Steve Shear <azur@netcom.com> writes:
For voice calls, it is not possible to merely insert a more secure crypto function (e.g., IDEA) inside the less secure A5 crypto, unless the GSM base station's protocol is aware the subscriber unit is using the more secure crypto. Otherwise, when the GSM base station unwraps the A5 encrypted data stream, which it assumes will contain digitized voice packets in the clear, it will not find what its looking for and will be unable to convert the packets to a circuit-switched voice signal.
The circuit-switched voice signal is digital also, and the voice packet payload is transferred byte for byte into the circuit switched link right? So to insert IDEA encryption inside the A5 layer, you'd need to packetize the IDEA ciphertext so that it conforms to the voice packet spec (to fool the unwrapping the GSM station performs into thinking the packets are voice packets). Is that possible? Is the packet switched protocol non-error corrected? (ie Is packet dropping expected?) If so you need to use an IDEA mode which does not rely on previous packets.
If, as I have previously stated, the subscriber instead uses the data port of his instrument and establishes a data link with his payload protected by the more the secure crypto this is entirely feasible.
If the above is infeasible. Probably this is a more practical approach anyway, as the data port has been already engineered to solve some of problems that would be encountered, and you would be duplicating effort. Adam -- print pack"C*",split/\D+/,`echo "16iII*o\U@{$/=$z;[(pop,pop,unpack"H*",<> )]}\EsMsKsN0[lN*1lK[d2%Sa2/d0<X+d*lMLa^*lN%0]dsXx++lMlN/dsM0<J]dsJxp"|dc`