HTTP                                                            R. Polli
Internet-Draft                         Team Digitale, Italian Government
Obsoletes: 3230 (if approved)                                  L. Pardue
Intended status: Standards Track                              Cloudflare
Expires: 26 November 2022                                    25 May 2022


                             Digest Fields
                  draft-ietf-httpbis-digest-headers-09

Abstract

   This document defines HTTP fields that support integrity digests.
   The Content-Digest field can be used for the integrity of HTTP
   message content.  The Repr-Digest field can be used for the integrity
   of HTTP representations.  Want-Content-Digest and Want-Repr-Digest
   can be used to indicate a sender's interest and preferences for
   receiving the respective Integrity fields.

   This document obsoletes RFC 3230 and the Digest and Want-Digest HTTP
   fields.

About This Document

   This note is to be removed before publishing as an RFC.

   Status information for this document may be found at
   https://datatracker.ietf.org/doc/draft-ietf-httpbis-digest-headers/.

   Discussion of this document takes place on the HTTP Working Group
   mailing list (mailto:ietf-http-wg@w3.org), which is archived at
   https://lists.w3.org/Archives/Public/ietf-http-wg/.  Working Group
   information can be found at https://httpwg.org/.

   Source for this draft and an issue tracker can be found at
   https://github.com/httpwg/http-extensions/labels/digest-headers.

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/.





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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
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   This Internet-Draft will expire on 26 November 2022.

Copyright Notice

   Copyright (c) 2022 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
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Document Structure  . . . . . . . . . . . . . . . . . . .   4
     1.2.  Concept Overview  . . . . . . . . . . . . . . . . . . . .   4
     1.3.  Obsoleting RFC 3230 . . . . . . . . . . . . . . . . . . .   5
     1.4.  Notational Conventions  . . . . . . . . . . . . . . . . .   6
   2.  The Content-Digest Field  . . . . . . . . . . . . . . . . . .   7
   3.  The Repr-Digest Field . . . . . . . . . . . . . . . . . . . .   8
     3.1.  Using Repr-Digest in State-Changing Requests  . . . . . .   9
     3.2.  Repr-Digest and Content-Location in Responses . . . . . .  10
   4.  Integrity preference fields . . . . . . . . . . . . . . . . .  10
   5.  Hash Algorithms for HTTP Digest Fields Registry . . . . . . .  11
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
     6.1.  HTTP Messages Are Not Protected In Full . . . . . . . . .  13
     6.2.  End-to-End Integrity  . . . . . . . . . . . . . . . . . .  13
     6.3.  Usage in Signatures . . . . . . . . . . . . . . . . . . .  13
     6.4.  Usage in Trailer Fields . . . . . . . . . . . . . . . . .  14
     6.5.  Usage with Encryption . . . . . . . . . . . . . . . . . .  14
     6.6.  Algorithm Agility . . . . . . . . . . . . . . . . . . . .  14
     6.7.  Resource exhaustion . . . . . . . . . . . . . . . . . . .  15
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
     7.1.  HTTP Field Name Registration  . . . . . . . . . . . . . .  15
     7.2.  Establish the Hash Algorithms for HTTP Digest Fields
           Registry  . . . . . . . . . . . . . . . . . . . . . . . .  16
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  16



