Fix unsupported hash algo exception in GCPSigner

GCPSigner is expected to raise UnsupportedAlgorithmError, if the hash
algorithm encoded in scheme is invalid. Switching from
securesystemslib.hash to hashlib, requires wrapping and re-raising to
maintain the expected behavior.

This is a temporary solution until #766.

Signed-off-by: Lukas Puehringer <[email protected]>

Author: lukpueh

securesystemslib/signer/_gcp_signer.py

@@ -180,7 +180,12 @@ def _get_hash_algorithm(public_key: Key) -> str:
             )
 
         # trigger UnsupportedAlgorithm if appropriate
-        _ = hashlib.new(algo)
+        # TODO: validate scheme/algo in constructor (#766)
+        try:
+            _ = hashlib.new(algo)
+        except (ValueError, TypeError) as e:
+            raise exceptions.UnsupportedAlgorithmError(algo) from e
+
         return algo
 
     def sign(self, payload: bytes) -> Signature: