GB1379740A - Method and apparatus for encoded messages transmission - Google Patents

Abstract

1379740 Digital transmission; ciphering systems GRETAG AG 7 Dec 1971 [8 Dec 1970] 56849/71 Heading H4P A message is ciphered by mixing clear text with the output from a ciphered pulse generator and a term derived from date and time. An identical generator is provided at the receiver for deciphering. As the initial state of the pulse generator is continually reset the commencing position of the generator is independent of any data supplied earlier. In a preferred embodiment a clock oscillator is used as the derived term source and synchronizing is performed between transmitter and receiver. A non- secret supplementary code is envisaged but is not particularly described in the Specification. An information source, e.g. a teleprinter 9, is connected to a transmission link 148, 148<SP>1</SP> through a mixer 8 which dependent on direction of transmission, adds/subtracts cipher sequences from generator 5 which has a long cycle and may comprise a number of storage units connectible in dependence on a secret code. Generator 5 is connected to a sync. stage 6 and has inputs from a date-time store 1, further store 2, and a mod-2 mixer 18, receiving in turn, output from a cipher store 3. The date-time store provides an output for example once per second representative of, say, date and time, and store 2 provides individual subscribers' addresses where a number of stations are connected into a system. Store 1 may additionally include a pseudorandom generator (16), Fig. 2 (not shown), providing a supplementary code which may also be supplied to mixer 18. In order to maintain sync. it is necessary for the sequence to be interrupted once per cycle for this to be effected. Sync. stage 6 comprises time comparison means 61, time difference store 62 and OR 63 connected through a time correcting means 4 feeding into date-time store 1. The time difference between a received signal and signals produced by generator 5 or a signal produced by the date-time store is measurable at 61 through a switch 23. The time difference may be utilised for resetting a receiver's own clock from unit 61 and this value may be stored in 62 which is cancelled at the end of a message. Store 3 may comprise a random access memory and/or may be connected to store 1 or 2 for further varying the cipher arrangement. The mixer 18 output is dependent on a secret code, date-time code, plus possible supplementary code, hence another subscriber having an identical generator commencing at the same position can decipher messages by means of a similar mixer 8. A suitable cipher generator is described in Specification 1,364,518. The timing clock which is incorporated in unit 1 may embrace a crystal oscillator and frequency divider which provides date-time information. Information from stores 1, 2 is supplied serially which necessitates serial/parallel conversion in (22), (26), Fig. 2, not shown. A separate synchronizing sequence sent at bit rate may be checked at a receiver by a generator which runs at a multiple of bit rate.