GB1388035A - Method of and apparatus for encoded communications transmission - Google Patents

Abstract

1388035 Digital transmission; cyphering systems GRETAG AG 19 April 1972 [19 April 1971] 18209/72 Heading H4P In a method of cyphering data, clear text is mixed with a first sequence developed by a programme and transmitted in alternation with a pseudo statistical sequence, the latter being compared on reception with an identical soquence to establish correlation and hence provide synchronization of the first sequence. The first sequence may be determined by first and supplementary codes and the pseudo statistical generator may produce a series of different sequences utilized sequentially; the pseudo statistical sequence may be inserted periodically or aperiodically. Data source 1, e.g. a teleprinter, is connected to line 4 through mixer 3 to which code computer 5 is connectable through switch S3. Cypher computer 5 may be designed to be variable and this must be brought into the same initial condition as a corresponding computer (5<SP>1</SP>) in the receiver. This is achieved by a base code, and a supplementary code generated in computer 8 preferably controlled by date and time. Computer 8 has substantially the same routine as 5 and the date time converter 7 has its output expressed in the decimal terms and feeds separate mixers 9a, 9b through a switch S1 which is closed periodically, e.g. once per minute. Code stores 6, 22 store first and second base codes respectively, the latter passing an output into mixers 9a, 9b which also receive an input from unit 12, the latter being provided for multiple network operation. Switch S1 is closed periodically, e.g. each minute, and output from 7 passes into 9a, 9b to be mixed with output from 22. Each sequence from 9a, 9b resets computer 8, causing it to generate a erypto synchronizing sequence of pseudo statistical form which may be shorter than that generated by 5 and may be selected from a longer sequence. To increase security stores 6, 22 are also controlled by 7 so that the code may be changed periodically, e.g. daily. The output from 8 is connected through store 13 and switch S2 to the input of mixer 14. When it is desired to reset the conditions in 5 a signal from the source 1 is recognized by 15 and is transferred by 16 which reverses switches S2, S3. Alternatively the synchronizing sequences may be inserted automatically into intervals in the data so that the receiver may be reset to the same condition as the transmitter. Decyphering is carried out in the reverse manner to cyphering. Clock 23 should be independent of the date/time converter 7. At the beginning of a transmission it may be desirous to reverse all the clocks connected to the network by say 2 minutes in order that three successive synchronizing sequences may be received. When communication is established clocks are then advanced again by two minutes by being run at a high speed for a determined period.