NO121285B - - Google Patents

Description

Key material generator for cryptographic teleprinter equipment.

The invention relates to a key material generator for cryptographic teleprinter equipment and in particular to a key material generator for producing "pseudo-random" sequences.

Many types of key material generators with mechanical rotors or with electric ring counters are known. However, such key material generators have not been found suitable to provide sufficiently long sequences in all cases.

The purpose of the invention here is to provide a key material generator which, with relatively simple circuits, is capable of producing very long "pseudo-random" sequences.

The peculiarity of the invention can be seen from the following

claim.

The above-mentioned purpose as well as the distinctive features specified in the requirements

of the invention is clear from the following detailed description of execution seen in connection with the drawings, where

Fig. la shows a functional block diagram for an embodiment of the invention, Fig. 1b shows a block diagram with the most important connections between the blocks, Fig. 1c-i and 2a-h show a detailed block diagram of the embodiment of the invention, Fig. 3 shows a detailed diagram above block 6 in Fig. 2a, a monostable pulse generator (OS), Fig. 4 shows a detailed diagram of block 5 in Fig. 2a, a reset circuit (RES) and timing pulse generator (25kHz OSC), Fig. 5 shows a detailed diagram of block 14 in Fig. 2a, a teleprinter driver amplifier (DA) and a readout circuit (RC), and Fig. 6 shows the various logic symbols used in

Fig. 1c-i and 2a-h,

Fig. 7 shows how Fig. 1c-i and 2a-h are to be put together for

to form a complete form.

Functional block diagram

Figure 1a shows a functional block diagram which is closely related to the detailed block diagram. The following main parts of the block diagram will be discussed in some detail: Generator for pseudo-random key characters. - Generator for 5-element number "P". --Count "C(P)"„ - Input register "Reg X". - Plug board. - Plug A. Then the start procedure as well as the encryption and decryption process and telex connection will also be discussed in connection with the functional block diagram.

Generator for pseudo-random key characters

The key generator comprises two non-linear feedback shift registers REG I and REG II. Both registers have 15 stages, 12 stages respectively in blocks 10 and 11 and 3 stages in block 12 which also contains the non-linear feedback circuits. The two outputs from the registers are added moduio-2 and the result is stored in REG Z in block 9. During encryption and decryption, the two registers will be affected so that there is always a new key character present in REG Z. All outputs from the two registers are connected individual columns on a plug board. Which outputs are to be used for the non-linear feedback functions are selected with the help of plugs on the plug board, rows F,G,H and K being decisive for REG I and rows S,T,V and W being decisive for REG II .

Generator for 5-element number "P"

"P" is a 5-element pseudo-random number generated by de

two main registers REG I and REG II using 5 outputs from each register, selected using plugs on the plug board. The plug board rows A - E and M,N,P,Q,R are fed to block 8 and the setting circuits for the binary counter "C(P)". Each of the five setting signals is a modulo-2 addition of an output signal from REG I and an output signal from REG II. These combined signals are also pseudo-random.

Counter "( Pl"

The binary counter in block 7 will always count "down". The function of the counter is to control the number of shift pulses that are supplied to the input register REG X. When encrypting, the number "P" is initially inserted into the binary counter, and this means that REG X is supplied with "P" shift pulses, but when decrypting, the number 31-"P" is set into the counter to begin with. When deciphering, the counter will also stop when it contains the number 1, instead of 0, so that the counter in this case counts 31 - "P" - 1 step, i.e. 30 - "P" step. The significance of this will be apparent from the description of the decryption process.

Entry regulations REG X

REG X in block 2 contains 5 equal steps, one of which is shown in fig. let. In addition, this register has a sixth stage which acts as a buffer stage. REG X has two modes of operation: a) It acts as a normal 5-element shift register that receives shift pulses from the input counter in block 4. The input to it

first step comes from the remote printer via the reading circuit in block 14. (This connection is not shown in the functional

block diagram).

b) It acts as a feedback shift register where each stage comprises a modulo-2 addition circuit and a flip-flop, where

any output can be connected to any input by means of solder connections on the coding plug shown on the left of the drawing, and both of these connections are made so that a maximum sequence of 2<5>- 1 = 31 appears. This is symbolized in Fig. la by an X that comes via plug A through the modulo-2 addition circuit in block 1 and

through the input logic in block 2 until the flip-flop.

It is also possible to set each element in the 5-element register REG X to the result of a modulo-2 addition of one element in REG X itself and a corresponding element .in REG Z. This is symbolized in figure la by the lower modulo- 2 addition circuit in block 1

with inputs x and z.

Plug board

The plug table is a table with 10 horizontal rows and 30 vertical columns that form a matrix with 300 intersection points. The vertical columns are connected to the 30 outputs from the two main registers. Randomly selected outputs from the two registers can be connected to the horizontal rows using plugs inserted in the junction points in the plug table. These random outputs will be fed to the non-linear feedback circuits and the number "P" setting circuits. It will also be possible, using row L on the plug board, to preset the two registers with a start content.

