signDigest method
Signs the SHA-256 digest of signedAttrs and returns a raw signature
(PKCS#1 v1.5 or ECDSA) to be embedded in the CMS.
Implementation
@override
Future<Uint8List> signDigest(Uint8List digest) {
throw UnsupportedError(
'ECDSA signer requer signSignedAttributes() para evitar double-hash.',
);
}