"signature";req and "signature-input";req are not recommended

Originally submitted by Takahiko Kawasaki (Takahiko Kawasaki) on 2024-09-20

FAPI 2.0 Message Signing, Section 5.6.2.1. Resource servers, the third clause says as follows:

if the request was signed, shall include the request signature and request signature input in the response signature input by means of the req boolean flag defined in Section 2.4 of [RFC9421];

If this clause means that "signature";req and "signature-input";req must be included in the signature base (and there seems to be no other way to interpret it), then this clause should be removed.

The third-to-last paragraph of Section 2.4. Signing Request Components in a Response Message in RFC 9421 HTTP Message Signatures states the following and explicitly says it is “NOT RECOMMENDED” to do so for security reasons.

While it is syntactically possible to include the Signature and Signature-Input fields of the request message in the signature components of a response to a message using this mechanism, this practice is NOT RECOMMENDED. This is because signatures of signatures do not provide transitive coverage of covered components as one might expect, and the practice is susceptible to several attacks as discussed in Section 7.3.7. An application that needs to signal successful processing or receipt of a signature would need to carefully specify alternative mechanisms for sending such a signal securely.

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Bitbucket status: open

Bitbucket origin: issue 718

Review FAPI 2.0 Message Signing §5.6.2.1 alongside RFC 9421 §2.4 and the linked security discussion. Determine whether the third clause requires "signature";req and "signature-input";req, then update the specification wording if that interpretation is confirmed and verify it addresses the cited RFC warning.