Plug A

When REG X acts as a feedback shift register, the five feedback signals will be fed back to the five inputs via plug A. When the cross-connections on this plug are correctly made, REG X will cycle through a maximum sequence, i.e. a cycle of 31 characters.

Other parts shown in the functional diagram

REG Z can optionally be set using a key tape reader (TAPE RDR) via the input logic in block 9. This mode of operation can be used if greater security is desired. The main registers REG I and REG II do not function in such cases, so that the input to REG Z is zero.

REG Y in block 9 is only used in the start phase and then as a buffer register in the encryption unit.

Block 3 comprises the following three circuits:

1) HUK FF is a flip-flop that remembers whether a CARRIAGE RETURN character is detected or not. 2) INSERT./ADD. is used during the start-up phase to reset the input register and the main registers. 3) The block K-^ - Kg is a normal shift register connected as a ring counter and is used for controlling the encryption and decryption processes. The functions of the counter are as follows: At K^, the teleprinter character is read into the register REG X. At K2, the contents of this register are tested to find out whether it contains NUMBER SHIFT or.CARRIAGE BACK. If a NUMBER SHIFT is detected, this character is transformed into CARRIAGE RETURN. If, however, CARRIAGE BACK is detected, this is converted to NEW LINE. The reason for this will become clear when the telex operation is described. At K^, a pseudo-random 5-element number "P" is fed into the binary counter "C(P)". At K^ the input register REG X is connected as a feedback shift register. The counter "C(P>" counts down to zero and the pulses from this counter are fed via FFg^ in block 4 to the input register in block 2. At K^ the contents of REG I

and REG II added modulo-2 and the result returned to REG X. Then the contents are tested to determine whether or not the resulting character is a character allowed in telex. If not, the modulo-2 addition is performed one more time. The resulting character will then always be an allowed character.

Block 7 contains two ring counters, i^-i^and j^-jg. These counters control the start phase. The main registers must always start from a random starting point for each new message to be sent. This means that they must be entered in six characters. The first three are supplied to REG I and the next three are supplied to REG II by means of the two ring counters.

In block 5 there is an oscillator with frequency 2 5 kHz which triggers a monostable multivibrator in block 6 and this in turn triggers the input counter in block 5. The monostable circuit can be set to different time delays with the help of a selector and different teleprinter speeds can thus be realised.

ADDITIONAL in block 5 is used to reset the main registers when sending plaintext or when encrypting with a key band.

TP.LOG. in block 4 it determines the signal delivered to the teleprinter's receiver coil. This signal consists of a start and a stop pulse supplied from the input counter, and five information elements supplied from REG X. In READY position this information comes from register stage 1 and in SEND or RECEIVE position from the buffer stage in REG X. These two inputs are symbolized by X-^ and Xg.

Encryption process

The encryption process starts when the readout circuits receive a start pulse from a plaintext character when operating the keyboard sensor or the automatic transmitter on a teleprinter. The readout circuit starts the input counter, which in turn supplies shift pulses to REG X, REG I and II and REG Z via flip-flop FFg2- In this first phase of the encryption process, REG X is connected as a normal shift register with information input to the first stage, whereby the plaintext character is entered into REG X, while the preceding encrypted character is sent to the teleprinter from register stage Xg via the teleprinter's drive amplifier (DR. FORST.) Now that REG X contains the five information elements, REG Z contains five new key elements, and "C( P)" is set to a new P number. Then REG X is connected as a feedback shift register with period 31, and pulses from 25 kHz (the oscillator) are supplied to "C(P)" and REG X. The supply of these 25 kHz shift pulses is cut off when "C(P)<n>= 0 , i.e. after P pulses. REG X has then been fed through P states and the current state is therefore a function of "P" which can be symbolized by the expression X(P). Finally, REG X is set to the result of the addition X(P) © Z, where Z is the stored output from the key generator If the result of this addition yields one of the two characters that are not allowed, the addition is performed once more, whereby X(P) again yields X(P) © Z © Z = X(P).. In this case X(P) is used as encrypted character.

Decryption process

The decryption process starts when the readout circuit receives a

start pulse from the encrypted character X(P) © Z, which is then shifted into REG X, while the previous deciphered character is read by the teleprinter via its drive amplifier. The contents of REG X and REG Z are then added modulo-2 and the result, X(P) © Z © Z = X(P), is inserted into REG X. The plaintext character X can now theoretically be recovered by advancing REG X up» steps backwards in the cycle, or by shifting the REG X 30-"P" step forward. The latter method is performed by setting C(P) to 31-"P"

and count down to 1. On the receiving side, of course, the addition REG X © REG Z is also performed twice if an unwanted character is detected.

