HTTP Problem Types for Digest Fields

Introduction 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 () 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 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.

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 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 . The terms "integrity fields" and "integrity preference fields" in this document are to be interpreted as described in . The term "problem type" in this document is to be interpreted as described in . The terms "request", "response", "intermediary", "sender", "client", and "server" are from . The problem types in this document are defined using JSON . They can be serialized into an equivalent XML format as outlined in .

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.

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 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. Example:

A request with sha-256 integrity fields, which are not supported by the server

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:

A request with a sha-256 integrity preference field, which is not supported by the server

Response indicating the problem and advertising the supported algorithms

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 , 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 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.

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

Response indicating that the provided digest is too short

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. For this problem type, the mismatching-digests extension member is defined, whose value is a 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 .
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.

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

Response indicating the mismatching digests

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.

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.

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:: of this document

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:: of this document

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:: of this document