Patent GB-1428469-A (GB)
Title: Method and apparatus for conversion of graphical clear information into ciphered graphical information and vice versa
IPC: G06K9/20
Filed: 1973-06-08
Abstract: 1428469 Ciphering and deciphering graphical data GRETAG AG 8 June 1973 [9 June 1972] 27514/73 Heading G5X To convert graphical information, e.g. a cheque card signature, into enciphered form, and to decipher the enciphered form to obtain a clear copy, two similar devices, or possibly one with combined facilities, scan the input optically by means of a flying spot television camera, synchronized with a second CRT displaying the converted data ready to be photographed on to a card in the enciphering application, or open to view when deciphering. The clear data converted to binary pulse form is enciphered by combining it via a modulo-2- mixer (producing pulse output for pulse on one but not both of inputs), with a secret code pulse train, itself selected from a larger stored train by a supplementary code number input via a separate terminal. In addition to the enciphered data the output display contains the supplementary code together with timing and synchronizing pulses. In deciphering, the timing and synchronizing pulses ensure that the device scans the image correctly, in spite of minor errors in position, orientation and scale, and the supplementary code enables the correct part of the secret code train to be drawn out of store. In both devices (Figs. 1, 3) the scanning spot of fixed brightness on CRT 1 is focused on the signature or enciphered card 4 in holder 35, the instantaneous brightness of the reflected spot being measured by photo-electric device 5, amplified 13 and sampled and clipped 14 to produce a pulse train in time with pulses from clock 12. This input train enters the encipher or decipher device at terminals A or D, and the converted train emerges at terminals B or E for possible remote transmission, or via brightness modulator 9 to CRT 2, on which the image is displayed. Both devices possess clock 12, master controller or program generator 11, 23, secret code store 20, code pulse computer 19, modulo- 2-mixer 22, 32 and shift register 15, 33. Synchronizing pulse generators 17 and 28 are similar but have slightly different functions, and supplementary code stores 18, 29 accept code from terminal C and from incoming optical data respectively. In addition the enciphering device (Fig. 1) has AND gate 16 and OR gate 21, while deciphering device (Fig. 3) has AND gates 26, 30, 31, clock synchronizing unit 27 and correlator 43. A signature can be scanned by 120 lines, while each tube displays 600, thus in enciphering the contents of the first of each five lines are stored in shift register 15 and copied four times after the original, forming a line set of five identical lines. Before scan of the signature begins, three line sets in group I (Fig. 2) and a further three in II are traversed in order to print on the enciphered card 6 the pulses in fields TR, SI1, IN, SI2 in groups I and II. When the data scan area III is reached, fields TR and SI1 are added before, and SI2 after, the data. Field TR contains 8 clock pulses in all cases, field IN contains clock pulses in group I, supplementary code as a binary number in group II, and enciphered data in group III. In all groups, fields SI1 and SI2 each occupy ten pulses, of which seven form a synchronizing train from generator 17 of binary form 1110100, starting in alternate line sets at pulse 1 and 4. Counting of pulses and lines to delimit fields and groups is controlled by program generator 11. In deciphering, scan takes place over a field larger than card 6, and as soon as pulses are detected, the pulses in field TR are used to set in clock synchronizer 27 frequency and phase of clock 12 to match the card, a process later reinforced by clock pulses in field IN. The phases (start at 1 or 4) of synchronizing pulses in fields SI1 and SI2 are compared and generate any necessary horizontal component added to vertical scan to correct misalignment of the card with the horizontal scan axis. The line in each line set used for deciphering purposes is the second after the first having SI1 and SI2 pulses in phase with each other, i.e. normally the middle line of five. The start of scan of the critical IN field must be perfectly synchronized with the switch-on of deciphering circuits at deflection Y3; this is accomplished by generating in 28 five synchronizing 1110100 trains, each delayed by one pulse. Only one of these will correlate perfectly, as detected by correlator 43, with the train read from card 7, and three pulses after the end of the selected train, or immediately if the card train starts at pulse 4, the deciphering circuits are activated. In group II these read the supplementary code and load it into store 18, from which it selects the correct block of secret code in store 20, and makes ready code pulse computer 19. On entering the seventh line set, i.e. group III, entry at Y3 into field IN initiates reading of the secret code, continued in each line set for the pre-fixed count of pulses in IN, and re-started at Y3 in the following line set. The secret code pulses are combined with the enciphered data pulses in modulo-2-mixer 32 to produce clear output, stored in shift register 33 and displayed in five successive lines on CRT 2. Use of the wrong secret code can be recognized by blank background areas of the signature field incorrectly deciphered as filled with random pulses.