HTTP Problem Types for Digest Fields
draft-ietf-httpapi-digest-fields-problem-types-06

Building Blocks for HTTP APIs                                  M. Kleidl
Internet-Draft                                               Transloadit
Intended status: Standards Track                               L. Pardue
Expires: 26 December 2026                                     Cloudflare
                                                                R. Polli
                                                                 Par-Tec
                                                            24 June 2026

                  HTTP Problem Types for Digest Fields
           draft-ietf-httpapi-digest-fields-problem-types-06

Abstract

   This document specifies HTTP problem types that servers can use in
   responses to problems encountered while dealing with a request
   carrying integrity fields and integrity preference fields.  Using an
   HTTP problem type, the server can provide machine-readable error
   details to aid debugging or error reporting.

About This Document

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

   The latest revision of this draft can be found at https://ietf-wg-
   httpapi.github.io/digest-fields-problem-types/draft-ietf-httpapi-
   digest-fields-problem-types.html.  Status information for this
   document may be found at https://datatracker.ietf.org/doc/draft-ietf-
   httpapi-digest-fields-problem-types/.

   Discussion of this document takes place on the Building Blocks for
   HTTP APIs Working Group mailing list (mailto:httpapi@ietf.org), which
   is archived at https://mailarchive.ietf.org/arch/browse/httpapi/.
   Subscribe at https://www.ietf.org/mailman/listinfo/httpapi/.

   Source for this draft and an issue tracker can be found at
   https://github.com/ietf-wg-httpapi/digest-fields-problem-types.

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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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   3
   3.  Problem Types . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Unsupported Hashing Algorithms  . . . . . . . . . . . . .   4
     3.2.  Invalid Digest Values . . . . . . . . . . . . . . . . . .   6
     3.3.  Mismatched Digest Values  . . . . . . . . . . . . . . . .   8
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
     5.1.  Registration of "digest-unsupported-algorithms" Problem
           Type  . . . . . . . . . . . . . . . . . . . . . . . . . .  10
     5.2.  Registration of "digest-invalid-values" Problem Type  . .  10
     5.3.  Registration of "digest-mismatched-values" Problem
           Type  . . . . . . . . . . . . . . . . . . . . . . . . . .  10
   6.  Normative References  . . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   [DIGEST] and [UNENC-DIGEST] define HTTP fields for exchanging
   integrity digests and preferences, but do not specify, require, or
   recommend any specific behavior for error handling relating to
   integrity by design.  The responsibility is instead delegated to
   applications.  This document defines a set of HTTP problem types
   ([PROBLEM]) that can be used by server applications to indicate that
   a problem was encountered while dealing with a request carrying

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   integrity fields and integrity preference fields.

   For example, a request message may include content alongside Content-
   Digest and Repr-Digest fields that use a digest algorithm the server
   does not support.  An application could decide to reject this request
   because it cannot validate the integrity.  Using an HTTP problem
   type, the server can provide machine-readable error details to aid
   debugging or error reporting, as shown in the following example.

   # NOTE: '\' line wrapping per RFC 8792

   HTTP/1.1 400 Bad Request
   Content-Type: application/problem+json
   Want-Content-Digest: sha-512=3, sha-256=10

   {
     "type": "https://iana.org/assignments/http-problem-types#\
       digest-unsupported-algorithms",
     "title": "Unsupported hashing algorithms",
     "unsupported_algorithms": [
       {
         "algorithm": "foo",
         "header": "Want-Content-Digest"
       }
     ]
   }

2.  Conventions and Definitions

   Some examples in this document contain long lines that may be folded,
   as described in [RFC8792].

   The terms "integrity fields" and "integrity preference fields" in
   this document are to be interpreted as described in [DIGEST] and
   updated in [UNENC-DIGEST].

   The term "problem type" in this document is to be interpreted as
   described in [PROBLEM].

   The terms "request", "response", "intermediary", "sender", "client",
   and "server" are from [HTTP].

   The problem types in this document are defined using JSON [JSON].
   They can be serialized into an equivalent XML format as outlined in
   Appendix B of [PROBLEM].