Starting procedure

As previously mentioned, 6 characters are needed to equip the main registers with initial information. These 6 characters are entered first in each message to be encrypted and are chosen either randomly or according to a list of long cycles in the key generator. These characters are encrypted for transmission by using a setting of the main registers determined from the plug board.

Telex connection

The ciphertext must not contain characters that cannot be printed

a normal teleprinter in telex installation or characters that will for-

control transmission on a telex channel, such as WHO'S THERE. Such characters are avoided by not using the characters ALL SPACE and NUMBER SHIFT as cipher text. The ciphertext alphabet thus contains 30 characters, while there are 31 permissible plaintext characters. A pure encryption of these 31 characters is therefore not possible. The problem has been solved by introducing restrictions when encrypting CART BACK and NEW LINE.

Non-permitted characters are very easily avoided by omitting the two critical characters in the input register REG X when this works as a normal linear feedback shift register. REG X is thus advanced along a 30-cycle containing all possible 5-element combinations except the two corresponding to the teleprinter characters ALL SPACE and

NUMBER CHANGE.

BLOCK CHART

Figure 1b shows a block diagram that includes the most important control threads, to give a better understanding of the detailed diagram.

GENERAL DESCRIPTION

In Fig. 1c-i and 2a-h, a detailed block drawing of the embodiment of the invention is shown. The circuit shown represents an "off-line" cryptographic device where the key material is not placed on a strip, which device is capable of encrypting and decrypting normal teleprinter messages in a manner comparable to standard telex operations.

The main blocks are indicated by broken lines, and the terminals for each main block are numbered with individual numbers. The terminals are also equipped with designations so that their interconnections are easy to understand.

In almost all the main blocks, suitable test points are indicated. These points are designated TP1, TP2 ... TP5 and will not be further described.

In the main blocks, logic symbols are used which are described in detail in connection with the diagram, above the symbols, Fig. 6. All the logic blocks are equipped with codes that are individual for each main block. The flip-flop circuits are additionally equipped with functionalist designations placed approximately in the middle of the logic symbol.

Block 1 in Fig. 1c, d contains five pairs of modulo-2 addition circuits. All the adder circuits except the bottom one consist of a NAND gate and an EXCLUSIVE-OR gate. The bottom circuit consists of three NAND gates Al, A2, A3. The outputs of each pair are connected to the inputs of an individual EXCLUSIVE-OR gate. By means of two gate signals K5 and K4, either the lower or upper addition circuit of each pair can have its output connected to the output of the five EXCLUSIVE-OR gates El, E2, Kl, K2, B2.

Block 2 in Fig. 1c,d contains six flip-flops, - Xg, with their respective circuits. The flip-flop circuits - Xg, constitute a feedback shift register (register X) which is used to store and process the information elements consisting of teleprinter characters. The flip-flop circuit Xg is used for delays. In addition to the input circuits for the flip-flop circuits X^-Xg, block 2 also contains four NAND gates A2, D2, Al, Dl to detect whether the contents of the flip-flop circuits X^-X, respectively correspond to one of the teleprinter characters NUMERIC SHIFT (1 ...), CARRIAGE RETURN (<), NEW LINE (s=) or ALL SPACE (Bl).

Block 3 in Fig. 2d,f contains various gate and flip-flop circuits, the most important of which are the memory flip-flop which is used to store the character NEW LINE during the decryption, the setting flip-flop which is used to generate the necessary signals for the initial setting of the key generator registers, and the main program counter (counter K) which consists of five counter stages k-^- k,..

Block 4 in Fig. 2c,b contains the input counter FFo, FFe, FFa, FFb, FFc, FFd and FF 0 with associated circuits. This is a binary counter used to control the input of teleprinter characters into and out of the equipment.

Block 5, Fig. 2a containing the 2 5 kHz OSC (timing pulse generator) and RES (reset circuit) blocks is shown in detail in Fig. 4. The reset circuit is used to provide DC signals to the main register in the key material generator.

Block 6 in Fig. 2a containing the block OS is shown in detail at

Fig. 3. This circuit is used for timing control of the input counter.

Block 7 in Fig. 2d,e,f,g,h contains a ring counter (counter 1) with three stages i2 - i 4 and another ring doubler (counter 3) also with three stages jl - j3. These two counters are used to regulate the starting process of the equipment. Block 7 also contains a binary counter (counter P) with five stages C^- Cg. The latter is a binary counter used to count down from a starting point received as DC signals from block 8.

Block 8 in Fig. 2e,g,h contains various gate circuits. The port circuits are used to supply the counter C-^- C&i block 7 with the necessary information from the generator for the key material in the

respectively the encryption and decryption phase.

Block 9 in Fig. 1e,f contains a key-character register (register Z) with five stages Z^-Z^. This register normally receives its information in serial form from the key generator on terminals J, K in stage Z. However, information can also come in parallel from an external tape reader with the designation TR. Block 9 also contains a main amplifier for the push pulses, which amplifier consists of gates C2, C3, C4, and F, G, H1 and H2. It also contains a shift register (register Y) Y\- Y5 which is used as an intermediate holding register during the initialization phase.

Blocks 10,11 and 12 of Fig.le,f,g,h,i constitute the key generator, which in turn contains two 15-element non-linear feedback shift registers, REG I1- REG I15- REG II1- REG H15> with trigger-flip -flops and associated output and input gate circuits.

Block 14 in Fig. 2a contains the blocks RC (Reading Circuit) and DA (Drive Amplifier) which are shown in detail in Fig. 5.

Plug A in Fig. 1c,d contains interconnected connections between its terminals which determine the pattern of the feedback on the feedback shift register X, - Xg, block 2, which is again used in the encryption and decryption process.

Block TR in Fig. 1f is a strip reader. This device is normally not connected, but can be connected to enable the use of disposable key strips as key material.

The grids with the designation PB in Fig. lg,h,i show schematically

a plug board used for inserting information regarding the key material in the equipment. The circles around the intersections of horizontal and vertical lines indicate that there is an electrical connection by means of a shorting contact.

In Fig. le, h, 2b, several change-over contacts are shown with the designation either S or ^. These are contacts on a switch with three positions. The middle position of the switch corresponds to the equipment being at rest or in the ready position. The other two positions correspond to sending and receiving or encryption and decryption, respectively.

Block TP in Fig. 1f suggests a teleprinter with its transmitting contact and receiving coil. This is also shown in Fig. 5.

Block TB1 (Fig. 2a) is a terminal strip. S4 (Fig. 2a) is a switch with two positions corresponding to single-current and double-current operation of the teleprinter. S3 (Fig. 2a) is a switch with two positions corresponding to twenty and thirty mA double current or forty and sixty mA single current to the teleprinter receiver magnet. The multi-position switch JB (Fig. 2a) is used to select the correct time constant for the monostable circuit in block 6 (Fig. 2a).

Description of the operation

Introduction

As mentioned, the ciphertext cannot contain characters that cannot be punched by a normal teleprinter in a telex installation, or characters that can interfere with the transmission in a telex channel, such as the character

WHO'S THERE.

The encryption process starts when the readout circuit receives the start pulse from character X in a clear text operated from the keypad or the automatic transmitter of a teleprinter. The readout circuit starts an oscillator, which in turn provides push pulses to the registers X(Fig..2c ,d), I (Fig.lg,i), II (Fig.lh,i) and Z (Fig.le).

In this way, the previous encrypted character is sent to the teleprinter from register X via the teleprinter's drive amplifier. When register X contains the 5 information items, register Z contains 5

"new" key elements and counter P (Fig. 2g,h) are set to a new P number. Now register X is connected as a feedback shift register of maximum length, and the shift pulses are supplied to counter P and register X. The supply of 2 5 kHz shift pulses is stopped when counter P = 0, i.e. after P pulses. Then X is connected as a normal shift register and set for information register X © register Z.

This is the encrypted form of the clear text that is sent out

when the next plaintext character is pushed into register X. This is the usual encryption process. To avoid certain difficulties in connection with the telex channels, the cipher text characters ALL/ SPACE and NUMBER SHIFT are not used. The former because some teleprinters do not

can perforate ALL SPACE. NUMERICAL SHIFT is avoided because disturbances in the transmission can occur for certain "number" characters.

Now when one of these two combinations is detected as ciphertext, register X is not set to register X © register Z,

but it remains at rest. In this way, the shifted clear text in register X is sent out as encrypted text.

This process is reversible, and this enables decryption.

The encryption phase

Let it first be assumed that the initialization process which will be explained later has been carried out and that normal encryption will take place. The character to be encrypted enters the equipment serially from the teleprinter's transmission connector TP (Fig. 2a) which has a power supply from the reading circuit RC in block 14. (Fig. 2a). In the sense circuit RC, the current that passes or does not pass through the transfer contact is changed to logic levels suitable for the rest of the equipment. The output signal from the reading circuit goes to block M-(port Al) (Fig. 2a) which contains the input counter FFo, FFe, FFa, FFb, FFc, FFd and the associated circuits. Upon receiving the start pulse from the character to be encrypted, this input counter will undergo a complete cycle at a rate determined by the monostable circuit OS in block 6, Fig. 2a. Signals written from the input counter are used to control the entry of the character into the input register in block 2, Fig. lc,d and also to feed the key generator registers in blocks 10, 11 and 12. When the input counter has completed its cycle, they the five information elements which are of teleprinter type, be stored in the input register

- X5i block 2.

The encryption process will then start. This process is controlled by a ring counter k1-kg in block 3 Fig. 2f called the counter K. The counter K remains in its rest position during the input counter cycle. At the end of the input counter cycle, i.e. when the last teleprinter-type information element has been fed into register X, the counter K will go to position K2.

