HTTP Problem Types for Digest Fields

Introduction by design does not define, require or recommend any specific behavior for error handling relating to integrity. 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. 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 term "request", "response", "intermediary", "sender", and "server" are from .

Problem Types

Unsupported Hashing Algorithm This section defines the "https://iana.org/assignments/http-problem-types#digest-unsupported-algorithm" problem type. A server MAY use this problem type if it wants to communicate to the client that one of the hashing algorithms referenced in the integrity or integrity preference fields present in the request is not supported. For this problem type, unsupported-algorithm is defined as the only extension member. It SHOULD be populated in a response using this problem type, with its value being the algorithm key of the unsupported algorithm from the request. The response can include the corresponding integrity preference field to indicate the server's algorithm support and preference. Example:

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

Response Advertising the Supported Algorithms This problem type is a hint to the client about algorithm support, which the client could use to retry the request with a different, supported, algorithm. Note that a request may contain more than one integrity field. This problem type can also be used when a request contains an integrity preference field. For example:

GET Request with Want-Repr-Digest

Response Advertising the Supported Algorithms

Invalid Digest Value This section defines the "https://iana.org/assignments/http-problem-types#digest-invalid-value" problem type. A server MAY 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 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. The server SHOULD include a human-readable description why the value is considered invalid in the title member. 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

Response indicating that the provided digest is too short 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.

Mismatching Digest Value This section defines the "https://iana.org/assignments/http-problem-types#digest-mismatching-value" problem type. A server MAY 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. Three problem type extension members are defined: the algorithm, provided-digest, and calculated-digest members. A response using this problem type SHOULD populate all members, with the value of algorithm being the algorithm key of the used hashing algorithm, with the value of provided-digest being the digest value taken from the request's integrity fields, and the value of calculated-digest being the calculated digest. The digest values MUST BE serialized as byte sequences as described in . 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 values.

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

Response indicating the mismatching digests 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.

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.

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-algorithm" Problem Type

Type URI:: https://iana.org/assignments/http-problem-types#digest-unsupported-algorithm
Title:: Unsupported Hashing Algorithm
Recommended HTTP status code:: 400
Reference:: of this document

Registration of "digest-invalid-value" Problem Type

Type URI:: https://iana.org/assignments/http-problem-types#digest-invalid-value
Title:: Invalid Digest Value
Recommended HTTP status code:: 400
Reference:: of this document

Registration of "digest-mismatching-value" Problem Type

Type URI:: https://iana.org/assignments/http-problem-types#digest-mismatching-value
Title:: Mismatching Digest Value
Recommended HTTP status code:: 400
Reference:: of this document