Building Blocks for HTTP APIs M. Kleidl
Internet-Draft Transloadit
Intended status: Informational L. Pardue
Expires: 13 December 2025 Cloudflare
R. Polli
Par-Tec
11 June 2025
HTTP Problem Types for Digest Fields
draft-ietf-httpapi-digest-fields-problem-types-02
Abstract
This document specifies problem types that servers can use in
responses to problems encountered while dealing with a request
carrying integrity fields and integrity preference fields.
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
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This Internet-Draft will expire on 13 December 2025.
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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. Mismatching Digest Values . . . . . . . . . . . . . . . . 7
4. Security Considerations . . . . . . . . . . . . . . . . . . . 9
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
5.1. Registration of "digest-unsupported-algorithms" Problem
Type . . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.2. Registration of "digest-invalid-values" Problem Type . . 10
5.3. Registration of "digest-mismatching-values" Problem
Type . . . . . . . . . . . . . . . . . . . . . . . . . . 10
6. Normative References . . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
[DIGEST] defines HTTP fields for exchanging integrity digests and
preferences, but does not specify, require or recommend any specific
behavior for error handling relating to integrity by design. The
responsibility is instead delegated to applications. This draft
defines a set of problem types ([PROBLEM]) that can be used by server
applications to indicate that a problem was encountered while dealing
with a request carrying integrity fields and integrity preference
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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 a 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
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.
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].
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].
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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].
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.
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.
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.
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Example:
POST /books HTTP/1.1
Host: foo.example
Content-Type: application/json
Accept: application/json
Accept-Encoding: identity
Repr-Digest: sha-256=:mEkdbO7Srd9LIOegftO0aBX+VPTVz7/CSHes2Z27gc4=:
Content-Digest: sha-256=:mEkdbO7Srd9LIOegftO0aBX+VPTVz7/CSHes2Z27gc4=:
{"title": "New Title"}
Figure 1: A request with sha-256 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, sha-256=0
Want-Content-Digest: sha-512=10, sha-256=0
{
"type": "https://iana.org/assignments/http-problem-types#\
digest-unsupported-algorithms",
"title": "Unsupported hashing algorithms",
"unsupported-algorithms": [
{
"algorithm": "sha-256",
"header": "Repr-Digest"
},
{
"algorithm": "sha-256",
"header": "Content-Digest"
}
]
}
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: sha=10
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Figure 3: A request with a sha-256 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": "sha",
"header": "Want-Repr-Digest"
}
]
}
Figure 4: Response indicating the problem and advertising the
supported algorithms
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 MUST NOT be used if the server is not able to parse
the integrity fields according to Section 4.5 of [STRUCTURED-FIELDS],
for example because of a syntax error in the field value.
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.
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* 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
# 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. Mismatching Digest Values
This section defines the "https://iana.org/assignments/http-problem-
types#digest-mismatching-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.
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For this problem type, the mismatching-digests extension member is
defined, whose value is a JSON [JSON] array of entries identifying
each mismatching 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 mismatching 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.
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
mismatching 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
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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-mismatching-values",
"title": "Mismatching digest values",
"mismatching-digests": [
{
"algorithm": "sha-256",
"provided-digest": ":RK/0qy18MlBSVnWgjwz6lZEWjP/lF5HF9bvEF8FabDg=:",
"header": "Repr-Digest"
}
]
}
Figure 8: Response indicating the mismatching digests
4. Security Considerations
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, a server could assess the risk of disclosing
error details and prefer a general problem type over a more specific
one.
When a server informs the client about mismatching digest values, it
should not expose the calculated digest to avoid exposing information
that 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
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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-mismatching-values" Problem Type
Type URI: https://iana.org/assignments/http-problem-types#digest-
mismatching-values
Title: Mismatching Digest Values
Recommended HTTP status code: 400
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>.
[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>.
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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>.
[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>.
Authors' Addresses
Marius Kleidl
Transloadit
Email: marius@transloadit.com
Lucas Pardue
Cloudflare
Email: lucas@lucaspardue.com
Roberto Polli
Par-Tec
Italy
Email: robipolli@gmail.com
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