In this position K2 wants an assessment and possible modification

of plain text in register X take place. This is necessary because the cipher text that is finally formed from the encryption process must not contain characters that cannot be transmitted without difficulty through a telex channel. In this equipment, the characters ALL SPACE and NUMERIC SHIFT must not appear in the encrypted text. The former is not present in the plain text, but the latter is. To get NUMERIC SHIFT removed from the encrypted text, this character in the plain text will be changed to CARRIAGE BACK. To prevent confusion, WAGON BACK in the clear text must be changed to NEW LINE. This is possible because it is assumed that in the clear text, the two characters CART BACK and NEW LINE always appear in pairs. If necessary, the changes to the clear text described above are carried out in the program counter position K2 by using a shift pulse in the input register stages - Xc. The program counter K then moves to position K3. In this position K3, the first step in the encryption process takes place. The input register X is now connected as a maximum length feedback register. It will receive a number of shift pulses determined by the contents of counter P, Fig. 2g,h, which is constituted by flip-flop C-^ - C5i block 7, Fig. 2g>h. This counter counts down from its starting position to zero using the signal from the main clock. The starting position is determined by a "pseudo-random" number P

which is transferred from the key generator REG I, REG II, Fig. lg,h,i to counter P, Fig. 2g,h, in parallel when the main program counter K,

Fig. 2f, is in position K2. When the counter P has reached the zero position, the program counter K will go from K3 to KM-, and step two of the encryption process will take place.

This second encryption operation consists of modulo-2 addition

in block 1 of the contents of input register X and an arbitrary number entering in parallel from register Z in block 9, Fig. le. As can be seen from the drawing, the register Z contains five stages Z-^to Zg which are connected as an ordinary shift register. The information that goes into register Z comes from the output of the two key generator registers, REG I, REG II at port D3 in block 12, Fig. li. Register Z is pushed together with the two key generator registers REG I, REG II during the input of the plain text character with the program counter in position At Modulo-2 the addition of the contents of the input register X and in the key character register Z is carried out as mentioned above in parallel without any transfer of mente information from step to step. Before the program counter K is switched to K5, the content of register X is checked to check whether it can be transferred to the line. Ports D1 and A2 in block 2, Fig-. 1, it is checked whether the register contains the characters ALL SPACE or NUMERIC SHIFT, respectively, and the modulo-2 additions will then be repeated.

The contents of register X will thus be as it was after

step one of the encryption process. Because ALL SPACE will never be present during the normal cycle of a maximum-length loopback register, and because the SHIFT position is automatically avoided in this equipment, the contents of register X after step one of the encryption process can always be transferred to the line. As can be seen, the second step in the encryption process is not used when it results in a character that cannot be transferred to the line.

The encrypted character which is now in register X is transferred to the teleprinter's receiver magnet TP, Fig. 2a, during the input of the next simple text character in register X, after which the above-mentioned process is repeated.

The start process mentioned earlier is necessary to ensure that the key generator registers REG I, REG II have different start points from message to message. During the encryption phase, this initial process consists of the input of 6 teleprinter characters, i.e. 30 items into the key generator registers. At the same time, these 6 characters are encrypted and transferred to the teleprinter's receiver magnet TP (and then to the line). On the receiving or deciphering side, the 6 characters will be deciphered and transferred into the decryption equipment's key generator registers. In this way, the same starting point is ensured for both the encryption and decryption equipment. The 6 characters that provide a starting point are taken during normal operation from a starting point table that is prepared in advance for a special feedback pattern in the key generator registers.

During the encryption, the starting point characters are not transferred directly from the reading circuit to the key generator registers, but they go through an intermediate storage register, register Y in block 9, Fig. 1e. Register Y receives shift pulses at the same time as the key generator registers REG I, REG II, and the above-mentioned key character register Z.

The necessary gate functions during the start process are kept track of by two three-stage ring counters I and J in block 7, Fig. 2f,h. Counter I containing steps i_ - i^ is skipped once for each complete cycle counter K, Fig. 2f, makes. The counter J- containing the steps j^-j^ is advanced once for each complete cycle the counter I makes.

The plug-in board PB, Fig. lg, h, i, stores information about how the output position should be for the registers REG I and REG II, Fig. le,

30 9

f, g, h, i. There are 2 10 possible settings. The initial settings are used for encryption of the 6 characters chosen to provide sufficient lengths of the cycles in the above registers. The counters ij- i^ and j^- jg in block 7 keep track of the start procedure and determine when the registers are to be fed with start information.

The signal from the main clock that controls all the functions in the equipment is written from a 2 5 kHz oscillator in block 5, Fig. 2a, which produces square waves. Block 5 also contains a reset circuit which provides the current necessary to set all the flip-flop circuits in the key generator registers to zero when the equipment is in ready or sleep mode.