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3.  Problem Types

   The following section defines three problem types to express common
   problems that occur when handling integrity or integrity preference
   fields on the server.  These problem types use the digest- prefix in
   their type URI.  Other problem types that are defined outside this
   document, yet specific to digest related problems, may reuse this
   prefix.

   Requests can include multiple integrity or integrity preference
   fields.  For example, they may use the Content-Digest and Repr-Digest
   fields simultaneously or express preferences for content and
   representation digests at the same time.  Therefore, similar problems
   can appear multiple times for one request.  The problem types defined
   in this document allow expressing multiple appearances, while each
   time identifying the corresponding header that contained the
   problematic value.

   Generally, clients that consume problem details cannot rely on the
   presence of any problem type field; see Section 3.1 of [PROBLEM].
   The problem types in this document define extension members
   (Section 3.2 of [PROBLEM]) that are optional for servers to populate
   and send.  While populating all extension members can improve the
   usefulness of the problem type response, omission of any extension
   member defined in this document has no special meaning.

3.1.  Unsupported Hashing Algorithms

   This section defines the "https://iana.org/assignments/http-problem-
   types#digest-unsupported-algorithms" problem type.  A server can use
   this problem type to communicate to the client that one or more of
   the hashing algorithms referenced in the integrity or integrity
   preference fields present in the request are not supported.

   For this problem type, the unsupported_algorithms extension member is
   defined, whose value is a JSON [JSON] array of entries identifying
   each unsupported algorithm.  Each entry in the array is a JSON object
   with the following members:

   *  The algorithm member is a JSON string containing the algorithm key
      of the unsupported algorithm.

   *  The header member is a JSON string containing the name of the
      integrity or integrity preference field that referenced the
      unsupported algorithm.

   The response can include the corresponding integrity preference field
   to indicate the server's algorithm support and preference.

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   This problem type is a hint to the client about algorithm support,
   which the client could use to retry the request with different,
   supported algorithms.

   The following example shows a request with the content {"title": "New
   Title"} (plus LF).  The digest fields use the MD5 algorithm, which is
   not supported by the server as the algorithm is deprecated.

   POST /books HTTP/1.1
   Host: foo.example
   Content-Type: application/json
   Accept: application/json
   Repr-Digest: md5=:Uwq9xB4MJtDTknVOSEE1WA==:
   Content-Digest: md5=:Uwq9xB4MJtDTknVOSEE1WA==:
   Unencoded-Digest: md5=:Uwq9xB4MJtDTknVOSEE1WA==:

   {"title": "New Title"}

        Figure 1: A request with md5 integrity fields, which are not
                          supported by the server

   # NOTE: '\' line wrapping per RFC 8792

   HTTP/1.1 400 Bad Request
   Content-Type: application/problem+json
   Want-Repr-Digest: sha-512=10, md5=0
   Want-Content-Digest: sha-512=10, md5=0
   Want-Unencoded-Digest: sha-512=10, md5=0

   {
     "type": "https://iana.org/assignments/http-problem-types#\
       digest-unsupported-algorithms",
     "title": "Unsupported hashing algorithms",
     "unsupported_algorithms": [
       {
         "algorithm": "md5",
         "header": "Repr-Digest"
       },
       {
         "algorithm": "md5",
         "header": "Content-Digest"
       },
       {
         "algorithm": "md5",
         "header": "Unencoded-Digest"
       }
     ]
   }

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       Figure 2: Response indicating the problem and advertising the
                            supported algorithms

   This problem type can also be used when a request contains an
   integrity preference field with an unsupported algorithm.  For
   example:

   GET /items/123 HTTP/1.1
   Host: foo.example
   Want-Repr-Digest: md5=10

      Figure 3: A request with a md5 integrity preference field, which
                       is not supported by the server

   # NOTE: '\' line wrapping per RFC 8792

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

   {
     "type": "https://iana.org/assignments/http-problem-types#\
       digest-unsupported-algorithms",
     "title": "Unsupported hashing algorithms",
     "unsupported_algorithms": [
       {
         "algorithm": "md5",
         "header": "Want-Repr-Digest"
       }
     ]
   }

