6. Conclusion

As already mentioned, the key distribution can be made simple and effective with smart cards which also contain the DES algorithm. The cards are customised by the operator and sent to each individual subscriber. The key stored in the card is uniquely related to the subscribers logical directory number, which again is uniquely related to the physical line. A smart card can thus only be used by the subscriber it is intended for. However, the keys must be protected so that an intruder can not misuse it to fabricate a counterfeited smart card. Counterfeits will, if successful, allow an intruder to connect to the line and masquerade as the real subscriber in the same way as shown in Figure 1. 5.5. Compatibility and Feature Interaction The AD is best suited to be used together with DTMF terminal equipment, but it will still be possible to use it with rotary dial sets or other older equipment using register signalling. It would then be necessary to define the state " Wait for Challenge " from Figure 8 to also be transparent so that conversation can take place. This is because rotary sets use alternated high and low impedance register signalling, which would cause the AD to jump between the first two states and then finally end up in " Wait for Challenge " when the last digit has been dialled. In addition, the local exchange must be able to receive both rotary and DTMF signalling during the same call. Most telephone exchanges allow DTMF equipment to use the " Recall " function which in fact is a calibrated period of high impedance. The Recall function can be used for many purposes. Sometimes a Recall alone can activate a service, and in other cases the subscriber receives a special dialtone as invitation to dial a new directory number or a special service code. Because the AD will be in " Wait for Challenge " after a Recall, it is now possible to challenge the AD for each new service or new directory number called, which would make it impossible for an intruder to take over a call which is already established, and use it to make further calls. We have analysed the problem of theft (misuse or resources) in analogue telephone access networks , and described how to solve the problem by introducing authentication of the legitimate subscriber by an authentication module. By …