C2PA exclusion range flaw: Why signature verification must check coverage

Signature coverage determines which bytes a cryptographic signature protects, and the text highlights a C2PA proof of concept by David Buchanan in which a valid claim signature and RFC 3161 timestamp covered an empty input because the manifest excluded the entire lottery-ticket image file from hashing. Existing C2PA tools reportedly accepted the file without warning because they checked signature validity but not whether meaningful image content was signed, allowing the photo to be altered after timestamping without invalidating its cryptographic proofs. The issue reflects broader verification failures seen in SAML signature wrapping, JWT algorithm confusion, and recent SAML bypasses, where software verifies one object but consumes different or unsigned content. The proposed defense is to explicitly calculate and enforce coverage, restrict format-specific exclusion ranges, bind applications to the exact verified bytes, reject empty or trivial hash targets, and treat timestamps as evidence only for the data actually signed. Although exclusions can be necessary in some formats because embedded signatures create circular dependencies, the text argues that verifiers need precise rules for permissible excluded regions rather than accepting broad or arbitrary exclusions.

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