Verifies the HMAC signature of an encrypted (SK, 46) payload using self.SK_ar

        :param data: bytes(payloads.SK())
        :raise IkeError: if calculated signature does not match the one in the payload

def verify_hmac(self, data):
        """
        Verifies the HMAC signature of an encrypted (SK, 46) payload using self.SK_ar

        :param data: bytes(payloads.SK())
        :raise IkeError: if calculated signature does not match the one in the payload
        """
        hmac = HMAC(self.SK_ar, digestmod=sha256)
        hmac_theirs = data[-MACLEN:]
        hmac.update(data[:-MACLEN])
        hmac_ours = hmac.digest()[:MACLEN]
        logger.debug('HMAC verify (ours){} (theirs){}'.format(
            binascii.hexlify(hmac_ours), binascii.hexlify(hmac_theirs)))
        if hmac_ours != hmac_theirs:
            raise IkeError('HMAC verify failed')