GB1605355A - improvements in or relating to a process and device for encyphering clear texts - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO A PROCESS AND DEVICE FOR ENCYPHERING CLEAR TEXTS VWe TEUEFUNBY G.M.s.H., a company recognized under German laws, of Berlin Charlouenburg 1, Ernst-Reuter-Platz, Germany do hereby declare the invention, for which Vwe pray that a patent may be granted to me/us, and the method by which it is to be performed, lo be particularly described in and by the following statement:- The mechanical accomplishment of encyphering can be divided into two large groups, addilion methods and substitution methods. In the addition methods a cypher number is added by any fixed rule to each of the clear text characters which are indicated by numbers. The cypher numbers, which should form as random as possible a numerical sequence, can be taken from a random generator or quasi-random generator.

The substitution method, which from the stand point of cryptology are in most cases considered more secure from unauthorised decyphering, consist essentially in allotting to each of the individual cypher steps, which correspond to the sequentially occurring characters of the clear text, a complete alphabet of cypher characters from which the corresponding cypher letters are chosen according to a fixed rule for each possible value of the clear text letter. Unlike the addition methods the cypher letter of a selected cyper step in this system depends not only on the ordinal number of this step but also upon the specially occurring clear text letter.

The invention relales lo a method of encyphering clear texts and decyphering of secret texts from an alphabet of n characters in which there is allotted lo each letter of the clear text a complete substitution alphabet of n characters which changes from letter to letter.

By a complete alphabet is understood one in which each of Lhe n characters occurs at least once and only once, so that an unambiguous representation between clear text and secret text is possible.

For a long time a mechanical apparatus for the production of a quasi random sequence of complete substitution alphabets has been known under the name ENIGMA. In this machine the 26 letters of the normal alphabet are permutated in a number of oppositely rotating substitution cylinders. The drawback of this and of similar mechanical apparatus is that, among other things, they can only operate at a low speed because of the moving masses.

In the invention the series of substitution alphabets are selected from a series of characters, hereinafter called a numerical worm, which are delivered by a random generator or quasi-random generator, hereafter both called a random generator, whereby, as occasion arises, superfluous characters are suppressed, as are duplicated characters occurring during the formation of the substitution alphabet. Hereby the device is so arranged that all the repetitions of characters occurring during the compilation of a substitution alphabet are suppressed. The complete substitution alphabet so produced can be stored in a suitable storage device. A storage position for example is allotted to each of the n characters of the clear text alphabet. In place of the clear text letter the substitution alphabet character in the corresponding storage cell is then used as a letter or character of the secret text.

In the invention the substitution alphabet is constructed preferably from the same characters as the clear text to be encyphered. If for instance teleprinter characters are to be encyphered which can be interpreted after the separation of the start and slop pulses as a binary representation of the figures 0 to 31 then the substitution alphabet will also be constructed from the figures 0 lo 31. If redundant codes are to be encyphered, in which more possible characters exist than are actually used, then care must be taken that for the substitution alphabet also selection rules are valid which allot to each alphabet only so many characters as are used in the clear text alphabeL It is essential that the random generator delivers during the transmission period of the clear text character (about In sec. in a teleprinter) a numerical worm of such length that there are present therein, with a probability of unity, all the characters of the substitution alphabet except one. The final missing character is unambiguously determined and can therefore be inserted without waiting for it to occur randomly in the numerical worm. The n character substitution alphabet is therefore complete when n-l of the n possible characters appear in the numerical worm. It can however happen now and then that the (n-l)th or even the (n-2)th character is not ready before the period required for compiling the substitution alphabet has already elapsed. Precautions are therefore taken so that after the lapse of a certain time, i.e. after the passage of a quite specific number of characters of the numerical worm, the characters which are still missing will be attached in arbitrary sequence, i.e. according to their value or alphabetical sequence, to the already completed portion of the substitution alphabet or else such an incomplete substitution alphabet will be rejected and measures inslituted which cause, in such very rare cases, the clear text letter itself to be transmitted.

A random generator, which is suitable for carrying out the invention, is described for example in British Patent Application 40817/58 of December 17th 1958. Such a generator is especially suitable for use in the invention because it allows a numerical worm of very long repetition period to be produced with relatively simple means and with such a high speed as is necessary for the method according to the invention bound by the above mentioned requirements.

Fig. 1 shows the main circuit diagram of an arrangement for carrying out the method according to the invention. The random generator 1 delivers from a previously compiled cypher step a random sequence of characters via a line or group of lines 2, said sequence being compiled from its alphabet of n characters. Line 2 leads to an encoding matrix 3 in which the characters of the numerical worm are transposed into a 1-from-n-code. This means that one of the output lines 4,, 4,... 4, of the encoding mauix is allotted to each of the n possible characters of the numerical worm. A blocking device is incorporated in each of the lines 4, all the said blocking devices, hereafter called barriers, being represented by block 5 in Fig. 1. These barriers are shown sehematically in Fig. 1 as switches Si, 52... 5,, These switches are closed in the output condition and are so designed that they are automatically opened when a pulse passes and block the appropriate line to the passage of further pulses, as shown by the arrows These switches can be designed as bistable elements known per se, e.g. flip-flops consisting of electronic valves or transistors, or as relays or arrangements containing magnetic cores with rectangular hysteresis curves, as explained in more detail below. During the build up of a complete substitution alphabel only one pulse can occur in each of the output lines 6,, 6,... 6, of barrier 5. Incidental repetitions of pulses on the same line, which are equivalent to repetitions of characters in the numerical worm, are suppressed so that a complete substitution alphabet results. The pulses sequentially arriving in line 6 are transposed into a secret text code in a coding device 7, each line 6 having a different code character allotted to it. The sequence in which these code characters appear and are delivered via the line or group of lines 8 to a storage device 9 depends upon the random sequence in which the switches 5,. 5,... 5, are closed. The code characters of the substitution alphabet, arranged according to its sequential occurrence, are accepted by the storage device into individual storage cells, each storage cell having allotted to it a character of the clear text alphabeL Encyphering of the previous clear text letter does not take place until the complete substitution alphabet is present in the storage device 9. After this complete alphabet has eventually built up in storage device 9 the random generator 1 is stopped, via a line 10 for example. Furthermore the barriers 5 are returned to their output state and the path for the clear text letter which is to be encyphered is opened via switching means 12. This clear text letter is fed via switch 13, which is set to "Encypher", to a coding device 14 which allots one of the n lines or paths 15 to each possible one of the n letters in a 1 -from-n-code. A special clear text letter means therefore a current or a pulse on one of the output lines 15 and this current or pulse causes the secret text letter allotted to it in the storage cell to be transmitted via line 16 to the switch 17 which is set to "Encypher". When the secret text letter has been successfully delivered the storage device 9 is again quenched and the random generator starts once again to prepare a new substitution alphabets If the alphabets of the clear text and of the incident generator are using the same n characters of an alphabet and are only permutations of one another then the coding device 7 can be dispensed with by branching the characters of the random generator off the line 2 and feeding them directly to storage device 9 via a suitable blocking device which can be constituted by the same switching means as those of barrier 5.

