def validate_key(self, activation_key):
try:<EOL><INDENT>username = signing.loads(<EOL>activation_key,<EOL>salt=self.key_salt,<EOL>max_age=conf.get('<STR_LIT>') * <NUM_LIT><EOL>)<EOL>return username<EOL><DEDENT>except signing.BadSignature:<EOL><INDENT>return None<EOL><DEDENT>
Verify that the activation key is valid and within the
permitted activation time window, returning the username if
valid or ``None`` if not.