The deciphering phase

The deciphering of a character starts with a transmission of a character from the teleprinter's transmission contact TP into the equipment as in the encryption phase. The start pulse from the character to be encrypted will also in this case start a cycle in the input counter in block 4, Fig. 2c. At the same time, the information elements of the ciphertext characters to be deciphered are fed into the input register X. This happens while the program counter K is in its rest position Kl. The two steps in the encryption process must now be carried out in reverse order. Therefore, in program counter K's position K2 modulo-2 addition of the contents of registers X and Z is performed. If the contents of register X after the modulo-2 addition is equal to ALL SPACE or NUMBER SHIFT, it is clear that the modulo-2 addition has been performed twice in during the encryption. For this reason, it is also repeated here, and this means that only the reverse process of step 1 in the encryption will be carried out. A five stage feedback register of maximum length has a cycle of 31. In this equipment the position corresponding to NUMBER SHIFT is automatically passed. Therefore, this register has an effective length of 30 positions. To reproduce the original clear text during the decryption process, it is necessary to advance register X 30-P steps. This is done, as previously mentioned under the explanation

of the encryption process, by setting P correctly. The shifting of register X during the decryption is done with the program counter in position K3. Finally, with the program counter in position K4, a modification must be carried out which cancels the one carried out during encryption. As explained above, the NUMBER SHIFT character was changed to CARRIAGE RETURN, and the CARRIAGE RETURN character was changed to NEW LINE. If therefore the encrypted character contained in register X is equal to CARRIAGE RETURN, it must be changed to NEW LINE. This is done by adding

a push pulse to the steps in register X with the program counter in position K4. If the contents of register X after receiving feedback is equal to NEW LINE, this may be due to either a NEW LINE character during the encryption or a changed CARRIAGE RETURN during the encryption. As mentioned above, it is assumed that the signs WAGON BACK and NEW LINE appear in pairs and in the same order as just mentioned. When the character NEW LINE first appears after the feedback switch operation, it is immediately changed to CARRIAGE BACK with the program counter in position K4. This change is also stored by setting the memory flip-flop in block 3, Fig. 2d. Provided that CARRIAGE BACK and NEW LINE are always transmitted in pairs, the next character formed from the return push operation during the decryption will again be NEW LINE. As the memory flip-flop is now set, the change of the character NEW LINE is looped with the program counter in position K4. This means that NEW LINE will be the encrypted character to be transferred to the teleprinter's receiving magnet TP, Fig. 2a, during the next input process. As you can understand, the sign CARRIAGE BACK has been made redundant in this way, and the teleprinter combination that is usually used to transfer this sign is used instead to transfer NUMERICAL SHIFT. This avoidance of the teleprinter combination NUMERICAL SHIFT from the ciphertext is introduced to prevent numerals in the ciphertext alphabet. This is necessary because in an unserviced telex station it cannot be allowed that the answering unit is triggered, while

the ciphertext is about to enter. As is known, the response unit is triggered by WHO THERE. By completely avoiding the NUMERIC SHIFT character in the cipher text, the teleprinter will never be put in "upper case" when it receives an encrypted message.

DESCRIPTION OF THE MAIN CIRCUITS

Key generator

The key generator contains two non-linear feedback shift registers, REG I and REG II (Blocks 10, 11, 12., Fig. lg,h,i), each with 15 stages, REG I^.-^g°g REG H3 respectively .-15'°S me<3

a feedback signal F (A-, A-, A, , A-,) © A, _, where F (A^, Aj, A^., A^) is a non-linear function whose combination table of the binary variables A., A., A, and A, are formed by the output signals from four of the register stages. i, j, k and 1 are all different and otherwise randomly chosen using plugs on a plug table.

This is shown in Fig. 1g,h,i where the feedback signal from

REG I is taken from steps No. 4, 14, 10 and 9, so that i, j, k and 1

in this case are equal to 4, 14, 10 and 9 respectively. These four output signals are fed to ports B4, Cl, B3 and C2 in block 12,

Fig. li, and the combined signal from port C2 is modulo-2

added in gates E4, Fl with the output signal A^j. from the last stage REG The feedback signal is supplied to the register REG I at gate Dl, block 10, Fig. le,g.

The feedback signal to the register REG II is taken in the same way from steps No. 7, 10, 3 and 6, combined in ports Bl, A1, B2 and A2, block 12, Fig. li. This combined signal is added modulo-2

to the output signal A15 from the last register stage REG H-^g in ports E3, F2 and transferred to the register input in port D2, block 11,

Fig. 1f, h.