This barrier allows the character delivered by the random generator to pass when, and only when, one of the barriers 5 is switched, if this character is not used in the compilation of the complete alphabet. For decyphering a secret text the secret text character received via line 18 is stored by means of storage device 19 and is compared step by step with the content of the substitution alphabet storage device 9. This is effected by selling switches 13 and 17 to "Decypher" and starting the random generator.

When the complete substitution alphabet has built up in storage device 9 a clear text generator 20 is started via line 10, said generator, in its normal state, feeding the characters of the clear text alphabet firstly to a delay storage device 21 and secondly to the encoding matrix 14 via switch 13. The delay storage device 21 stores the already arrived clear text character only until the arrival of the next clear text character. As in encyphering the currents or pulses flowing in the lines 15 carry the contents of the cells of storage device 9 via line 16 lo switch 17 from whence they are rcgistcrcd in storage means 22. These storage means arc, like the delay storage device 21, so designed that they store the character which has arrived only until the next one is delivered. It is determined in the comparison device 23 whether the contents of storage devices 19 and 22 coincide, which must be the case for exactly one of the n steps. If there is coincidence then the clear text character in the delay storage device 21 is released by a criterion in the line 24 and the decyphering of the next secret text character can begin.

As already mentioned above it happens with relatively great probability, where randomly occurring characters are delivered by the random generator in the numerical worm, that a character does not appear for some time, but, however, as already stated a complete substitution alphabet is not present until each of the n possible characters of the random generator has occurred at least once, then it is recommended that the barrier 5 should be somewhat modified as shown in Fig. 2. For this purpose the output lines 6 are combined by suitable decoupling diodes 30 and connected to a counter 31 of length n-i. If this counter has reached the state n-l it means that the first n-l characters of the substitution alphabet have been read off the numerical worm and that only the nth character is missing. This nth character is unambiguously determined by the preceding n-l characters. The counter 31 then sends, via the decoupling diodes 32 for example, a pulse to all the input lines of barrier 5 so that only the one as yet unopened switch delivers a pulse to the appropriate line 6 and the last missing code character is formed in the coding device 7. In order to attain an unambiguous result the random generator 1 in this embodiment is stopped by the closing pulse of the above described counter 31. In the same way the rcturn of the barrier 5 into the output setting, i.e.

closing of switches 51. 52.. 5 can be effected via a suitable delay circuit 33 from this counler.

Fig. 3 shows a further embodiment for an arrangement for carrying out the system according to the invention in which the storage device for the complete substitution alphabet is simplificd insofar that only one single character of the substitution alphabet necds to be stored at any one time. This embodiment is especially adapted to the encyphering of teleprinter characters which are represented as five unit binary numbers. The number n of the letters of the clear text alphabet, as well as of the individual substitution alphabets, amounls to n = 32. The random generator is again shown at 1 and feds a random sequence of five unit binary numbers to encoder 3 via line 2. Oul of the 32 output lines 41,4.... 432 only one al a time is actuated by each of the 32 pulse combinations fed via line 2 and causes one of the 32 flip-flops conlained in barrier 5 to trip over from the zero to the unity condilion. The trip-over of a flip-flop causes a pulse to be transmitted via the condcnscr 40 to one of the corresponding 32 input lines 6,, 62... 632 of encoder 7 so that a corresponding pulse combination results in the five output lines 8. So far the arrangement is similar to that described in Fig. 1. Each of the 32 input lines 6, ... 632 is connected via a decoupling diode to a common line 41 which leads to a counter 42. The counter 42 represents therefore the number of characters of the actual substitution alphabet which have already been delivered via line 8, just as counter 31 of Fig. 2 does.

The output lines 8 of the encoder are connected to a five-step register 43 in which that character of the substitution alphabet, which is defined by the last line 6 to be affected by a pulse, is stored.

To encypher clear text into secret text there are provided moreover the clear text input register 44 and the secret text output register 45 which also contain a five unit binary number, thus a clear text letter or a secret text letter.

Between the corresponding positions of the five unit binary counter 42 and the clear text input register 44 is a coincidence circuit 46 which transmits a pulse to the line 47 when the binary number in register 44 corresponding to the clear text letter coincides with the binary number in counter 42. At this moment the register 43 is fillcd via lines 8 with the corresponding character of the substitution alphabet The pulse delivered from the coincidence circuit 46 via line 47 is fed to a gating circuit 48 which connects the cells of register 43 with the corresponding positions of the secret text output register 45 so that the substitution letter passes out of register 43 into register 45 and from there is fed to the output terminal 49.

Aftcr the transmission of the secret text letter the counter 42 goes on counting to the end, that is until all the 32 flip-Rops of barrier 5 have trippcd over and it then transmits over line 50 a pulse which causes all the flip-flops of barrier 5 to rctum to the output condition. Simultaneously the random generator can be stopped by this pulsc. By this time the next clear text letter can be fed from terminal 51 to the register 44 and, after the rcncwed start of the random generator 1 via terminal 52, starts the encyphering of the next letter in the same way. It is advantageous to make use of the separated stop pulse from the teleprinter character for starting the random generator because the occurrence of the stop pulse cnsures that the five pulse positions of the teleprinter character are recorded in the register 44.

For decyphering a secret text into a clear text two further five unit binary registers are provided, namely the secret text input register 60 and the clear text output register 61. The Icuers of the secret text are individually passed form terminal 62 and recorded in register 60.

