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Versions: 00 01 02 03 04 05 06                                          
HTTP                                                            R. Polli
Internet-Draft                         Team Digitale, Italian Government
Obsoletes: 3230 (if approved)                                  L. Pardue
Intended status: Standards Track                              Cloudflare
Expires: 31 March 2022                                 27 September 2021


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

Abstract

   This document defines HTTP fields that support integrity checksums.
   The Digest 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 and Want-Content-
   Digest can be used to indicate a sender's desire to receive integrity
   fields respectively.

   This document obsoletes RFC 3230.

Note to Readers

   _RFC EDITOR: please remove this section before publication_

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

   The source code and issues list for this draft can be found at
   https://github.com/httpwg/http-extensions (https://github.com/httpwg/
   http-extensions).

Status of This Memo

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

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

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



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   This Internet-Draft will expire on 31 March 2022.

Copyright Notice

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

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Document Structure  . . . . . . . . . . . . . . . . . . .   4
     1.2.  Concept Overview  . . . . . . . . . . . . . . . . . . . .   4
     1.3.  Replacing RFC 3230  . . . . . . . . . . . . . . . . . . .   5
     1.4.  Notational Conventions  . . . . . . . . . . . . . . . . .   6
   2.  Representation Digest . . . . . . . . . . . . . . . . . . . .   6
   3.  The Digest Field  . . . . . . . . . . . . . . . . . . . . . .   7
   4.  The Content-Digest Field  . . . . . . . . . . . . . . . . . .   8
   5.  Want-Digest and Want-Content-Digest Fields  . . . . . . . . .   8
   6.  Digest Algorithm Values . . . . . . . . . . . . . . . . . . .   9
   7.  Using Digest in State-Changing Requests . . . . . . . . . . .  13
     7.1.  Digest and Content-Location in Responses  . . . . . . . .  13
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
     8.1.  Digest Does Not Protect the Full HTTP Message . . . . . .  13
     8.2.  Digest for End-to-End Integrity . . . . . . . . . . . . .  14
     8.3.  Usage in Signatures . . . . . . . . . . . . . . . . . . .  14
     8.4.  Usage in Trailer Fields . . . . . . . . . . . . . . . . .  15
     8.5.  Usage with Encryption . . . . . . . . . . . . . . . . . .  15
     8.6.  Algorithm Agility . . . . . . . . . . . . . . . . . . . .  15
     8.7.  Duplicate digest-algorithm in field value . . . . . . . .  16
     8.8.  Resource exhaustion . . . . . . . . . . . . . . . . . . .  16
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  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 . . . . . . . . . . . . . . .  17
     9.4.  Changes Compared to RFC5843 . . . . . . . . . . . . . . .  17
     9.5.  Want-Digest Field Registration  . . . . . . . . . . . . .  17
     9.6.  Digest Field Registration . . . . . . . . . . . . . . . .  18
     9.7.  Want-Content-Digest Field Registration  . . . . . . . . .  18
     9.8.  Content-Digest Field Registration . . . . . . . . . . . .  18



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   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  18
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  18
     10.2.  Informative References . . . . . . . . . . . . . . . . .  20
   Appendix A.  Resource Representation and Representation-Data  . .  22
   Appendix B.  Examples of Unsolicited Digest . . . . . . . . . . .  24
     B.1.  Server Returns Full Representation Data . . . . . . . . .  24
     B.2.  Server Returns No Representation Data . . . . . . . . . .  24
     B.3.  Server Returns Partial Representation Data  . . . . . . .  25
     B.4.  Client and Server Provide Full Representation Data  . . .  25
     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. . . . . . . . . . . . . . . . . . . . .  27
     B.7.  POST Response does not Reference the Request URI  . . . .  27
     B.8.  POST Response Describes the Request Status  . . . . . . .  28
     B.9.  Digest with PATCH . . . . . . . . . . . . . . . . . . . .  29
     B.10. Error responses . . . . . . . . . . . . . . . . . . . . .  30
     B.11. Use with Trailer Fields and Transfer Coding . . . . . . .  30
   Appendix C.  Examples of Want-Digest Solicited Digest . . . . . .  31
     C.1.  Server Selects Client's Least Preferred Algorithm . . . .  31
     C.2.  Server Selects Algorithm Unsupported by Client  . . . . .  32
     C.3.  Server Does Not Support Client Algorithm and Returns an
           Error . . . . . . . . . . . . . . . . . . . . . . . . . .  32
   Appendix D.  Changes from RFC3230 . . . . . . . . . . . . . . . .  32
     D.1.  Deprecate Negotiation of Content-MD5  . . . . . . . . . .  33
     D.2.  Obsolete Digest Field Parameters  . . . . . . . . . . . .  33
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  33
   FAQ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  33
   Code Samples  . . . . . . . . . . . . . . . . . . . . . . . . . .  35
   Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  36
     Since draft-ietf-httpbis-digest-headers-05  . . . . . . . . . .  36
     Since draft-ietf-httpbis-digest-headers-04  . . . . . . . . . .  37
     Since draft-ietf-httpbis-digest-headers-03  . . . . . . . . . .  37
     Since draft-ietf-httpbis-digest-headers-02  . . . . . . . . . .  37
     Since draft-ietf-httpbis-digest-headers-01  . . . . . . . . . .  37
     Since draft-ietf-httpbis-digest-headers-00  . . . . . . . . . .  38
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  38

