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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def pre_dissect(self, s):
 """ Decrypt, verify and decompress the message. """

        if len(s) < 5:
            raise Exception("Invalid record: header is too short.")

        if isinstance(self.tls_session.rcs.cipher, Cipher_NULL):
            self.deciphered_len = None
            return s
        else:
            msglen = struct.unpack('!H', s[3:5])[0]
            hdr, efrag, r = s[:5], s[5:5 + msglen], s[msglen + 5:]
            frag, auth_tag = self._tls_auth_decrypt(efrag)
            self.deciphered_len = len(frag)
            return hdr + frag + auth_tag + r