After the registering of a leuer the random generator 1 is set in motion via the starting terminal 52 and the letters of the substitution alphabet are fed, in the manner already described, sequentially into register 43 via lines 8. Between register 43 and line 64 is a coincidence circuit 63 which delivers a pulse to line 64 when the substitution letter already fed into 43 coincides with the secret letter which is stored in register 60 and which is to be decyphered. Simultaneously the counter 42 has, in the manner already described, counted off the ordinal number of the substitution letter in the appropriate substitution alphabet and also the binary number of the corresponding clear text letter. Pulse line 64 leads to a gating circuit 65 which, when the contents of registers 43 and 60 coincide, causes the content of counter 42 to be transmitted into the clear text output register 61.

This is however exactly that clear text letter, corresponding to the secret letter, which can be taken out of register 61 via terminal 66.

Also in this decyphering process the counter 42 goes on counting up lo binary number 32 and delivers via line 50 a pulse for re-setting the flip-flop of barrier 5 and for stopping the random generator. Thereupon the decyphering of the next secret text letter fed to terminal 62 can be begun with the next substitution alphabet Furthermore the arrangement described with reference to Fig 3 can, in order to save time, be so set up that the 31st letter of the substitution alphabet stops the random generator and simultaneously trips over that flip-nop of barrier 5 which is the last one slill in the commencing state. For this to happen the line 50 must receive from counter 42 a pulse at the time of number 31. This pulse is fed via decoupling diodes not to the right, as shown, but lo the left inputs of the flip flops of barrier 5 so that the last nip-flop trips over. The re-setting of the flip.flops can then be effected by the same pulse via a delay line just as in the embodiment of Fig. 2.

Fig. 4 shows a modification of the arrangement of Fig. 3 in which, instead of the flip-flops of barrier 5, magnetic rectangular cores with almost rectangular hysteresis curves, more especially ferrite cores, are used.

Such ring cores, which are also called rectangular cores for short because of the rectangular hysteresis curve of their material, have, as is known, two stable magnetic states and can be changed from one to the other stale by means of electrical pulses whereby the latter state is mainlained until a reverse pulse restores the first state. Fig. 5 shows the known characteristic of such rectangular cores. Two positive pulses of amplitude 1/2i, bring the core in question from the zero condition to the unity condition and a negative pulse of amplitude -iO returns the core from the unit condition to the zero condition. Using rectangular cores as bistable elements in barrier 5 simplifies, as a study of Fig. 4 shows, the coding effected by the encoding matrix 3 in Fig. 3. This is because the rectangular cores are arranged in the form of a switching matrix which can be used in known fashion for a part of the encoding.

In place of the encoder 3 of Fig. 3, which encodes in a l-from-32 code the five bits fed via lines 2, two encoders 3' and 3" are provided of which the first encodes 3 bits in a I-from-8 code and the second encodes the remaining two bits in a from4 code.

The outputs of encoders 3' and 3" are fed to the column or line wires of a coincidence matrix consisting of rectangular ferrite cores 70 in which only one ferrite core at a time is made to trip over magnetically by the coincidence of the pulses in a selected line wire and in a selected column wire. Thus each binary number, which can take the values zero to 31, delivered by the random generator corresponds to the trip over of a quite definite ferrite core 70 from the zero to the unity state. If the same binary number appears again in the same substitution alphabet the core has already tripped over and so nothing further happens. Each core is, not shown in Fig.

4, provided with an output line 6, as shown for a single core 70, in Fig. 6. Thus 32 such output lines 6,, .... 6, lead from the whole matrix of Fig. 4 to the encoder 7, not shown, as already explained with reference to Fig. 3. Moreover a line 71 is threaded through all the cores 70, said line delivering a pulse every time there is a trip over of a core 70. This corresponds to the pulse delivered via line 41 in the arrangement of Fig.

3, which pulse is fed to counter 42. Counter 42 thus counts the number of rectangular cores already tripped over and thus the number of substitution alphabet letters already available.

Moreover it is connected, in the same way as the arrangement of Fig. 3, via a coincidence circuit with the clear text input register 44 and by means of a gating circuit 65 with the clear text output register 51. These registers, as also registers 60, 43 and 45, are again represented in Fig. 4 in the same way as encoder 7 as they have the same functions and therefore need not be described again.

Regisler 42 in Fig. 4 has an output line 72 which delivers a pulse to a pulse former 73 after the 31 counting steps. This pulse former 73 passes the pulse as a positive pulse to line 71 so that on the one hand the last rectangular core 70, which has not yet tripped over, is brought into adjustment and also the counter 42 is further set to a unit, i.e. to number 32. This causes counter 42 to transmit a further pulse via line 74 which is formed in a pulse former into a negative pulse which is also fed to line 71. This negative pulse, to which the counter 42 is insensitive, is amplified in pulse former 75 to such an extent that il throws all the cores 70 via line 71 into their original state, that is into the zero state, so that barrier 5, which is comprised of cores 70, is again open at all its 32 outputs and the compilalion of the next substitution alphabet can begin.

In Fig. 7 is shown an embodiment for the barrier 5 in which relays A, B ... N are used as switches. The remaining parts of the encyphering device are the same as in Fig. 3 and are therefore not shown again. Here it is prcsumed that - as in the example of Fig. 3 - at the 31st character of the substitution alphabet the counter 42 delivers a pulse for closing the switch corresponding to the last letter of the substitution alphabeL The lines 4,, 4,... 432 lead to relays A, B ... or N, each of which has contact, which is automatically held closed denoled by the corresponding small letter and by the index 1 and a contact denoted by the index 2 which delivers an output pulse to the appropriate lines 6,. 6,... or 6;. For re-setting all the relays A ... N a relay R is provided which is actuated by the 32nd pulse of counter 42 and opens the quiescent current contact r so that all the relays A...Nopen.

Fig. 8 shows an embodiment for barrier 5 in which gas discharge tubes ........ R,2 are used as switches which are fired at their grids via lines 4,, 42.. 433 in arbitrary sequence. The last gas discharge tube is, as already explained in the example of Fig. 4, once again fired by the pulse former 73 and all the relays are discharged via contact r of a Relay R, not shown, as soon as the 32nd pulse from counter 42 arrives.

In order further to simplify the arrangement described with reference to Fig. 3 or the modified arrangements of Figs. 2, 7 or 8, the secret text registers 45 and 60 on the one hand and clear text registers 44 and 61 on the other hand can each be combined in a single register, whcreby switch-over means are provided, lo alter correspondingly their logical function for encyphering and decyphering.