1.  Introduction

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






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   This document defines two digest integrity 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 transport integrity, offering the
   potential to detect programming errors and corruption of data in
   flight or at 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.  Finally, they can be used to validate integrity when
   reconstructing a resource fetched using different HTTP connections.

   This document obsoletes [RFC3230].

1.1.  Document Structure

   This document is structured as follows:

   *  Section 2 describes concepts related to representation digests,

   *  Section 3 defines the Digest request and response header and
      trailer field,

   *  Section 4 defines the Content-Digest request and response header
      and trailer field,

   *  Section 5 defines the Want-Digest and Want-Content-Digest request
      and response header and trailer field,

   *  Section 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 using Digest and
      Want-Digest.

1.2.  Concept Overview

   This document defines the Digest request and response header and
   trailer field; see Section 3.  At a high level, the value contains a
   checksum, computed over selected representation 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 requires some
   sort of manipulation to be considered a representation or conveys a



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   partial representation of a resource, such as Range Requests (see
   Section 14.2 of [SEMANTICS]).

   To support use-cases where a simple checksum of the content bytes is
   required, this document introduces the Content-Digest request and
   response header and trailer field; see Section 4.

   Digest and Content-Digest support algorithm agility.  The Want-Digest
   and Want-Content-Digest fields allows endpoints to express interest
   in Digest and Content-Digest respectively, and preference of
   algorithms in either.

   Digest field calculations are tied to the Content-Encoding and
   Content-Type header fields.  Therefore, a given resource may have
   multiple different checksum values when transferred with HTTP.  To
   allow both parties to exchange a simple checksum with no content
   codings (see Section 8.4.1 of [SEMANTICS]), two more digest-
   algorithms are added ("id-sha-256" and "id-sha-512").

   Digest fields do not provide integrity for HTTP messages or fields.
   However, they can be combined with other mechanisms that protect
   metadata, such as digital signatures, in order to protect the phases
   of an HTTP exchange in whole or in part.

   This specification does not define means for authentication,
   authorization or 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 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 "instance" has been superseded by
   selected representation.

   This document replaces [RFC3230].  The Digest 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 field
   parameters are obsoleted Appendix D.2 and the algorithm table has
   been updated to reflect the current state of the art.





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1.4.  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   This document uses the Augmented BNF defined in [RFC5234] and updated
   by [RFC7405] along with the "#rule" extension defined in
   Section 5.6.1 of [SEMANTICS] 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
   (e.g.  SHA-1, CRC32c) whereas digest-algorithm tokens are quoted
   (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.

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

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

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

   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 8.3).



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   The checksum is computed using one of the digest-algorithms listed in
   the HTTP Digest Algorithm Values Registry (see Section 6) and then
   encoded in the associated format.

3.  The Digest Field

   The 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==

   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 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 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-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.





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4.  The Content-Digest Field

   The Content-Digest field 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.






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   Want-Digest indicates the sender's desire to receive a representation
   digest on messages associated with the request URI and representation
   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 indicates the sender's digest-algorithm preferences.
   Section 12.4.2 of [SEMANTICS]) describes qvalue usage and semantics.