                 Figure 4: Response indicating the problem

3.2.  Invalid Digest Values

   This section defines the "https://iana.org/assignments/http-problem-
   types#digest-invalid-values" problem type.  A server can use this
   problem type when responding to a request, whose integrity fields
   include a digest value that cannot be generated by the corresponding
   hashing algorithm.  For example, if the digest value of the sha-512
   hashing algorithm is not 64 bytes long, it cannot be a valid SHA-512
   digest value and the server can skip computing the digest value.
   This problem type cannot be used if the server is not able to parse
   the integrity fields and obtain a value according to Section 4.5 of
   [STRUCTURED-FIELDS], for example because of a syntax error.

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   For this problem type, the invalid_digests extension member is
   defined, whose value is a JSON [JSON] array of entries identifying
   each invalid digest.  Each entry in the array is a JSON object with
   the following members:

   *  The algorithm member is a JSON string containing the algorithm
      key.

   *  The header member is a JSON string containing the name of the
      integrity field that contained the invalid digest value.

   *  The reason member is a JSON string containing a human-readable
      description why the value is considered invalid.

   This problem type indicates a fault in the sender's calculation or
   encoding of the digest value.  A retry of the same request without
   modification will likely not yield a successful response.

   The following example shows a request with the content {"hello":
   "world"} (plus LF), but the digest has been truncated.  The
   subsequent response indicates the invalid SHA-512 digest.

   PUT /items/123 HTTP/1.1
   Host: foo.example
   Content-Type: application/json
   Repr-Digest: sha-512=:YMAam51Jz/jOATT6/zvHrLVgOYTGFy1d6GJiOHTohq4:

   {"hello": "world"}

      Figure 5: A request with a sha-512 integrity field, whose digest
                       has been truncated to 32 bytes

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   # NOTE: '\' line wrapping per RFC 8792

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

   {
     "type": "https://iana.org/assignments/http-problem-types#\
       digest-invalid-values",
     "title": "Invalid digest values",
     "invalid_digests": [
       {
         "algorithm": "sha-512",
         "header": "Repr-Digest",
         "reason": "digest value is not 64 bytes long"
       }
     ]
   }

    Figure 6: Response indicating that the provided digest is too short

3.3.  Mismatched Digest Values

   This section defines the "https://iana.org/assignments/http-problem-
   types#digest-mismatched-values" problem type.  A server can use this
   problem type when responding to a request, whose integrity fields
   include a digest value that does not match the digest value that the
   server calculated for the request content or representation.

   For this problem type, the mismatched_digests extension member is
   defined, whose value is a JSON [JSON] array of entries identifying
   each mismatched digest.  Each entry in the array is a JSON object
   with the following members:

   *  The algorithm member is a JSON string containing the algorithm key
      of the hashing algorithm.

   *  The provided_digest member is a JSON string containing the digest
      value taken from the request's integrity fields.  The digest value
      is serialized as a byte sequence as described in Section 4.1.8 of
      [STRUCTURED-FIELDS].

   *  The header member is a JSON string containing the name of the
      integrity field that contained the mismatched digest value.

   The problem type intentionally does not include the digest value
   calculated by the server to avoid attackers abusing this information
   for oracle attacks.

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   If the sender receives this problem type, the request might be
   modified unintentionally by an intermediary.  The sender could use
   this information to retry the request without modification to address
   temporary transmission issues.

   The following example shows a request with the content {"hello":
   "woXYZ"} (plus LF), but the representation digest for {"hello":
   "world"} (plus LF).  The subsequent response indicates the mismatched
   SHA-256 digest value.