Such an arrangement is shown in Fig. 9 where, instead of the secret text registers 45 and 60, the common register 80 is used for encyphering and decyphering, said register 80 being combined with register 43, by means of the And-circuits 48 as also by the And-circuits 63, in the same way as has already been explained with reference to Fig. 3. In the same way inslcad of clear text registers 44 and 61 of Fig. 3 there is the common clear text register 81 for cncyphering and decyphering, and said register 81 can be combined likewise with counter 42 by And-circuit 46 and also by Andcircuit 65 as explained with reference to Fig. 3.

Furthermore a switching arrangement 82, 83, 84, 85, 86 and 87 is provided in which all the switches are activated simultaneously.

In the lower position, in which switches 82, 84 and 86 are closed, the arrangement is set for encyphcring. The clear text input (terminal 51) is switched lo register 81 and the secret text output (Lerminal 49) is taken from register 80.

Simultaneously the And-circuit combinations 46 and 48 are connected together by switch 84.

In the upper switch position, in which switches 83, 85 and 87 are closed, the arrangement is set to cypher The secret text is fed from terminal 62 via switch 83 lo register 80 and the clear text is fed via switch 87 from register 81 to terminal 66. Simultaneously the And-circuit combination 63 is connected to And-circuit combination 65. The above switches can of course consist of clcclronic switching elements. It must be understood that in the embodiment examples schematic circuits are shown which can be realised by electronic means in the most divers ways.

Fig. 10 shows a block diagram of a complete equipment for encyphering and decyphering teleprinter signals. It contains an encyphering circuit 101 with a synchroniser, the details of which will be explained with reference to Figs.

11 and 15, a character preparing arrangement 103 for the clear text characters produced by a teleprinter or for the encyphered characters received from the encyphering circuit 101, which will be explained in detail with reference to Fig. 12, a preparing equipment 104 for the encyphered teleprinter characters arriving from the teleprint content of a teleprinter character from the character preparing device 103 (Fig. 10) into the acceptance register 111 via a multiple line 120 a multiple Andtircuit 121 is used together with multiple Orcircuit 119; for acceptance of the information of a teleprinter character from the preparing equipment 104 via a line 122 a multiple And-circuit is used in like manner together with Or-circuit 119.

The encyphering circuit of Fig. 11 contains also a cypher counter 124 which counts out at its output T6 the number of the substitution letters delivered from the cypher generator 1 during the encyphering of a single clear text letter including the letters delivered more than once and which delivers a signal at the ninety sixlh letter (setting 1,100,000) to a bistable switch 127 via a decyphering And-circuit 125 and an Or-circuit 126. At setting 1,111,111, that is at the hundred and twenty-seventh letter, the cypher counter delivers a signal via the decyphering And-circuit 128 to a bistable switch 129. Multiple And-circuits 130 and 131 are used for delivering the encyphered or decyphered information of a teleprinter character to the character preparing equipment 103 or 104 from acceptance register 111. The other elements of the encyphering circuit of Fig. 11 will be explained in more detail below with the description of the function because they can only be understood in conjunction with the function.

Fig. 12 shows an example of the design of a character preparing equipment 103 of Fig. 10. lot contains a teleprinter 201 and a teleprinter receiver 202 of normal design as well as a transmitlreeeive switch 203 by which a negative potential is applied during transmission to a line S and during reception to a line e in order to open the signal path during transmission or reception via appropriate And-circuits.

Moreover the character preparing equipment contains a two-part shift register consisting of a total of seven bistable switches, five of which form the register portion 204 which accepts the five information bits of a teleprinter character.

The second part of the shift register 205 consisting of two bistable switches is used for passing on the teleprinter character to receiving machine 202 or for inserting, before the passing on, the starting step in the teleprinter character via line T, The outputs of the five bistable switches or register part 204 are connected lo the multiple line 120 which leads via the Andcircuit 121 and the Or-circuit 119 to the acceptance register 111 of Fig. 11. The inputs of register portion 204 are connected via multiple line 206 with the multiple And-circuit 103 of Fig. 11, via which the output of acceptance register 111 can be taken in parallel to the shift register 204. Furthermore the shift register 204 has an input line 207 through which the teleprinter signals produced by teleprinter 201 are passed in sequence via the And-circuit 208 to a smoothing circuit 209 and an amplifier 210 in the shift register. The bistable switch 211 is used for producing the shift synchronism, said switch being closed from the amplifier via differential unit 212, by the leading edge of the teleprinter character start pulse or by a pulse from the Andcircuit 213 via Orcircuil 214, and an astable multivibrator 215 is thus switched on, said vibrator producing pulses in the normal teleprinter system synchronism and these pulses being formed into short pulses in a monostable flip flop 216, amplified m amplifier 217 and fed as shift pulses to the shift register 204, 205. For reading off the shift pulses there is a three stage binary counter 218 which switches out the switch 211 via decoding Andcircuit 219 and line 220 and thereby interrupts the production of further shift pulses when five information bits are present in register portion 204. Line 220 leads also to the encyphering circuit of Fig. 11 there to effect the acceptance of the contents of register 204 into acceptance register 111 and to effecl further functions.

Fig. 13 shows the design of the character preparing device 104 of Fig. 19 which is in circuit between the distant line and the encyphering circuit 101. The design is essentially the same as that of the circuit in Fig.

12.

331 is the receiver relay which closes the receiving contact 332 corresponding to the encyphered teleprinter character arriving via line 333. 334 is the transmitting relay which closes the transmitting contact 335 for the outgoing transmission line 336. This arrangement is otherwise designed similarly to the arrangement of Fig. 12, the receiving contact 332 taking the place of teleprinter 201 and the transmilting relay 334 taking the place of the teleprinter receiver.

The arrangement also contains a shift register 304, 305 corresponding to the shift register 204, 205 of Fig. 12 as well as the other parts already explained with reference to Fig. 12; the corresponding pans of Fig. 13 each being given a reference higher by 100.

The outputs of shift register 304 are connected with the multiple line 122 which, via the And-circuit 123 and the Or-circuit 119 of Fig. 11, permits an encyphered teleprinter character present in shift register 304 to be accepted into the acceptance register 111. The transmission of an information from the acceptance register 111 to the shift register 304 is made through the multiple And-circuit 130 and the multiple line 306.

Fig. 14 shows an embodiment for the character barrier 105 of Fig. 10 which is designed similarly to the character barrier described already with reference to Fig. 4.

Unlike the latter however the encyphering of the character by the encyphering matrix of Fig. 3 is dispensed with because in the present embodiment the cypher letter is taken from the register 112 directly to the acceptance register 111 (Fig. 11) via the multiple And-circuit 117 and multiple Or-circuit 119. The five bit information taken from the cypher register 112 in parallel along multiple line 132 is decyphered in the decoding matrices 133 and 134 and whenever an enquiry pulse T, arrives via amplifier 135 the corresponding magnetic core of matrix 136 trips over and produces on the reading wire 137 an output pulse which is fed via amplifier 138 and line 139 to And-circuit 140 in Fig. 11. This output pulse on line 139 occurs only if the cypher letter present in cypher register 112 had not yet appeared in the current encyphering cycle for only then can one of the 32 cores of matrix 136 trip over. The character barrier of Fig. 14 is also provided with an erasing wire 141 threaded through all the cores of the matrix 136, said wire returning all the cores to their output-(zero)-state with the arrival of a pulse T, via amplifier 142.

In Fig. 14a is shown the threading of the individual lines for a single core 143 of matrix 136. 144 is the line lead, 145 the column lead. It will be seen that the lines 144, 145 and 137 are wound through the ring core in the same sense whilst line 141, which effects the return tripover of the cores, is threaded in the opposite direction.

Before describing in detail the function of the encyphering equipment shown schematically in Fig. 10 and the working of its individual parts with reference to Figs. 11, 12, 13 and 14 a more detailed explanation will now be given of the circuit symbols and preferred embodiments of the same mentioned in the figures. Bistable switches of known design are used as switches and more particularly as register elements, said bistable switches being designed in the manner of an Eccles-Jordan circuit Fig. 16a shows the symbols employed in the above figures for such a switch with the information inputs 151 and 152 and the corresponding outputs 153 and 154.

Moreover the circuit contains two synchronising inputs 155 and 156 which are indicated by broken arrows. By means of a pulse applied to the synchronising input in question the information present as a D.C. potential at the appropriate information input is transmitted to the appropriate output.

Fig. 16b shows a circuit with two transistors 159, 160 for carrying out this function whereby the corresponding input and output terminals are given the same references as in Fig. 16a.

Between the application of an input potential to one of the terminals 151 or 152 and the appearance of the corresponding output potentials a specific synchronising pulse must appear al a given time during which the appropriate pre-storage condenser 157 or 158 is charged, the charge of said condenser then being transmitted by the tailing edge of the synchronising pulse lo the base of the appropriate transistor and so switching over the switch. In the circuit shown as also in what follows, il is assumed that the Yes-condilion is represented by a negative potential of about -7 volts and the No-condition by a potential of about 0 volts. Everything in the circuits given in the following has reference to this meaning of the potentials.

Fig. 1 7a shows the symbols used in the circuit diagrams for simple And-circuits with inputs 161, 162, 163 and output 164 and also shows a favourable embodiment of such an Andcircuit with two diodes 165 and 166 which are so connected that a negative potential applied to terminal 163 via resistance 167 will be transmitted to output terminal 164 only when the terminals 161 and 162 are also negative.

In Fig. 17b is shown the symbol for an Orcircuit with the input terminals 171 and 172 and output terminal 173 and also their function for negative pulses with diodes 174 and 175 the anodes of which are connected together at output terminal 173.

Fig. 17c shows the symbol used in the circuit diagrams for a multivibrator which is used as a pulse generator al several positions. When one of the preceding potentials is applied to the input terminal 177 a pulse train of the preceding frequency continues to be produced at output terminal 178 until the input potential is again switched off. Such circuits are generally known as astable flip-flops.

Fig. 17d shows the symbol used in the circuit diagrams for a pulse former which can for example be a monostable flip-flop of known type. Finally Fig. 17e shows the symbol for a pulse amplifier of any known design.

The composite circuit of an And-circuit, an Or-circuit and a bistable switch, as used at several positions in the embodiment example of the invention, is shown in Fig. 18; on the left as a symbolic circuit diagram and on the right in a technical form. The references are the same as those of Figs. 16 and 17. The switch is assumed to be Off, i.e. it shows al its terminal 154 a potential of about -7 volts and at its terminal 153 a potential of 0 volts. In order to switch On a negative potential of -20 volts is applied for a specific period, e.g. from a further generator element described below, lo terminal 163. At the terminals 161 and 162 there are negative potentials of -7 volts which represent the further conditions for the switching On. Permanently at the synchronising input terminal 156 is a synchronising pulse whose quiescent potential is 0 volts and whose pulse peak is at -7 volts for example. During a synchronising pulse the condenser 157 is charged via diode 180 and the stored charge is transmitted during the trailing edge of the synchronising pulse via diode 181 to the base of transistor 159, so that the lalter cuts off and the potential at its output terminal 153 jumps from about 0 volts to -7 volts. The switching Off of this switch takes place in similar manner by the action of transistor 160.

This is not shown in Fig. 18.

Fig. 19 shows on the left the symbol and below on the right a circuit for a generating element which, like the register element of Fig.

16, has a pre-storage device in which its switching-over is prepared. The switching over is effected by synchronising pulses which are fed to terminals 185 and 186. The generating switch is switched on by means of a synchronising pulse applied to terminal 186 via an And-circuit 188, i.e. the potential at its output terminal 189 jumps from zero to -20 volts. The output voltage is returned to 0 volts by the application of a synchronising pulse via Andcircuit 187. Such a generating switch should, in comparison, be low ohmically loaded at its output terminal 189. It is designed as a fliefilop from transistors 190 and 191 which have feedback via RCcombinations. The feed-back path from the base of transistor 191 to the collector of transistor 190 contains a further transistor 192 which, by means of circuit leads, connects the output terminal 189 directly to the ballery voltage and thus brings about the low ohmic resistance of the output. The rest of the operation is similar lo that of the register elemcnt of Fig. 16.

In the top left of Fig. 20 is given the symbol occurring in Figs. 11, 12 and 13 for a counting register in which the binary information can be fed in sequence via line 401 and from which the information can be read off via the multiple line 402. Fig. 20 shows beneath the symbol a circuit example for such a counting register using the previously mentioned switch (Fig. 16) in which the same refcrcnces are used for the input and output. A more detailed explanation is therefore not necessary.

Fig. 21 shows on the left the symbol and below right a circuit example for a shift register as is used in Fig. 12 for example as register 204 or in Fig. 13 as register 304. The information can be fed to this register either in parallel via the multiple line 411 or in sequence via the twin line 412. Thc shift synchronisms are fed to terminal 413 and shift the information present in the register one place to the right and from the last position out of the register via output lines 414. The register can be read off in parallel at any time via the multiple line 415.

Fig. 22 shows at the lop the symbol used in Fig. 11 for example of a multiple And-circuit and of a multiple Or-circuit and underneath a circuit example for such a connecting circuit using the symbols of Fig. 17. The information on lines 421 offered in parallel from a register, not shown, is then transmitted to the output lines 422 when the condition lines 423, 424, 425 and 426 all carry negative voltages of -7 volts for example. The lines 428 can be connected with a similar combination of And-circuits (not shown).

At the lop in Fig. 23 is shown the symbol used in Fig. 11 for the comparison circuits 115 and 116 and underneath a circuit example. Only when Lhe opposing register elements of registers 431 and 432 show at a given time, in pairs, the same output voltages (-7 volts or 0 volts) as their L-outputs and at their 0output docs a voltage of -7 volts appear on the reading off line 433, said voltage being used as a positive criterion to show that the contents of the two registers are the same. In all other cases the voltage of the reading off line is 0 volts.

Now that the essential circuits of the encyphering equipment of Fig. 10 have been described the function of the equipment with reference to an example will be described in which a telegraph signal keyed in the teleprinter 201 (Fig. 12) is delivered to the distant line 336 (Fig. 13). Switch 203 (Fig. 12) is at S (transmit) so that all the Andcircuits with an input S receive there a negative potential. If for example the key is pressed for letter A of teleprinter 201 then a pulse is fed via Andcircuit 208 and smoothing circuit 209 to amplifier 210, said pulse having the shape somewhat like that shown in Fig. 24a. The signal consists of a starting step, which is always a mark, five information bits (in the case of "A" two marks and three spaces) and a stop step which is always a space. The bistable switch 211 is switched on by the first negative edge, i.e. the leading edge, of the starting step via the differentiating unit 212, the output voltage of said bistable switch being shown in Fig. 24b.

The output of switch 211 controls the astable multivibrator 215 which delivers a pulse train.

The monostable flip-flop 216 is driven by the positive edges of this pulse train and produces the narrow synchronising pulse of Fig. 24c. The spacing d between positive edge and positive edge of this pulse train T, adapts itself to the teleprinter and is synchronised with the latter's inlemal synchronising frequency. In the German teleprinter network this spacing is 20 m.sec.

With the aid of this pulse train T, the teleprinter character is shifted bit for bit from amplifier 210 into shift register 204. The output voltage of amplifier 210 is simultaneously fed via an Andcircuit 240 and an Or-circuit 241 to an amplifier 242 which actuates the receiving coil of teleprinter 202 and causes the same letter which was keyed at 201 to be transcribed at 202.

The pulse train of amplifier 217 also drives the counter 218 which is so designed that it returns to its rest position after every 7 input beats. After six pulses the And-circuit responds and produces an output signal as in Fig. 24d.

This causes, via line 220, the bistable switch 211 to return again to its position of rest with the next synchronising pulse T, and the astable multivibrator 215 thus to stop. The same startstop mechanism is maintained in this period conlrol as is usual in teleprinter machines. The astable multivibrator 215 must therefore be freshly switched on with each starting step of a teleprinter letter.

With the response of And-circuit 219, that is after the sixth step of the teleprinter signal has been shifted into the register 204/205 and thus the five information steps of the teleprinter signal are present in register 204, the content of register 204 is taken with the pulse into register 111 via the multiple And-circuits 121 (Fig. 11).

When the astable multivibrator 215 is switched off the clear text signal transmitted by teleprinter 201 is present in register 111. This instanl of time is denoted by tin Fig. 24. The output signal of the Andcircuit 219 (Fig. 24d) arrives via line 220 and an And-circuit 243 as well as an Or-circuit 244 (Fig. 11) at the bistable switch 129 which is switched on at the instant t1, that is simultaneously with the switching off of the bistable switch 211.

With the switching on of the bistable switch 129 at time t, the actual encyphering process begins, i.e. the substitution of the clear text letter present in the register 111 by a substitution letter. Switch 129 switches on a multivibrator 245 which produees a synchronising pulse To which is amplified in the amplifier 246 and fed to synchronising generator 102 (Fig. 10). The synchronising generator, which controls the periodic course of the process in the encyphering circuit of Fig. 11, is shown in Fig.

15. It consists of a four stage binary counter 250 in a known circuit (similar to Fig. 20) whose individual bistable switches 251, 252, 253 and 254 have output voltages which are reproduced in the curves b, c, d and e of Fig. 25. Curve a represents the input synchronism To of the counter. The counter outputs, as shown in Fig.

15, are connected via And-circuits to the inputs of the generating elements 255, 256, 257 and 258 to all of whose synchronising inputs is fed the synchronising pulse TD. The generating elements are designed as in Fig. 19 as already described. By means of the connection, shown in Fig. 15, of counter 250 lo their inputs there appear at the outputs of generating elements 255, 256, 257 and 258 synchronising pulses T3, T,, T3 and T6 which are shown in Fig. 25 in the curves f, g, h, j. Synchronising pulse To and thereby also pulse T3, T4, T5 and T6 continue for as long as the counter 124 (Fig. 11) goes on counting, i.e. until the synchronising pulse T6 has occurred 127 times. With the next T6-pulse the switch 129 switches off and stops the multivibrator 245. Simultaneously there is produced in a monostable flip-nop 259 a single occurring pulse T,, (Fig. 25k) which indicates the completion of the encyphering and effects the acceptance into register 304 (Fig. 13) of the substitution letter now present in register 111. It also sets switch 127 to L (On) and initiates the selection of the next substitution letter.

This will now be described with reference to Fig. 11. With the aid of synchronising pulse T3 a random character of five bits is taken from the random generator 1 via the And-circuit 113 into register 112. The position of the five bistable switches of this register is decoded in the decyphering matrices 133 and 134 (Fig. 14) and so amplified that one of the four horizontal wires and one of the right vertical wires of the ferrite core storer 136 can carry a current. When the synchronising pulse T, becomes negative (Fig. 25g) the current can now in the two selected wires via amplifier 135 and a specific core of the ferrite core storer 136 is selected.

