protected function _algoToCipher(CipherAlgorithmIdentifier $algo): string
    {
        $oid = $algo->oid();
        if (array_key_exists($oid, self::MAP_CIPHER_OID_TO_NAME)) {
            return self::MAP_CIPHER_OID_TO_NAME[$oid];
        }
        if (AlgorithmIdentifier::OID_RC2_CBC == $oid) {
            if (!$algo instanceof RC2CBCAlgorithmIdentifier) {
                throw new \UnexpectedValueException('Not an RC2-CBC algorithm.');
            }
            return $this->_rc2AlgoToCipher($algo);
        }
        throw new \UnexpectedValueException(
            sprintf('Cipher method %s not supported.', $algo->name()));
    }