| 摘要 |
1,249,520. Digital transmission systems. SIEMENS A.G. 31 July, 1969 [1 Aug., 1968], No. 38373/69. Heading H4P. In a teleprinter system comprising a number of out-stations and a central station, e.g. an exchange, after setting up a connection between an out-station and the central station, e.g. by dialling, the out-station sends its identifying signal which, at the central station, is stored and selects a cipher key generator unique to the out-station. Subsequently a cipher code is transmitted by the out-station. This code consists of the output of a cipher key generator identical to that at the central station and comprising a shift register whose output is fed back and added to its input and to selected stages in the register. Alternatively the cipher code consists of a number of telegraph characters generated by the keyboard: in this case the code is also fed to the input of the out-station's cipher key generator. At the central station the cipher code sets up the bistables in the selected generator so that the generators at the two stations have identical settings. The out-station now transmits the identifying signal in ciphered' form. At the central station the signal is deciphered and compared with the stored plain language identifying signal. If the signals are identical, ciphered transmission can proceed. Characters from the keyboard are modulo-2 added to the cipher key generator output, the ciphered characters being transmitted in Start-Stop form. At the central station the ciphered characters are modulo-2 added to the output of the selected generator, the resulting deciphered signals being printed. The transmitted ciphered characters may be added to the output of the generators and fed back to the input of both generators for extra security. At the end of a message, or if transmission error is detected, the connection reverts to plain language mode. The feed back connections in the generators may be altered periodically
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