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     8.2.  Informative References  . . . . . . . . . . . . . . . . .  18
   Appendix A.  Resource Representation and Representation Data  . .  19
   Appendix B.  Examples of Unsolicited Digest . . . . . . . . . . .  21
     B.1.  Server Returns Full Representation Data . . . . . . . . .  22
     B.2.  Server Returns No Representation Data . . . . . . . . . .  22
     B.3.  Server Returns Partial Representation Data  . . . . . . .  23
     B.4.  Client and Server Provide Full Representation Data  . . .  24
     B.5.  Client Provides Full Representation Data, Server Provides
            No Representation Data . . . . . . . . . . . . . . . . .  24
     B.6.  Client and Server Provide Full Representation Data  . . .  25
     B.7.  POST Response does not Reference the Request URI  . . . .  26
     B.8.  POST Response Describes the Request Status  . . . . . . .  26
     B.9.  Digest with PATCH . . . . . . . . . . . . . . . . . . . .  27
     B.10. Error responses . . . . . . . . . . . . . . . . . . . . .  28
     B.11. Use with Trailer Fields and Transfer Coding . . . . . . .  29
   Appendix C.  Examples of Want-Repr-Digest Solicited Digest  . . .  29
     C.1.  Server Selects Client's Least Preferred Algorithm . . . .  30
     C.2.  Server Selects Algorithm Unsupported by Client  . . . . .  30
     C.3.  Server Does Not Support Client Algorithm and Returns an
           Error . . . . . . . . . . . . . . . . . . . . . . . . . .  30
   Appendix D.  Migrating from RFC 3230  . . . . . . . . . . . . . .  31
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  32
   Code Samples  . . . . . . . . . . . . . . . . . . . . . . . . . .  32
   Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  33
     Since draft-ietf-httpbis-digest-headers-08  . . . . . . . . . .  33
     Since draft-ietf-httpbis-digest-headers-07  . . . . . . . . . .  34
     Since draft-ietf-httpbis-digest-headers-06  . . . . . . . . . .  34
     Since draft-ietf-httpbis-digest-headers-05  . . . . . . . . . .  34
     Since draft-ietf-httpbis-digest-headers-04  . . . . . . . . . .  34
     Since draft-ietf-httpbis-digest-headers-03  . . . . . . . . . .  34
     Since draft-ietf-httpbis-digest-headers-02  . . . . . . . . . .  35
     Since draft-ietf-httpbis-digest-headers-01  . . . . . . . . . .  35
     Since draft-ietf-httpbis-digest-headers-00  . . . . . . . . . .  35
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  36

1.  Introduction

   HTTP does not define the means to protect the data integrity of
   content or representations.  When HTTP messages are transferred
   between endpoints, lower layer features or properties such as TCP
   checksums or TLS records [RFC2818] can provide some integrity
   protection.  However, transport-oriented integrity provides a limited
   utility because it is opaque to the application layer and only covers
   the extent of a single connection.  HTTP messages often travel over a
   chain of separate connections, in between connections there is a
   possibility for unintended or malicious data corruption.  An HTTP
   integrity mechanism can provide the means for endpoints, or
   applications using HTTP, to detect data corruption and make a choice



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   about how to act on it.  An example use case is to aid fault
   detection and diagnosis across system boundaries.

   This document defines two digest integrity mechanisms for HTTP.
   First, content integrity, which acts on conveyed content (Section 6.4
   of [SEMANTICS]).  Second, representation data integrity, which acts
   on representation data (Section 3.2 of [SEMANTICS]).  This supports
   advanced use cases such as validating the integrity of a resource
   that was reconstructed from parts retrieved using multiple requests
   or connections.

   This document obsoletes RFC 3230 and therefore the Digest and Want-
   Digest HTTP fields; see Section 1.3.

1.1.  Document Structure

   This document is structured as follows:

   *  Section 2 defines the Content-Digest request and response header
      and trailer field,

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

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

   *  Section 5 describes algorithms and their relation to the fields
      defined in this document,

   *  Section 3.1 details computing representation digests,

   *  Appendix B and Appendix C provide examples of using Repr-Digest
      and Want-Repr-Digest.

1.2.  Concept Overview

   The HTTP fields defined in this document can be used for HTTP
   integrity.  Senders choose a hashing algorithm and calculate a digest
   from an input related to the HTTP message, the algorithm identifier
   and digest are transmitted in an HTTP field.  Receivers can validate
   the digest for integrity purposes.  Hashing algorithms are registered
   in the "Hash Algorithms for HTTP Digest Fields" (see Section 5).

   Selecting the data on which digests are calculated depends on the use
   case of HTTP messages.  This document provides different headers for
   HTTP representation data and HTTP content.




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   There are use-cases where a simple digest of the HTTP content bytes
   is required.  The Content-Digest request and response header and
   trailer field is defined to support digests of content (Section 3.2
   of [SEMANTICS]); see Section 2.

   For more advanced use-cases, the Repr-Digest request and response
   header and trailer field (Section 3) is defined.  It contains a
   digest value computed by applying a hashing algorithm to selected
   representation data (Section 3.2 of [SEMANTICS]).  Basing Repr-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 partial
   representation of a resource, such as Range Requests (see
   Section 14.2 of [SEMANTICS]).

