Claim Signature
Definition
An X.509-based digital signature over the hashes of all assertions in a claim plus the content binding hash. Produced by the signer's private key and verified against the signer's certificate, which must chain to a root in the C2PA trust list.
- Basis
- X.509-based digital signature
- Covers
- Hashes of all claim assertions plus content binding hash
- Produced with
- Signer's private key
- Verified against
- Signer's certificate chaining to a C2PA trust list root
Common questions
What happens if the signer's certificate does not chain to a trust list root?+
Verification software will flag the claim as untrusted or unverifiable, because the signature can still be mathematically valid while the certificate issuer is not one a relying party has chosen to trust, so trust list membership is a separate check from cryptographic validity.
Does the claim signature prove the image content itself is unaltered?+
It proves the claim's recorded assertions and the content binding hash were not altered after signing, so any edit made after the claim was signed either invalidates the signature or generates a new claim, but it cannot vouch for edits made before the credential was created.
Why sign a hash rather than the content credentials data directly?+
Hashing the assertions and content binding first produces a fixed-size digest to sign, which is computationally efficient and lets verification confirm nothing in the claim or the bound content changed without needing to re-transmit or re-sign the full underlying data each time.
Related terms
- Assertion
- A single provenance statement inside a C2PA claim. Examples include the camera make and model, GPS coordinates at capture, an AI-training or...
- C2PA Manifest
- The structured provenance record embedded in or associated with a media file. Contains one or more signed claim blocks, each holding a...
- C2PA Trust List
- A curated list of trusted root certificate authorities whose chains can anchor a C2PA claim signature. Maintained by the C2PA organisation. A...
- Hard Binding
- A content binding that stores SHA hashes of specific byte ranges of the media file inside the signed manifest. Any modification to...
- Soft Binding
- A content binding using a perceptual fingerprint or watermark embedded in the signal rather than the file bytes. Designed to survive format...