   PUT /items/123 HTTP/1.1
   Host: foo.example
   Content-Type: application/json
   Repr-Digest: sha-256=:RK/0qy18MlBSVnWgjwz6lZEWjP/lF5HF9bvEF8FabDg=:

   {"hello": "woXYZ"}

       Figure 7: A request with a sha-256 integrity field, which does
                      not belong to the representation

   # NOTE: '\' line wrapping per RFC 8792

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

   {
     "type": "https://iana.org/assignments/http-problem-types#\
       digest-mismatched-values",
     "title": "Mismatched digest values",
     "mismatched_digests": [
       {
         "algorithm": "sha-256",
         "provided_digest": \
           ":RK/0qy18MlBSVnWgjwz6lZEWjP/lF5HF9bvEF8FabDg=:",
         "header": "Repr-Digest"
       }
     ]
   }

            Figure 8: Response indicating the mismatched digests

4.  Security Considerations

   All security considerations from Section 5 of [PROBLEM], Section 6 of
   [DIGEST], and Section 7 of [UNENC-DIGEST] apply as well.

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   Disclosing error details could leak information such as the presence
   of intermediaries or the server's implementation details.  Moreover,
   they can be used to fingerprint the server.

   To mitigate these risks, server operators could assess the risk of
   disclosing error details and prefer a general problem type over a
   more specific one.

   There is no method defined for the server to communicate a digest
   value that it calculated for the purpose of validation.  Such
   information can be abused for oracle attacks.

5.  IANA Considerations

   IANA is asked to register the following entries in the "HTTP Problem
   Types" registry at https://www.iana.org/assignments/http-problem-
   types (https://www.iana.org/assignments/http-problem-types).

5.1.  Registration of "digest-unsupported-algorithms" Problem Type

   Type URI:  https://iana.org/assignments/http-problem-types#digest-
      unsupported-algorithms

   Title:  Unsupported Hashing Algorithms

   Recommended HTTP status code:  400

   Reference:  Section 3.1 of this document

5.2.  Registration of "digest-invalid-values" Problem Type

   Type URI:  https://iana.org/assignments/http-problem-types#digest-
      invalid-values

   Title:  Invalid Digest Values

   Recommended HTTP status code:  400

   Reference:  Section 3.2 of this document

5.3.  Registration of "digest-mismatched-values" Problem Type

   Type URI:  https://iana.org/assignments/http-problem-types#digest-
      mismatched-values

   Title:  Mismatched Digest Values

   Recommended HTTP status code:  400

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   Reference:  Section 3.3 of this document

6.  Normative References

   [DIGEST]   Polli, R. and L. Pardue, "Digest Fields", RFC 9530,
              DOI 10.17487/RFC9530, February 2024,
              <https://www.rfc-editor.org/rfc/rfc9530>.

   [HTTP]     Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Semantics", STD 97, RFC 9110,
              DOI 10.17487/RFC9110, June 2022,
              <https://www.rfc-editor.org/rfc/rfc9110>.

   [JSON]     Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,
              <https://www.rfc-editor.org/rfc/rfc8259>.

   [PROBLEM]  Nottingham, M., Wilde, E., and S. Dalal, "Problem Details
              for HTTP APIs", RFC 9457, DOI 10.17487/RFC9457, July 2023,
              <https://www.rfc-editor.org/rfc/rfc9457>.

   [RFC8792]  Watsen, K., Auerswald, E., Farrel, A., and Q. Wu,
              "Handling Long Lines in Content of Internet-Drafts and
              RFCs", RFC 8792, DOI 10.17487/RFC8792, June 2020,
              <https://www.rfc-editor.org/rfc/rfc8792>.

   [STRUCTURED-FIELDS]
              Nottingham, M. and P. Kamp, "Structured Field Values for
              HTTP", RFC 9651, DOI 10.17487/RFC9651, September 2024,
              <https://www.rfc-editor.org/rfc/rfc9651>.

   [UNENC-DIGEST]
              Pardue, L. and M. West, "HTTP Unencoded Digest", Work in
              Progress, Internet-Draft, draft-ietf-httpbis-unencoded-
              digest-05, 17 June 2026,
              <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
              unencoded-digest-05>.

Authors' Addresses

   Marius Kleidl
   Transloadit
   Email: marius@transloadit.com

   Lucas Pardue
   Cloudflare

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   Email: lucas@lucaspardue.com

   Roberto Polli
   Par-Tec
   Italy
   Email: robipolli@gmail.com

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