: foo.example
   Range: bytes=1-7

   Response:
   Want-Digest: sha;q=1

                  Figure 34: GET Request with Want-Digest

   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 400 Bad Request
   Want-Digest: sha-256, sha-512

                    Figure 35: Response with Want-Digest

Appendix D.  Changes from RFC3230
D.1.  Deprecate Negotiation of Content-MD5

   This RFC deprecates the negotiation of Content-MD5 as it has been
   obsoleted by [RFC7231].

   See Section 4 for a new checksum negotiation mechanism for HTTP
   message 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

D.2.  Obsolete Digest Field Parameters

   Sections 4.1.1 and 4.2 of [RFC3230] defined field parameters.  This
   document obsoletes the semantics 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 HTTP response might decouple the
   effective request URI from input to validate the enclosed representation.  In checksum, an attacker
   could alter them to steer the
   example response below, validation behavior.

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

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 enclosed representation refers MICE content coding.

   Thanks to Julian Reschke for his valuable contributions to this
   document, and to the resource available at
   "/authors/123".

   Request:

   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. following contributors that have helped improve
   this specification by reporting bugs, asking smart questions,
   drafting or reviewing text, and evaluating open issues: Mike Bishop,
   Brian Campbell, Matthew Kerwin, James Manger, Tommy Pauly, Sean
   Turner, and Erik Wilde.

FAQ

   _RFC Editor: Please remove this section before publication._

   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" Digest with "PATCH" PATCH method?

       See Section 6. 7.

   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"? Want-Digest?

       The contentMD5 token defined in Section 5 of [RFC3230] is
       deprecated by Section 7. Appendix D.1.

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

   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._

Since draft-ietf-httpbis-digest-headers-05

   *  Reboot digest-algorithm values registry #1567

   *  Add Content-Digest #1542
   *  Remove SRI section #1478

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

   *  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" Content-Location

Authors' Addresses

   Roberto Polli
   Team Digitale, Italian Government
   Italy

   Email: robipolli@gmail.com

   Lucas Pardue
   Cloudflare

   Email: lucaspardue.24.7@gmail.com