def _get_key_alias_from_cache(self, key_arn):
for alias in self.KEY_METADATA:<EOL><INDENT>if self.KEY_METADATA[alias]['<STR_LIT>']['<STR_LIT>'] == key_arn:<EOL><INDENT>return alias<EOL><DEDENT><DEDENT>return None<EOL>
Find a key's alias by looking up its key_arn in the KEY_METADATA
cache. This function will only work after a key has been lookedup by
its alias and is meant as a convenience function for turning an ARN
that's already been looked up back into its alias.