Verify an M2Crypto-compatible signature

    :param Crypto.PublicKey.RSA._RSAobj key: The RSA public key object
    :param str message: The signed message to verify
    :rtype: str
    :return: The message (or digest) recovered from the signature, or an
        empty string if the verification failed

def public_decrypt(pub, message):
    '''
    Verify an M2Crypto-compatible signature

    :param Crypto.PublicKey.RSA._RSAobj key: The RSA public key object
    :param str message: The signed message to verify
    :rtype: str
    :return: The message (or digest) recovered from the signature, or an
        empty string if the verification failed
    '''
    if HAS_M2:
        return pub.public_decrypt(message, salt.utils.rsax931.RSA_X931_PADDING)
    else:
        verifier = salt.utils.rsax931.RSAX931Verifier(pub.exportKey('PEM'))
        return verifier.verify(message)