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

   Content-Digest and Repr-Digest are collectively termed Integrity
   fields.  Want-Content-Digest and Want-Repr-Digest are collectively
   termed Integrity preference fields.

   Integrity fields are tied to the Content-Encoding and Content-Type
   header fields.  Therefore, a given resource may have multiple
   different digest values when transferred with HTTP.

   Integrity 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.  For example, HTTP
   Message Signatures [SIGNATURES] could be used to sign Integrity
   fields, thus providing coverage for HTTP content or representation
   data.

   This specification does not define means for authentication,
   authorization or privacy.

1.3.  Obsoleting RFC 3230

   [RFC3230] defined the Digest and Want-Digest HTTP fields for HTTP
   integrity.  It also coined the term "instance" and "instance
   manipulation" in order to explain concepts that are now more
   universally defined, and implemented, as HTTP semantics such as
   selected representation data (Section 3.2 of [SEMANTICS]).





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   Experience has shown that implementations of [RFC3230] have
   interpreted the meaning of "instance" inconsistently, leading to
   interoperability issues.  The most common mistake being the
   calculation of the digest using (what we now call) message content,
   rather than using (what we now call) representation data as was
   originally intended.  Interestingly, time has also shown that a
   digest of message content can be beneficial for some use cases.  So
   it is difficult to detect if non-conformance to [RFC3230] is
   intentional or unintentional.

   In order to address potential inconsistencies and ambiguity across
   implementations of Digest and Want-Digest, this document obsoletes
   [RFC3230].  The Integrity fields (Section 3 and Section 2) and
   Integrity preference fields (Section 4) defined in this document are
   better aligned with current HTTP semantics and have names that more
   clearly articulate the intended usages.

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].

   This document uses the following terminology from Section 3 of
   [STRUCTURED-FIELDS] to specify syntax and parsing: Boolean, Byte
   Sequence, Dictionary, Integer, and List.

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

   Hashing algorithm names respect the casing used in their definition
   document (e.g.  SHA-1, CRC32c) whereas hashing algorithm keys are
   quoted (e.g. "sha", "crc32c").

   The term "checksum" describes the output of the application of an
   algorithm to a sequence of bytes, whereas "digest" is only used in
   relation to the value contained in the fields.

   Integrity fields: collective term for Content-Digest and Repr-Digest





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   Integrity preference fields: collective term for Want-Repr-Digest and
   Want-Content-Digest

2.  The Content-Digest Field

   The Content-Digest HTTP field can be used in requests and responses
   to communicate digests that are calculated using a hashing algorithm
   applied to the actual message content (see Section 6.4 of
   [SEMANTICS]).  It is a Dictionary (see Section 3.2 of
   [STRUCTURED-FIELDS]) where each:

   *  key conveys the hashing algorithm (see Section 5) used to compute
      the digest;

   *  value is a Byte Sequence (Section 3.3.5 of [STRUCTURED-FIELDS]),
      that contains the output of the digest calculation.

   For example:

   NOTE: '\' line wrapping per RFC 8792

   Content-Digest: \
     sha-512=:WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+TaPm+AbwAgBWnrI\
     iYllu7BNNyealdVLvRwEmTHWXvJwew==:

   The Dictionary type can be used, for example, to attach multiple
   digests calculated using different hashing algorithms in order to
   support a population of endpoints with different or evolving
   capabilities.  Such an approach could support transitions away from
   weaker algorithms (see Section 6.6).

   NOTE: '\' line wrapping per RFC 8792

   Repr-Digest: \
     sha-256=:4REjxQ4yrqUVicfSKYNO/cF9zNj5ANbzgDZt3/h3Qxo=:,\
     sha-512=:WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+TaPm+AbwAgBWnrI\
     iYllu7BNNyealdVLvRwEmTHWXvJwew==:

   A recipient MAY ignore any or all digests.  This allows the recipient
   to choose which hashing algorithm(s) to use for validation instead of
   verifying every digest.

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






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   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].

3.  The Repr-Digest Field

   The Repr-Digest HTTP field can be used in requests and responses to
   communicate digests that are calculated using a hashing algorithm
   applied to the entire selected representation data (see Section 8.1
   of [SEMANTICS]).