The position of each core corresponds to the 32 possible random characters which can be represented by the five unit register 112 (Fig.

11).

The object of the ferrite core storer is to determine whether the appropriate random character arriving in register 112 has appeared there for the first time or whether it had already been there once. At the beginning of a cycle all the cores are in the output position 0. With the arrival of a random character the core corresponding to this letter is selected via the decyphering 133 and 134 and is magnetised into its other stale. If at any later time during the cycle the same character enters register 112 then a core is selected which is already magnetised and so will deliver no output voltage via line 137 to the reading amplifier 138. If the reading amplifier 138 delivers an output signal during the synchronising pulse T5 (Fig. 25h) then the And-circuit 140 (Fig. 11) is also opened. The comparison circuit 115 has in the meantime tested whether the clear text character present in register 111 is identical with the random character present at the same time in register 112. If they are not identical then the output signal of comparator 115 remains at 0 volts. The And-circuits 118 and a further And-circuit 261 can therefore not respond. The And-circuit 140 then simply drives an amplifier 262 with whose output signal the five unit counter register 114 counts up to one. This register thus registers how many different letters have so far been delivered from the random generator.

This substitution in register 111 occurs when the comparison circuit 115 determines that the content of register 112 is identical with the clear text character previously present in register 111.

At any time when 115 shows a coincidence between 111 and 112 the content of counter register 114 will - during a synchronising pulse T5 - be taken via And-circuits 118 into register 111 and the clear text character of the teleprinter, prcscrved up to then in the register 111, is erased. The new content of acceptance register 111 now represents the cypher character which will be transmitted in place of the clear text character Simultaneously with acceptance a bistable switch 264 is switched off via Andcircuit 261 and an Or-circuit 263, the negative output voltage on line 265 of said bistable switch having up to now kept open the Andcircuits 118 and 261. At this time no further cypher character can be fed from register 114 into register 111 even if the comparison circuit 115 should respond again.

With synchronising pulse T6, which occurs only once during each cycle of the counter 250 (Fig. 15), the counter register 124 is set one place forward each time. If this counter has reached setting 96, then And-circuit 125 responds and resets the bistable switch 127 with the aid of synchronising pulse T6 via Or-circuit 126. The switching off of the L-output voltage of switch 127 prevents further characters from being taken into register 112 from random generator 1 because the And-circuit 266 responds during synchronising times T3. With the help of the amplified output signal of this Andcircuit the register 112 is used via a special input as a counter register and raises its content up one place. Thus it can be effected that all the cyper characters. which have not been randomly delivered from the cypher generator 1 during the preceding 96 synchronising steps, are now produced by a single count through of register 112 and thus each core of the ferrite storer 136 is reversely magnetised. After the passage of 126 pulses, which are counted in counter register 124, the binary counter 114 is again in its 0 output state for then all the 32 cores of storer 136 have uipped over.

It is also quite possible that the random generator has already delivered 31 of the possible 32 random characters before the passage of 96 pulses T6. Then an Andtircuit 267 responds which resets the bistable switch 127 via the Or-circuit 126 and a further prevents any further random characters being taken from random generator 1. If the counter register 124 has arrived at content 127 then And-circuit 128 responds and causes the bistable switch 129 to be again reset with the next synchronising pulse T6 and so stops the multivibrator 245. The monoslable nip nop 259 is switched on by the switching-off edge of switch 129 via a differential unit, the output of said nip-nop delivering pulse T,. This pulse drives the amplifier 142 (Fig. 14) which ensurcs that all the ring cores of ferrite core storer 136 are returned to their output state. Also the cypher letter already present in register 111 is taken by pulse T via And-circuil 131 lo register 304 (Fig. 13).

Whilst now the above described proccdure for the next clear text letter lo be cncyphcrcd is taking place in encyphering circuit of Fig. 11 by means of pulse T, the cypher letter present in register 304 must be prepared in the preparing circuit of Fig. 13 as a teleprinter signal with start and stop pulses. For this purpose the penultimate bistable switch in shift register portion 305 is activalcd by pulse T, and set to One which contains the starting step of a teleprinter signal. The bistable switch 311 is simultaneously r

Claims (15)