This is a register type that has not yet been fully mathematically explained in the literature, but it is clear that a long cycle from such a register always produces a "pseudo-random" pattern. Furthermore, the sequences usually have different lengths which means that the physical lengths of the key generator registers can be equal. There are still certain possibilities for a sequence to be very short. Therefore, the starting points in this system are not completely arbitrary, but they are chosen to give sufficient lengths to the cycles. In the current equipment which has two 15-element registers, 6 characters must be selected for each message to be sent. The output signals from the two registers are modulo-2 added in gates D1, D2, D3 and D4, Fig. 11, to give the desired sequence of "pseudo-random" key elements. Such sequences are stored in a 5-element register Z^- Zj., block 9,

Fig. le. The number of different connections is

In the two registers REG I and REG II, "trigger" flip-flops are used. In a "trigger" flip-flop, the content after a time pulse depends on the input signal as well as on the signal present in the flip-flop circuit itself. The following set of equations therefore describes the register's performance:

A^ is the signal located in register stage No. 1 before a time pulse, and A-^1 is the signal that is in the same step after the time pulse. It is the same for the other stages, and the performance is similar for the two registers.

Generator for "P"

P is a 5-element "pseudo-random" number taken from the two registers REG I and REG II, Fig. 1g, h, i, in the manner shown in block 8, Fig. 2e,g,h. Each element is the result of a modulo-2 addition of the output signals from randomly selected elements in both registers. Block 8, Fig. 2e,g,h contains 5 gate circuits to each of which two signals are supplied from the plug board PB. To the first port complex consisting of ports Cl, C2, C3, C4, B1, B2, B3 and D2, block 8, Fig. 2e, signals from the plug board multiples PB/A and PB/M are supplied. As can be seen from Fig. 1g,h, PB/A is associated with REG I, while PB/M is associated with REG II. A similar connection is assigned to the other four gate circuits. The number of different usable connections is:

The output from each of the gate circuits which together make up the number P is transferred to a counter - Cg, block 7, Fig. 2g,h.

During the encryption, the number P is fed into the binary counter

C-^ - Cg, but during the decryption this counter receives the number

31 - P. In both cases the counter counts down to zero at the same time as it counts the pulses from the 2 5 kHz supply.

Entry register

Reg X (Block 2, Fig. lc,d) which consists of the steps X. 1 , - X oc has two operational phases. a) It acts as a normal 5-element shift register and receives shift pulses at 50 Hz (for 5 0 Baud teleprinter speed). b) It acts as a feedback shift register where each stage contains a modulo-2 addition circuit, i.e. the gate Hl, and a flip-flop, i.e. X^. Any output can be connected to any input by means of connections soldered to a plug, PLUG A, and these connections are made according to a polynomial, and they result in a sequence of maximum length 2 5 - 1 = 31 .The 5-element combination that forms the character NUMERICAL SHIFT is removed from the cycle. In this way, REG X undergoes a 30-cycle containing all possible 5-element combinations except ALL SPACE and NUMBER SHIFT. There are approximately 15,000 different cycles to choose from. The shift pulses in this case come from the 2 5 kHz source, and the number of pulses is equal to the number of steps that the counter C, 1 - Cac, block 7 counts.

The plug table

The plug board (PB), Fig. lg,h,i, is a board with "horizontal" and "vertical" terminal rows arranged in matrix form with approximately 300 crossing points (holes). A plug inserted into a junction point will interconnect the horizontal and vertical terminal rows at that particular junction point. Plugs on the plug board determine which outputs should be used for the feedback functions, and which outputs should be used to provide the number P, and which flip-flop circuits are set first.

Monostable Multivibrator

Fig. 3 shows the detailed schematic diagram of block 6 in Fig. 2a. This circuit OS provides the timing reference for the teleprinter's input and output signals. Defined time intervals are achieved using a monostable multivibrator containing the transistors

Tsl and Ts2 with the associated components. In the rest phase, transistor Tsl conducts while Ts2 is disconnected. A "trigger" pulse supplied through a diode D3 will cut out transistor Tsl. In the case of regenerative action of the circuit, transistor Ts2 will again conduct. This state will last until the capacitor Cl is discharged through resistor R3. Transistor Tsl will start to conduct again and Ts2 will turn off. Transistor Ts4 conducts for a short time when the circuit is switched back to the rest state. In this way, the timing capacitor Cl is quickly charged. This results in a very short reset time. The circuit's pulse length is unaffected by high load. The transistors Ts3 - 4

and Ts5 - 6 form the input circuit. Triggering of the circuit occurs every time transistor Ts3 is switched on. Resistor R8 connected to the base of Ts3 is used to prevent possible trigger pulses when the circuit is in its apparent steady state. The output signal is taken from transistor Ts7's collector. In the rest phase, as previously established, transistor Ts2 is not current-carrying. Ts7 will then also not be current-carrying, and the output will be powerful. When the circuit is set to its apparent steady state, Ts2 is current-carrying, and on

same way Ts7. The output signal will then be low, i.e. almost equal to■the earth potential. The timing capacitor Cl contains four elements labeled Cia - Cld. The purpose of this arrangement is to achieve different pulse lengths by means of external connections, so that the circuit can be adapted to different teleprinter speeds. Connecting terminals 3 - 4 gives a speed of 75 Bauds, 6-4 corresponds to 50 Bauds, and 5 - 4 corresponds to 45 Bauds.