   Representations take into account the effect of the HTTP semantics on
   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 HTTP representation concepts, several examples are
   provided in Appendix A.

   When a message has no representation data it is still possible to
   assert that no representation data was sent by computing the digest
   on an empty string (see Section 6.3).

   Repr-Digest is a Dictionary (see Section 3.2 of [STRUCTURED-FIELDS])
   where each:

   *  key conveys the hashing algorithm (see Section 5) used to compute
      the digest;

   *  value is a Byte Sequence that contains the output of the digest
      calculation.

   For example:

   NOTE: '\' line wrapping per RFC 8792

   Repr-Digest: \
     sha-512=:WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+TaPm+AbwAgBWnrI\
     iYllu7BNNyealdVLvRwEmTHWXvJwew==:

   The Dictionary type can be used, for example, to attach multiple
   digests calculated using different hashing algorithms in order to
   support a population of endpoints with different or evolving
   capabilities.  Such an approach could support transitions away from
   weaker algorithms (see Section 6.6).






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   NOTE: '\' line wrapping per RFC 8792

   Repr-Digest: \
     sha-256=:4REjxQ4yrqUVicfSKYNO/cF9zNj5ANbzgDZt3/h3Qxo=:,\
     sha-512=:WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+TaPm+AbwAgBWnrI\
     iYllu7BNNyealdVLvRwEmTHWXvJwew==:

   A recipient MAY ignore any or all digests.  This allows the recipient
   to choose which hashing algorithm(s) to use for validation instead of
   verifying every digest.

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

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

3.1.  Using Repr-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 3).

   In responses,

   *  if the representation describes the status of the request, Repr-
      Digest MUST be computed on the enclosed representation (see
      Appendix B.8 );

   *  if there is a referenced resource Repr-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 Repr-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 Repr-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



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   Section 2 of [PATCH]).  In responses, instead, the representation
   digest will be computed on the selected representation of the patched
   resource.

3.2.  Repr-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 Repr-Digest is computed accordingly.  An example is
   given in Appendix B.7.

4.  Integrity preference fields

   Senders can indicate their interest in Integrity fields and hashing
   algorithm preferences using the Want-Content-Digest or Want-Repr-
   Digest fields.  These can be used in both requests and responses.

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

   Want-Repr-Digest indicates that the sender would like to receive a
   representation digest on messages associated with the request URI and
   representation metadata, using the Repr-Digest field.

   If Want-Content-Digest or Want-Repr-Digest are used in a response, it
   indicates that the server would like the client to provide the
   respective Integrity field on future requests.

   Want-Content-Digest and Want-Repr-Digest are of type Dictionary where
   each:

   *  key conveys the hashing algorithm (see Section 5);

   *  value is an Integer (Section 3.3.1 of [STRUCTURED-FIELDS]) that
      conveys an ascending, relative, weighted preference.  It must be
      in the range 0 to 10 inclusive. 1 is the least preferred, 10 is
      the most preferred, and a value of 0 means "not acceptable".

   Examples:

   Want-Repr-Digest: sha-256=1
   Want-Repr-Digest: sha-512=3, sha-256=10, unixsum=0
   Want-Content-Digest: sha-256=1
   Want-Content-Digest: sha-512=3, sha-256=10, unixsum=0






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5.  Hash Algorithms for HTTP Digest Fields Registry

   The "Hash Algorithms for HTTP Digest Fields", maintained by IANA at
   https://www.iana.org/assignments/http-dig-alg/
   (https://www.iana.org/assignments/http-dig-alg/), registers
   algorithms for use with the Integrity and Integrity preference fields
   defined in this document.

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

   Registrations MUST include the following fields:

   *  Algorithm Key: the Structured Fields key value used in Content-
      Digest, Repr-Digest, Want-Content-Digest, or Want-Repr-Digest
      field Dictionary member keys

   *  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; "insecure" when the
         algorithm is insecure; "reserved" when the algorithm references
         a reserved token value

   *  Description: a short description of the algorithm

   *  Reference(s): a set of pointers to the primary documents defining
      the algorithm and key

   Insecure hashing algorithms MAY be used to preserve integrity against
   corruption, but MUST NOT be used in a potentially adversarial
   setting; for example, when signing Integrity fields' values for
   authenticity.

