Known methods for authenticating communication participants add a code word to a command signal, which code word is a crytographic function of the command signal and a key value which in its turn is a function of a master key value and a change code word. The master key value has a permanently fixed...http://www.google.com.tw/patents/US5325434?utm_source=gb-gplus-share專利 US5325434 - Method for authenticating communications participants, system for application of the method and first communications participant and second communication participant for application in the system