The transistors can be of the following types:

Tsl: 2S 201; Ts2 - 3; 2S 302; Ts4; 2N 1711; Ts5-7; BSY 95A or equivalent components, and all the diodes can be of the type IS 92 0 or similar.

Time pulse generator

Fig. 4 shows a detailed sketch of two different circuits in block 5, Fig. 2a. The upper one is the time-pulse generator, 25 kHz OSC. This is an astable multivibrator containing the transistors Tsl

and Ts2 plus associated components. Transistors Ts3 and Ts4 are the circuit's output amplifiers. The purpose of the diodes D1, D2 and the smoothing capacitor C3 is to avoid a stable state where both transistors are conducting at the same time, which can occur with a conventional astable multivibrator.

The reset circuit

A reset circuit is shown in the lower part of Fig. 4

RES, whose task is to supply a direct current signal to the feedback registers' key generator. When one of the input transistors Ts6 or Ts7 is current-carrying, the output transistor Ts5 will also be current-carrying and supply current to the load. If both transistors are in a disconnected state, the output transistor Ts5 will also not be current-carrying. The transistors can be of the following types: Ts5; 2S 302; Tsl-4, 6-7; BSY 27 or equivalent components, and the diodes Dl-2 can be of the type IS 920 or similar.

The readout circuit

Fig. 5 shows a detailed sketch of the circuits that exist, i

block 14 on Fig. 2a. In the upper part of Fig. 5, the reading circuit RC is shown. A current of about 40 mA is supplied to the reading circuit RC. from the + 50 V source through the resistors R17 via terminals 6 and 3 through the teleprinter contact TP and back. If the teleprinter contact is closed, the input transistor Ts9 will be energized. If the teleprinter connector is open, Ts9 will not be live. The output signal taken from the collector of Ts9 will be low when the teleprinter contact is closed, which corresponds to MARK, and it will be high when the teleprinter contact is open, which corresponds to SPACE. In order to have the output signal available in the opposite phase, an inverting stage consisting of Ts8 and associated components has been added to the readout circuit.

The drive amplifier

In the lower part of Fig. 5, a detailed sketch of the teleprinter's drive amplifier DA is shown. This circuit contains the input transistors Tsl, Ts2 and Ts3 with associated components, two constant current generators, Ts6 and Ts7 with associated components and two teleprinter transistors Ts4 and Ts5 for current connection and the associated components. By means of external connections, S3, the circuit can be adapted to work for single or double current and at two different current levels. When operating with single current, the two generators with constant current are connected in parallel. In the MARK state, transistor Ts5 therefore conducts, while Ts4 is non-conducting. The current from the constant current generators thus passes through the remote recorder's receiver magnet M. In the SPACE state, the transistor Ts4 conducts, while Ts5 is non-conducting. No current therefore passes through the remote recorder's receiver magnet M. On the input side, MARK corresponds to a high input signal and SPACE to a low input signal. The transistors can be of the following types Tsl, Ts2, Ts3, Ts8, Ts9; BSY 95A; Ts4, Ts5;

2N 1893; Ts6, Ts7; 2N 2904A or equivalent components, while the diodes Dl, D2 can be of the type ZF 5.6 or similar.

Logical Symbols

Fig. 6 shows the different symbols that represent the different logical functions in the main diagram. These are: Inverter, 2-input NAND gate, 3-input NAND gate, 5-input NAND gate, JK flip-flop with preset and an EXCLUSIVE-OR gate.

These logic symbols and their function are described in detail in the leaflet "Series 73 Solid Circuit Semiconductor Networks", Bulletin No. DL-S 567650, July 1965, which is issued by Texas Instruments Incorporated. The symbols shown from the top in Fig. 6 correspond to the following TI network: 1/4 SN 7350, 1/4 SN 7360, 1/3 SN 7331, 1/2

SN 7 311, 1/2 SN 7 302 and 1/2 SN 7 370. The symbols corresponding to 1/4

SN 7350, 1/2 SN 7302 and 1/2 SN 7370 are drawn somewhat differently for reasons of simplicity.

Claims (2)

1. Key material generator for cryptographic teleprinter equipment comprising several shift registers, characterized in that for each shift register the outputs from a selection of its different stages are combined and the resulting signal fed back to the relevant register's input and that a modulo-2 addition circuit has its inputs directly connected to the outputs of the last steps in each of the shift registers, so that when the contents of the shift registers are circulated, this content is combined in the modulo-2 addition circuit, and that an output register with its input is connected to the output of the modulo-2 addition circuit, so that when the shift registers are operated by supplying feed pulses, a key combination is produced in the output register. 2. Key generator according to claim 1, characterized in that each register comprises several trigger flip-flop circuits, each of which has its control inputs connected to the output of the preceding circuit, whereby the register works so that the next content of an arbitrary flip-flop is equal to modulo 2 the sum of the current contents of this flip-flop and the previous flip-flop.