**WARNING** start of CLMS field may overlap end of DESC **. 126. The switching off of the L-output voltage of switch 127 prevents further characters from being taken into register 112 from random generator 1 because the And-circuit 266 responds during synchronising times T3. With the help of the amplified output signal of this Andcircuit the register 112 is used via a special input as a counter register and raises its content up one place. Thus it can be effected that all the cyper characters. which have not been randomly delivered from the cypher generator 1 during the preceding 96 synchronising steps, are now produced by a single count through of register 112 and thus each core of the ferrite storer 136 is reversely magnetised. After the passage of 126 pulses, which are counted in counter register 124, the binary counter 114 is again in its 0 output state for then all the 32 cores of storer 136 have uipped over. It is also quite possible that the random generator has already delivered 31 of the possible 32 random characters before the passage of 96 pulses T6. Then an Andtircuit 267 responds which resets the bistable switch 127 via the Or-circuit 126 and a further prevents any further random characters being taken from random generator 1. If the counter register 124 has arrived at content 127 then And-circuit 128 responds and causes the bistable switch 129 to be again reset with the next synchronising pulse T6 and so stops the multivibrator 245. The monoslable nip nop 259 is switched on by the switching-off edge of switch 129 via a differential unit, the output of said nip-nop delivering pulse T,. This pulse drives the amplifier 142 (Fig. 14) which ensurcs that all the ring cores of ferrite core storer 136 are returned to their output state. Also the cypher letter already present in register 111 is taken by pulse T via And-circuil 131 lo register 304 (Fig. 13). Whilst now the above described proccdure for the next clear text letter lo be cncyphcrcd is taking place in encyphering circuit of Fig. 11 by means of pulse T, the cypher letter present in register 304 must be prepared in the preparing circuit of Fig. 13 as a teleprinter signal with start and stop pulses. For this purpose the penultimate bistable switch in shift register portion 305 is activalcd by pulse T, and set to One which contains the starting step of a teleprinter signal. The bistable switch 311 is simultaneously rcset by pulse T via And-circuit 313 and Orcircuit 314 and the astable multivibrator 315 is switched on, its pulse train being rc-formcd in the monostable stage 316 into short synchronising pulses T, and amplified in amplifier 317. Synchronism T which is dimensioned exactly like T1 and limilcd by counter 318 to seven pulscs, shifts Lhe cypher letter into the last bistable switch of register portion 305 whose output gocs to amplifier 342 via And-circuit 370. This amplifier drives the transmit relay 334 whose contact 335 can operate directly on the distant linc. Counter 318 is shifted one setting further by each shift pulse. Andtircuit 319 responds to the pulse so that with the next synchronising pulse T8 to arrive the bistable switch 311 is again switched off and the astable multivibrator is stopped. The decyphering process at the receiving station will now be briefly described. For reception of an encyphered text and for decyphering the same the transmit/receive switch 203 (Fig. 12) is set to e. A letter received from the receiver relay 331 (Fig. 13) is passed on to amplifier 310 via Andcircuit 308 and smoothing circuit 309. The first negative output edge of this amplifier 310 drives switch 311 via the differential unit 312 and the Or-circuit 314, said switch 311 causing synchronising pulse T; tobeproduced and so taking care of the acceptance of the received cypher character into the shift register. With the response of Andcircuit 123 (Fig. 11) the information is taken with the next pulse T1 from register 304 into register 111. Simultaneously the bistable switch 129 is switched on via And-circuit 343 and Orcircuit 244. The subsequent drive procedure takes place as in the encyphering of a character to be transmitted except only that the comparison circuit 116, which compares the cypher letter present in register 111 with the setting of counter register 114, comes into action and determines whcn the information present in register 112 is to be taken via And-circuit 117 in to register 111. Simultaneously with acceptance the And-circuit 361 responds and switches off the bistable switch 264 via Or-circuit 263 and so prevents a repeated transmission from register 112 into register 111. When the counter register 124 has switched off the bistable switch 129 via And-circuit 128 the content of register 111 is transmitted with pulse T, via And-circuit 130 to register 204 (Fig. 12) and simultaneously the bistable switch 211 is switched on via Andcircuit 213 and Or-circuit 214. Also the pulse T, sets the penultimate switch of shift register 204/ 205 to Onc. This makes the starting step of the teleprinter character availablc. This starting step appears after the first shift pulse in the last bistable switch of register portion 205, the output of which drives amplifier 242 via the opened And-circuit 270 and the Or-circuit 241, said amplifier activating the receive coil of the teleprinter 202. It is now stated that the pulse frequency Tod of multivibrator 245 can be made so high that the time between two pulses T, is not greater than the time necessary for the transmission of a teleprinter character. Thus encyphering and decyphering can proceed without the transmission speed being afrected. WHAT WE CLAIM IS:

1. Process for encyphering of clear texts formcd from n characters of an alphabet and for decyphering the corresponding secret text in which a complete substitution alphabet of n characters changing from character to character is allotted to each character of the clear text

characterised by the sequence of substitution alphabets being selected from a series of characters delivered from a random or quasirandom generator wherein characters occurring more than once during the compilation of the substitution alphabet are suppressed.

2. Process as in claim 1 characterised by the substitution alphabets being composed of the same characters as the clear text alphabet (pure permutation).

3. Process as in claim 1 characterised by its application to the encyphering and decyphering of texts of teleprinter characters.

4. Arrangement for carrying out the process as in claim 1 characterised by the provision of a blocking device with n open pulse gates in the quiescent state each of which is allotted, via a special output of an encoder, to a character of the random generator connected to the input of the encoder and each of which is provided with a device by which the pulse gate is, with the passage of a pulse, blocked to the transit of further pulses.

5. Arrangement as in claim 4 characterised by relay switches being used as pulse gates.

6. Arrangement as in claim 4 characterised by bistable flip-fiop circuits consisting of electron valves or transistors being used as pulse gates.

7. Arrangement as in claim 4 characterised by magnetic ring cores with two stable magnetic states arranged in a coincidence matrix being used as pulse gates, each of said cores being threaded not only with a line and a column wire but also with a re-seuing wire threading all the cores and with a separate output wire for each core.

8. Arrangement as in claim 4 characterised by the provision of a counter which counts the number of pulses allowed to pass through the various pulse gates and which after reaching a predetermined number delivers a pulse which re-sets all the n pulse gates in their quiescent state and which is used to stop the random generator.

9. Arrangement as in claim 8 characterised by the provision of a substitution character register for putting into the selected coding for the transmission of the secret text the character of the substitution alphabet defined by the pulse gate which is the last one activated by the random generator.

10. Arrangement as in claim 9 characterised by the provision of a coincidence circuit between the counter and a clear text register containing the clear text character to be encyphered by means of which, when the counter condition agrees with the content of the register, a gating circuit is opened in order to feed to the secret text delivery register as an encyphered character the character of the substitution alphabet contained in the substitution character register.

11. Arrangement as in claim 9 characterised by the provision of a coincidence circuit between the substitution character register and a secret text input register containing the secret character to be decyphered whereby in the event of the contents of these two registers coinciding a gating circuit is opened so as to feed a binary number held in the counter as a clear text character to a clear text output register.

12. Arrangement as in claims 10 and 11 characterised by the provision in each case of a single register as secret text input and output registers on the one hand and as clear text input and output registers on the other hand and by the provision of switch-over means for connecting these registers with the rest of the circuit in accordance with their logical function in encyphering and decyphering.

13. Arrangement for carrying out the process of claim 1 characterised by the following: (a) An acceptance register to which, at the beginning, is fed the clear text character to be encyphered or the secret text character to be decyphered and from which, at the end, is read off the encyphered clear text character or the decyphered secret text character.

(b) A cypher register to which is fed from a random generator a series of random characters.

(c) A counter register which counts the number of different random characters fed to the cypher register.

(d) A first comparison circuit which in the event of encyphering when the content of cypher register coincides with the content of the acceptance register causes the content of the counter register to be transmitted to the acceptance register.

(e) A second comparison circuit which in the event of decyphering when the content of the counter register coincides with the content of the acceptance register causes the content of cypher register to be transmitted to the acceptance register.

14. Encyphering and decyphering processes substantially as described herein.

15. Encyphering and decyphering arrangemcnts substantially as described herein with reference to the accompanying drawings.