   The entries in Table 1 are registered by this document.














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   +===========+==========+============================+==============+
   | Algorithm | Status   | Description                | Reference(s) |
   | Key       |          |                            |              |
   +===========+==========+============================+==============+
   | sha-512   | standard | The SHA-512 algorithm.     | [RFC6234],   |
   |           |          |                            | [RFC4648],   |
   |           |          |                            | this         |
   |           |          |                            | document.    |
   +-----------+----------+----------------------------+--------------+
   | sha-256   | standard | The SHA-256 algorithm.     | [RFC6234],   |
   |           |          |                            | [RFC4648],   |
   |           |          |                            | this         |
   |           |          |                            | document.    |
   +-----------+----------+----------------------------+--------------+
   | md5       | insecure | The MD5 algorithm.  It is  | [RFC1321],   |
   |           |          | vulnerable to collision    | [RFC4648],   |
   |           |          | attacks; see [NO-MD5] and  | this         |
   |           |          | [CMU-836068]               | document.    |
   +-----------+----------+----------------------------+--------------+
   | sha       | insecure | The SHA-1 algorithm.  It   | [RFC3174],   |
   |           |          | is vulnerable to collision | [RFC4648],   |
   |           |          | attacks; see [NO-SHA] and  | [RFC6234]    |
   |           |          | [IACR-2020-014]            | this         |
   |           |          |                            | document.    |
   +-----------+----------+----------------------------+--------------+
   | unixsum   | insecure | The algorithm used by the  | [RFC4648],   |
   |           |          | UNIX "sum" command.        | [RFC6234],   |
   |           |          |                            | [UNIX], this |
   |           |          |                            | document.    |
   +-----------+----------+----------------------------+--------------+
   | unixcksum | insecure | The algorithm used by the  | [RFC4648],   |
   |           |          | UNIX "cksum" command.      | [RFC6234],   |
   |           |          |                            | [UNIX], this |
   |           |          |                            | document.    |
   +-----------+----------+----------------------------+--------------+
   | adler     | insecure | The ADLER32 algorithm.     | [RFC1950],   |
   |           |          |                            | this         |
   |           |          |                            | document.    |
   +-----------+----------+----------------------------+--------------+
   | crc32c    | insecure | The CRC32c algorithm.      | [RFC4960]    |
   |           |          |                            | appendix B,  |
   |           |          |                            | this         |
   |           |          |                            | document.    |
   +-----------+----------+----------------------------+--------------+

                     Table 1: Initial Hash Algorithms





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6.  Security Considerations

6.1.  HTTP Messages Are Not Protected In Full

   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 corruption.

   Integrity 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 (for example, HTTP Message Signatures
   [SIGNATURES]).

6.2.  End-to-End Integrity

   Integrity 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 a user agent can validate that resource retrieval succeeded
   before handing off to a HTML parser, video player etc. for parsing.

   Note that using these mechanisms 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 6.3.

6.3.  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 Integrity fields are included in this
   set.

   There are no restrictions placed on the type or format of digitial
   signature that Integrity fields can be used with.  One possible
   approach is to combine them with HTTP Message Signatures
   [SIGNATURES].










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   Digests explicitly depend on the "representation metadata" (e.g. the
   values of Content-Type, Content-Encoding etc).  A signature that
   protects Integrity fields 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 3.2.

   Signatures are likely to be deemed an adversarial setting when
   applying Integrity fields; see Section 5.  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 Integrity fields, including digests' de-duplication
   or combining different field values (see Section 5.2 of [SEMANTICS])
   might affect signature validation.

6.4.  Usage in Trailer Fields

   Before sending Integrity 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 Integrity 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 hashing 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]).

6.5.  Usage with Encryption

   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.

6.6.  Algorithm Agility

   The security properties of hashing algorithms are not fixed.
   Algorithm Agility (see [RFC7696]) is achieved by providing
   implementations with flexibility to choose hashing algorithms from
   the IANA Hash Algorithms for HTTP Digest Fields registry; see
   Section 7.2.