   Senders can provide multiple digest-algorithm items with the same
   qvalue.

   Examples:

   Want-Digest: sha-256
   Want-Digest: sha-512;q=0.3, sha-256;q=1, unixsum;q=0
   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; digest-algorithm token values MUST be compared in a
   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 can be used to
      reserve token values




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   *  Status: the 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 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

   *  Description: the description of the digest-algorithm and its
      encoding

   *  Reference: a set of pointers to the primary documents defining the
      digest-algorithm

   The associated encoding for 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-512 algorithm [RFC6234].  The output of
         this algorithm is encoded using the base64 encoding [RFC4648].

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

      *  Status: standard

   sha-256
      *  Digest Algorithm: sha-256

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

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

      *  Status: standard

   md5
      *  Digest Algorithm: md5






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      *  Description: The MD5 algorithm, as specified in [RFC1321].  The
         output of this algorithm is encoded using the base64 encoding
         [RFC4648].  This digest-algorithm 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 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: 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: deprecated





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   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, the following additional digest-
   algorithms are defined:

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

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

      *  Status: standard

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

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



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      *  Status: standard

7.  Using Digest in State-Changing Requests

   When the representation enclosed in a state-changing request does not
   describe the target resource, the representation digest MUST be
   computed on the 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
      MUST be computed on the enclosed representation (see Appendix B.8
      );

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

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

   For example, in PATCH requests, the representation digest 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 responses, instead, the representation
   digest will be computed on the selected representation of the patched
   resource.

7.1.  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.  An example is given
   in Appendix B.7.

8.  Security Considerations

8.1.  Digest Does Not Protect the Full HTTP Message

   This document specifies a data integrity mechanism that protects HTTP
   representation data or content, but not HTTP header and trailer
   fields, from certain kinds of accidental corruption.




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   Digest fields are 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.

8.2.  Digest for End-to-End Integrity

   Digest fields can help detect representation data or content
   modification due to implementation errors, undesired "transforming
   proxies" (see Section 7.7 of [SEMANTICS]) or other actions as the
   data passes across multiple hops or system boundaries.  Even a simple
   mechanism for end-to-end representation data integrity is valuable
   because user-agent can validate that resource retrieval succeeded
   before handing off to a HTML parser, video player etc. for parsing.

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

   Note that using digest fields 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 8.3.

8.3.  Usage in Signatures

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

   Since digest fields are hashes of resource representations, they
   explicitly depend on the representation metadata (e.g. the values of
   Content-Type, Content-Encoding etc).  A signature that protects
   Digest but not other representation metadata can expose the
   communication to tampering.  For example, an actor could manipulate
   the Content-Type field-value and cause a digest validation failure at
   the recipient, preventing the application from accessing the
   representation.  Such an attack consumes the resources of both
   endpoints.  See also Section 7.1.

   Digest fields 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.



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   Using signatures to protect the 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 trailer section, the sender should
   consider that intermediaries are explicitly allowed to drop any
   trailer (see Section 6.5.2 of [SEMANTICS]).

   When digest fields are used in a 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 suitable for use in the trailer
   section, some may require to pre-process the whole payload before
   sending a message (e.g. see [I-D.thomson-http-mice]).

8.5.  Usage with Encryption

   Digest fields 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 checksum 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.

8.6.  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 9.1.

   To help endpoints distinguish 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.





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   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 or Want-Content-Digest (see Section 5) 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).

8.7.  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.

8.8.  Resource exhaustion

   Digest fields 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.

9.  IANA Considerations

9.1.  Establish the HTTP Digest Algorithm Values Registry

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

   IANA is asked to update the "Reference" for this registry to refer
   this document and to inizialize the registry with the tokens defined
   in Section 6.

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






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9.2.  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/) registry:

   *  Digest Algorithm: contentMD5

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

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

   *  Status: obsoleted

9.3.  Changes Compared to RFC3230

   The contentMD5 digest-algorithm token defined in Section 5 of
   [RFC3230] has been added to the HTTP Digest Algorithm Values Registry
   with the "obsoleted" status.

