Patent GB-926227-A (GB)
Title: Improvements in or relating to telegraphic communication systems
IPC: H04L9/12
Filed: 1961-02-20
Abstract: 926,227. Ciphering apparatus. SOC. D'APPLICATIONS GENERALES D'ELECTRICITE ET DE MECANIQUE. Feb. 20, 1961 [Feb. 18, 1960], No. 6182/61. Class 40 (3). The invention provides a two-way telegraph system in which plain language at the transmitter is ciphered by means of a key, e.g. a cipher tape, the ciphered signals being deciphered at the receiver by a similar key. The keys at the receiver and transmitter must be in phase agreement and in order to verify that this is so signals characteristic of beginning and ending of ciphering are transmitted. If there is lack of phase agreement the keys are adjusted. General arrangement.-In the embodiment described, transmission of a first group of signals causes subsequent character signals to be ciphered. Immediately following the first group a second group is transmitted, this group being. ciphered. At the receiver it is deciphered. At the end of the message a third group is transmitted, which causes ciphering to cease. A fourth group, which follows directly, is therefore not ciphered before transmission and should not be " deciphered " at the receiver. By recognition of the various groups at the receiver the system detects whether or not the keys are in phase agreement. If they are not, each key is brought to a new position, in which they are in phase, and an alarm is initiated. Detailed operation of the system, Fig. 1.-Plain language telegraph signals from a transmitter 1 of station A pass via a signal group detector 2, an example of which is described with reference to Fig. 2 (not shown), and which recognizes the groups, to a switch 3 controlled by the detector. If unit 2 detects the first group referred to above switch 3 passes all subsequent signals through a ciphering device 4 and via line 101 to a distant receiver at station B. On detection of the third group switch 3 routes signals to by-pass unit 4, so that the fourth group is not ciphered before transmission. At the receiver the signals pass via a group detector 22<SP>1</SP> to a switch 23. A second group detector 22 receives plain language signals via the by-pass line 124, and on detection of the first group changes switch 23 so that received signals are passed via a start simulator (to permit step-by-step advance of the cipher tape in the event of fade out) to the deciphering device 24. The deciphered signals are passed through group detector 22 to a telegraph receiver 21. If the second group is not detected by unit 22 immediately following the first group a signal is sent to step the deciphering key to correct its phase, and also to an anomaly transmitter 25 which then generates a fifth group of signals. Detector 22 also detects the third group, which causes the cessation of ciphering, and causes switch 23 to by-pass the deciphering device 24. Detector 22<SP>1</SP> responds to the fourth group, but if the cipher keys are out of phase detector 22 will not have received the third group and therefore the fourth group signals will be passed through the deciphering device 24 instead of by-passing it. Detector 22<SP>1</SP>, on detecting the fourth group, sends a signal to gate 26, which is open when lines 125 and 126 are connected through switch 23. If the gate is open a signal sets the anomaly transmitter 25 in operation, reverses switch 23, and corrects the phase of the cipher key at 24. The output of the anomaly transmitter, the fifth group, passes to a gate 47, and from thence into the transmitting side of the station B, this being identical to the transmitting side, described above, of station A. Gate 47 is open when lines 145 and 144 are joined, i.e. when the ciphering device 44 is not in use, and the fifth group signals pass in clear to the transmission line 102. They are detected by a detector 62 in the receiver of station A, identical to that of station B, and passed to a transmission control unit 5. This unit shuts down the telegraph transmitter 1, causes correction of the phase of the cipher unit 4, and initiates an alarm 6. Ciphering unit and phase correction, Fig. 3.- Telegraph signals from the transmitter pass to the ciphering unit 4 via line 106. The pulses are passed to a series-to-parallel converter 224 controlled by the output of a time-base unit 208. The cipher tape 201 is moved in normal manner by means of a toothed wheel 202 and motor 203. The motor is controlled by pulses developed on line 229 by the telegraph start pulses, or by local pulses on line 230, such pulses passing via OR gate 214 and amplifier 215. Feelers 205 detect the perforations in the cipher tape and the signals derived therefrom coact in mixer 205 with the signals from translator 224, the resultant ciphered signals being converted to series form in a translator 225 prior to transmission on line 101 to the distant station B. If the fifth group of signals is received, a signal indicative of lack of phase equality reaches the unit 5. Unit 5 controls several units, as mentioned above, and also restarts the time-base 208, there being now no signals on line 106 as the transmitter has been shut down. Under these conditions the time-base supplies pulses at 210 which move the cipher tape step-by-step until the time-base is stopped by detection of a reference mark 221 on the tape.