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   The "standard" algorithms listed in this document are suitable for
   many purposes, including adversarial situations where hash functions
   might need to provide resistance to collision, first-preimage and
   second-preimage attacks.  Algorithms listed as "insecure" either
   provide none of these properties, or are known to be weak (see
   [NO-MD5] and [NO-SHA]).

   For adversarial situations, which of the "standard" algorithms are
   acceptable will depend on the level of protection the circumstances
   demand.  As there is no negotiation, endpoints that depend on a
   digest for security will be vulnerable to attacks on the weakest
   algorithm they are willing to accept.

   Transition from weak algorithms is supported by negotiation of
   hashing algorithm using Want-Content-Digest or Want-Repr-Digest (see
   Section 4) or by sending multiple digests 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).

   While algorithm agility allows the migration to stronger algorithms
   it does not prevent the use of weaker algorithms.  Integrity fields
   do not provide any mitigiations for downgrade or substitution attacks
   (see Section 1 of [RFC6211]) of the hashing algorithm.  To protect
   against such attacks, endpoints could restrict their set of supported
   algorithms to stronger ones and protect the fields value by using TLS
   and/or digital signatures.

6.7.  Resource exhaustion

   Integrity 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.

7.  IANA Considerations

7.1.  HTTP Field Name Registration

   IANA is asked to update the "Hypertext Transfer Protocol (HTTP) Field
   Name Registry" registry ([SEMANTICS]) according to the table below:










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     +=====================+===========+============================+
     | Field Name          | Status    | Reference                  |
     +=====================+===========+============================+
     | Content-Digest      | permanent | Section 2 of this document |
     +---------------------+-----------+----------------------------+
     | Repr-Digest         | permanent | Section 3 of this document |
     +---------------------+-----------+----------------------------+
     | Want-Content-Digest | permanent | Section 4 of this document |
     +---------------------+-----------+----------------------------+
     | Want-Repr-Digest    | permanent | Section 4 of this document |
     +---------------------+-----------+----------------------------+
     | Digest              | obsoleted | [RFC3230], Section 1.3 of  |
     |                     |           | this document              |
     +---------------------+-----------+----------------------------+
     | Want-Digest         | obsoleted | [RFC3230], Section 1.3 of  |
     |                     |           | this document              |
     +---------------------+-----------+----------------------------+

                                 Table 2

7.2.  Establish the Hash Algorithms for HTTP Digest Fields Registry

   This memo sets this specification to be the establishing document for
   the Hash Algorithms for HTTP Digest Fields
   (https://www.iana.org/assignments/http-structured-dig-alg/) registry
   defined in Section 5.

   IANA is asked to initialize the registry with the entries in Table 1.

8.  References

8.1.  Normative References

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

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

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



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   [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>.

   [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>.



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   [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>.

   [STRUCTURED-FIELDS]
              Nottingham, M. and P-H. Kamp, "Structured Field Values for
              HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021,
              <https://www.rfc-editor.org/rfc/rfc8941>.

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

8.2.  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.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-SHA]   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>.




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

   [RFC6211]  Schaad, J., "Cryptographic Message Syntax (CMS) Algorithm
              Identifier Protection Attribute", RFC 6211,
              DOI 10.17487/RFC6211, April 2011,
              <https://www.rfc-editor.org/rfc/rfc6211>.

   [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>.

   [SIGNATURES]
              Backman, A., Richer, J., and M. Sporny, "HTTP Message
              Signatures", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-message-signatures-09, 6 March 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
              message-signatures-09>.

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








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   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

   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




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   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

Appendix B.  Examples of Unsolicited Digest

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








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   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, Content-Digest and Repr-Digest can be returned
   independently.

B.1.  Server Returns Full Representation Data

   In this example, the message content conveys complete representation
   data.  This means that in the response, Content-Digest and Repr-
   Digest are both computed over the JSON object {"hello": "world"}, and
   thus have the same value.

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

                     Figure 10: GET request for an item

   NOTE: '\' line wrapping per RFC 8792

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

   {"hello": "world"}

     Figure 11: Response with identical Repr-Digest and 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 Content-Digest field-value is computed on empty content.
   Repr-Digest is calculated over the JSON object {"hello": "world"},
   which is not shown because there is no payload data.