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

9.4.  Changes Compared to RFC5843

   The digest-algorithm tokens for "MD5", "SHA", "SHA-256", "SHA-512"
   have been updated to lowercase.

   The status of "MD5" and "SHA" has been updated to "deprecated", and
   their description has been modified accordingly.

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

9.5.  Want-Digest Field Registration

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

   Field name: Want-Digest

   Status: permanent

   Specification document(s): Section 5 of this document







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9.6.  Digest Field Registration

   This section registers the Digest field in the "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 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 5 of this document

9.8.  Content-Digest Field Registration

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

   Field name: Content-Digest

   Status: permanent

   Specification document(s): Section 4 of this 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 Shambles", 5
              January 2020, <https://eprint.iacr.org/2020/014.pdf>.






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

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

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

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

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

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

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

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

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

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






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   [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 in 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 RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.

   [SEMANTICS]
              Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
              Semantics", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-semantics-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 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 for
              HTTP", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-header-structure-19, 3 June 2020,
              <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
              header-structure-19>.









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

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

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

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

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

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

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

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

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

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





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Appendix A.  Resource Representation and Representation-Data

   The following examples show how representation metadata, payload
   transformations and method impacts on the message and content.  When
   the content contains non-printable characters (e.g. when it is
   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 same content conveys a malformed JSON object, because the
   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








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   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 method can also alter the content.  For example, the response to
   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 HEAD request (empty content)

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



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Appendix B.  Examples of Unsolicited Digest

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

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

   Checksum mechanisms defined in this document are media-type agnostic
   and do not provide canonicalization algorithms for specific formats.
   Examples are calculated inclusive of any space.  While examples can
   include both fields, Digest and Content-Digest can be returned
   independently.

B.1.  Server Returns Full Representation Data

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

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

                     Figure 10: GET request for 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.





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   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 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 Digest

B.4.  Client and Server Provide Full Representation Data

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

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

   For presentation purposes, the response body is displayed as a
   Base64-encoded string because it contains non-printable characters.



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   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






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B.6.  Client and Server Provide Full Representation Data, Client Uses
      id-sha-256.

   The response contains two digest values:

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

   *  one taking into account the Content-Encoding.

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

   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 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.








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

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

   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"}




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                    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 [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











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   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 same
   Digest field-value would have been legitimate too.

B.10.  Error responses

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

   In the following example, a client sends the same request from
   Figure 25 to patch the resource located at /books/123.  However, the
   resource does not exist and the server generates a 404 response with
   a body that describes the error in accordance with [RFC7807].

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

   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 Transfer Coding

   An origin server sends Digest as trailer field, so it can calculate
   digest-value while streaming content and thus mitigate resource
   consumption.  The Digest field-value is the same as in Appendix B.1
   because Digest is designed to be independent from the use of one or
   more transfer codings (see Section 2).




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   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 demonstrate interactions where a client
   solicits a Digest using Want-Digest.  The behavior of Content-Digest
   and Want-Content-Digest is identical.

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

   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 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



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

   {"hello": "world"}

                Figure 31: Response with Different Algorithm

C.2.  Server Selects Algorithm Unsupported by Client

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

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

                  Figure 32: GET Request with Want-Digest

   HTTP/1.1 200 OK
   Content-Type: application/json
   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 "sha" Digest, the server advises "sha-256" and
   "sha-512".

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

                  Figure 34: GET Request with Want-Digest

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

                    Figure 35: Response with Want-Digest

Appendix D.  Changes from RFC3230






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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.

D.2.  Obsolete Digest Field Parameters

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

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

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

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.

   Thanks to Julian Reschke for his valuable contributions to this
   document, and to the 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.




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   2.  Why remove references to instance manipulation?

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

   3.  How to use Digest with PATCH method?

       See Section 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?

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

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

   7.  Does this specification change supported algorithms?

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

   8.  What about mid-stream trailer fields?




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       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.

Code Samples

   _RFC Editor: Please remove this section before publication._

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

   The following python3 code can be used to generate digests for JSON
   objects using 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.






























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  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




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   *  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




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   *  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

Authors' Addresses

   Roberto Polli
   Team Digitale, Italian Government
   Italy

   Email: robipolli@gmail.com


   Lucas Pardue
   Cloudflare

   Email: lucaspardue.24.7@gmail.com









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