decrypt a MACsec frame for this Secure Association

def decrypt(self, orig_pkt, assoclen=None):
        """decrypt a MACsec frame for this Secure Association"""
        hdr = copy.deepcopy(orig_pkt)
        del hdr[MACsec].payload
        pktlen = len(orig_pkt)
        if self.send_sci:
            hdrlen = NOSCI_LEN + SCI_LEN
        else:
            hdrlen = NOSCI_LEN
        if assoclen is None or not self.do_encrypt:
            if self.do_encrypt:
                assoclen = hdrlen
            else:
                assoclen = pktlen - self.icvlen
        iv = self.make_iv(hdr)
        assoc, ct, icv = MACsecSA.split_pkt(orig_pkt, assoclen, self.icvlen)
        decryptor = Cipher(
            algorithms.AES(self.key),
            modes.GCM(iv, icv),
            backend=default_backend()
        ).decryptor()
        decryptor.authenticate_additional_data(assoc)
        pt = assoc[hdrlen:assoclen]
        pt += decryptor.update(ct)
        pt += decryptor.finalize()
        hdr[MACsec].type = struct.unpack('!H', pt[0:2])[0]
        hdr[MACsec].payload = Raw(pt[2:])
        return hdr