signer function
Extracts the signer's fingerprint from a COSE_Sign1 signature without verifying it.
This allows looking up the appropriate verification key before attempting full signature verification.
signature: The serialized COSE_Sign1 structure
Returns the signer's fingerprint from the protected header's kid field.
Throws if the envelope is malformed.
Implementation
xdsa.Fingerprint signer({required Uint8List signature}) =>
xdsa.FingerprintInternal.wrap(ffi.coseSigner(signature: signature));