7.2.6. Multiple Signature Confusion

7.2.6. Multiple Signature Confusion

Since multiple signatures can be applied to one message (Section 4.3), it is possible for an attacker to attach their own signature to a captured message without modifying existing signatures. This new signature could be completely valid based on the attacker's key, or it could be an invalid signature for any number of reasons. Each of these situations needs to be accounted for.

A verifier processing a set of valid signatures needs to account for all of the signers, identified by the signing keys. Only signatures from expected signers should be accepted, regardless of the cryptographic validity of the signature itself.

A verifier processing a set of signatures on a message also needs to determine what to do when one or more of the signatures are not valid. If a message is accepted when at least one signature is valid, then a verifier could drop all invalid signatures from the request before processing the message further. Alternatively, if the verifier rejects a message for a single invalid signature, an attacker could use this to deny service to otherwise valid messages by injecting invalid signatures alongside the valid signatures.