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

                    Figure 12: HEAD request for an item







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   NOTE: '\' line wrapping per RFC 8792

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

       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.

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

                   Figure 14: Request for partial content

   NOTE: '\' line wrapping per RFC 8792

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

   "hello"

    Figure 15: Partial response with both Content-Digest and Repr-Digest

   In the response message above, note that the Repr-Digest and Content-
   Digests are different.  The Repr-Digest field-value is calculated
   across the entire JSON object {"hello": "world"}, and the field is

   Repr-Digest: sha-256=:X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=:

   However, since the message content is constrained to bytes 1-7, the
   Content-Digest field-value is calculated over the byte sequence
   "hello", thus resulting in





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   NOTE: '\' line wrapping per RFC 8792

   Content-Digest: \
     sha-256=:Wqdirjg/u3J688ejbUlApbjECpiUUtIwT8lY/z81Tno=:

B.4.  Client and Server Provide Full Representation Data

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

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



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

   {"hello": "world"}

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

                   Figure 18: Empty response with Digest

B.6.  Client and Server Provide Full Representation Data

   The response contains two digest values using different algorithms.

   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
   Repr-Digest: sha-256=:X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=:

   {"hello": "world"}

                     Figure 19: PUT Request with Digest

   NOTE: '\' line wrapping per RFC 8792

   HTTP/1.1 200 OK
   Content-Type: application/json
   Content-Encoding: br
   Content-Location: /items/123
   Repr-Digest: \
     sha-256=:4REjxQ4yrqUVicfSKYNO/cF9zNj5ANbzgDZt3/h3Qxo=:,\
     sha-512=:pxo7aYzcGI88pnDnoSmAnaOEVys0MABhgvHY9+VI+ElE60jBCwnMPyA/\
     s3NF3ZO5oIWA7lf8ukk+5KJzm3p5og==:

   iwiAeyJoZWxsbyI6ICJ3b3JsZCJ9Aw==

             Figure 20: Response with Digest of Encoded Content




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

   The request Repr-Digest field-value is computed on the enclosed
   representation (see Section 3.1).

   The representation enclosed in the response refers to the resource
   identified by Content-Location (see Section 6.4.2 of [SEMANTICS]).
   Repr-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
   Repr-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
   Repr-Digest: 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
   Repr-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 Repr-Digest field-value is computed on the enclosed
   representation (see Section 3.1).

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

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



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

   {"title": "New Title"}

                    Figure 23: POST Request with Digest

   HTTP/1.1 201 Created
   Content-Type: application/json
   Repr-Digest: 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].

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

   The response Repr-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
   Repr-Digest: sha-256=:bWopGGNiZtbVgHsG+I4knzfEJpmmmQHf7RHDXA3o1hQ=:

   {"title": "New Title"}




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                    Figure 25: PATCH Request with Digest

   HTTP/1.1 200 OK
   Content-Type: application/json
   Repr-Digest: 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
   Repr-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 Repr-Digest field-value is computed on this enclosed
   representation.

   HTTP/1.1 404 Not Found
   Content-Type: application/problem+json
   Repr-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










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B.11.  Use with Trailer Fields and Transfer Coding

   An origin server sends Repr-Digest as trailer field, so it can
   calculate digest-value while streaming content and thus mitigate
   resource consumption.  The Repr-Digest field-value is the same as in
   Appendix B.1 because Repr-Digest is designed to be independent from
   the use of one or more transfer codings (see Section 3).

   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
   Repr-Digest: sha-256=:X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=:

                  Figure 29: Chunked Response with Digest

Appendix C.  Examples of Want-Repr-Digest Solicited Digest

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






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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-Repr-Digest: sha-256=3, sha=10

                Figure 30: GET Request with Want-Repr-Digest

   HTTP/1.1 200 OK
   Content-Type: application/json
   Repr-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 because that is the only algorithm
   it supports.  The server is not obliged to produce a response
   containing a "sha" digest, it instead uses a different algorithm.

