See db.Database.

def ReadSignedBinaryReferences(
      self, binary_id
  ):
    """See db.Database."""
    binary_key = _SignedBinaryKeyFromID(binary_id)
    try:
      references, timestamp = self.signed_binary_references[binary_key]
    except KeyError:
      raise db.UnknownSignedBinaryError(binary_id)
    return references.Copy(), timestamp.Copy()