   GET /items/123 HTTP/1.1
   Host: foo.example
   Want-Repr-Digest: sha=10

                Figure 32: GET Request with Want-Repr-Digest

   NOTE: '\' line wrapping per RFC 8792

   HTTP/1.1 200 OK
   Content-Type: application/json
   Repr-Digest: \
     sha-512=:WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+TaPm+AbwAgBWnrI\
     iYllu7BNNyealdVLvRwEmTHWXvJwew==:

   {"hello": "world"}

               Figure 33: Response with Unsupported Algorithm

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

   Appendix C.2 is an example where a server ignores the client's
   preferred digest algorithm.  Alternatively a server can also reject
   the request and return an error.



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   In this example, the client requests a "sha" Repr-Digest, and the
   server returns an error with problem details [RFC7807] contained in
   the content.  The problem details contain a list of the hashing
   algorithms that the server supports.  This is purely an example, this
   specification does not define any format or requirements for such
   content.

   GET /items/123 HTTP/1.1
   Host: foo.example
   Want-Repr-Digest: sha=10

                Figure 34: GET Request with Want-Repr-Digest

   HTTP/1.1 400 Bad Request
   Content-Type: application/problem+json

   {
     "title": "Bad Request",
     "detail": "Supported hashing algorithms: sha-256, sha-512",
     "status": 400
   }

          Figure 35: Response advertising the supported algorithms

Appendix D.  Migrating from RFC 3230

   HTTP digests are computed by applying a hashing algorithm to input
   data.  RFC 3230 defined the input data as an "instance", a term it
   also defined.  The concept of instance has since been superseded by
   the HTTP semantic term "representation".  It is understood that some
   implementations of RFC 3230 mistook "instance" to mean HTTP content.
   Using content for the Digest field is an error that leads to
   interoperability problems between peers that implement RFC 3230.

   For the uncertainty of doubt, RFC 3230 was only ever intended to use
   what HTTP now defines as selected representation data.  The semantic
   concept of digest and representation are explained alongside the
   definition of Representation-Digest Section 3.

   While the syntax of Digest and Repr-Digest are different, the
   considerations and examples this document gives to Repr-Digest apply
   equally to Digest because they operate on the same input data.  See
   Section 3.1, Section 6 and Section 6.3.

   RFC 3230 could never communicate the digest of HTTP message content
   in the Digest field; Content-Digest now provides that capability.





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Acknowledgements

   This document is based on ideas from [RFC3230], so thanks to J.
   Mogul and A.  Van Hoff for their great work.  The original idea of
   refreshing RFC3230 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, Justin Richer, and Erik Wilde.

Code Samples

   _RFC Editor: Please remove this section before publication._

   How can I generate and validate the Repr-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 | hashing algorithm | digest-value")
   print("Identity | sha256 |", digest(item))
   # Encoding | hashing algorithm | digest-value
   # Identity | sha256 | X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=

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

   print("Encoding | hashing algorithm | digest-value")
   print("Identity | sha512 |", digest(item, algorithm=hashlib.sha512))
   print("Brotli | sha512 |", digest(item, algorithm=hashlib.sha512,
                                       encoding=brotli.compress))
   # Encoding | hashing algorithm | digest-value
   # Identity | sha512 |b'WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+TaPm'
   #                     '+AbwAgBWnrIiYllu7BNNyealdVLvRwEmTHWXvJwew=='
   # Brotli | sha512 | b'pxo7aYzcGI88pnDnoSmAnaOEVys0MABhgvHY9+VI+ElE6'
   #                   '0jBCwnMPyA/s3NF3ZO5oIWA7lf8ukk+5KJzm3p5og=='

Changes

   _RFC Editor: Please remove this section before publication._

Since draft-ietf-httpbis-digest-headers-08

   *  Add note about migrating from RFC 3230. #1968, #1971




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   *  Clarify what Want-* means in responses. #2097

   *  Editorial changes to structure and to align to HTTP style guide.

Since draft-ietf-httpbis-digest-headers-07

   *  Introduced Repr-Digest and Want-Repr-Digest, and deprecated Digest
      and Want-Digest.  Use of Structured Fields. #1993, #1919

   *  IANA refactoring. #1983

   *  No normative text in security considerations. #1972

Since draft-ietf-httpbis-digest-headers-06

   *  Remove id-sha-256 and id-sha-512 from the list of supported
      algorithms #855

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




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   *  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



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   *  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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