Method and device for the coded transmission of messages
BACKGROUND OF THE INVENTION 
     The invention relates to a method and apparatus for the coded transmission of messages by splitting up the clear (i.e. uncoded) signals to be transmitted into elements of equal length, which are transposed at the transmitting end by being delayed by at least partially different times and are transposed back at the receiving end by being further delayed by at least partially different times. The consecutively numbered elements e 1 , e 2 , e 3 , . . . of the clear signal x have coinciding lengths T o  (see FIG. 1), and their transposition in time leads, for example, to the coded signal y, of which the first element e 4  appears undelayed at the moment t = 3T o , while the other elements appear with varying delay. After transmission of the signal y at a receiver, the elements are restored to their original position by retransposition in order to recover the original clear signal. 
     The elements e 1 , e 2 , . . . may, as shown in FIG. 2, be pulses of the duration T o , which are keyed between -1 and +1 or between 0 and 1 in accordance with a telegraphic message. Each element may, however, also comprise a plurality of individual pulses of a data signal s, as shown in FIG. 3. The pulses may also be quantized in a plurality of stages. The formation of elements, the amplitude of which corresponds to the scanned values, formed at intervals T o , of a continuously variable clear signal s (t), is shown in FIG. 4. Instead, however, sections of the clear signal s (t) of constant length T o  may be formed as elements e 1 , e 2 , . . . as shown in FIG. 5. FIG. 5 also indicates that, instead of these continuously variable signal sections, a train of short individual pulses c (t) is suitable for forming the elements (see element e.sub. 3). Now, as a result of the encodingprocess, the sequence of such elements in time is altered, while the nature of the individual elements can remain unaltered. 
     Methods and devices for time coding, that is to say for the transposition in time of message elements, have become known for example through the Swiss Pat. No. 212,742 and 232,786, which describe how omissions and also repetitions of individual elements are avoided by periodically actuated switches. A periodic repetition of the transposition program effected at short intervals is undesirable, however, for cryptologic reasons. Accordingly, in the Swiss Pat. No. 518,658, a method is described which renders possible the control of the transposition process by random signals, as a result of which, periodic repetitions of the transposition program during a transmission are avoided. This control is achieved by means of a separate position register which, however, considerably increases the total expenditure necessary. The total expenditure on known devices is also comparatively heavy because the storage devices used are generally only partially filled with message elements wherein at least 50% of the stored locations remain unoccupied at any moment. 
     BRIEF DESCRIPTION OF THE INVENTION AND OBJECTS 
     According to the invention, these disadvantages are avoided by transposition in pairs of two elements at a time, which have a specific mutual spacing, at the transmitting end and retransposition of the same elements in pairs at the receiving end, the pairs of elements being transposed or retransposed at the transmitting end and at the receiving end being determined by irregular trains of control pulses which coincide at the two ends, and the elements which do not belong to the pairs of elements being delayed at the transmitting and receiving ends by a fixed time T, while the element of each pair which arrives first is delayed by double the time 2T at the transmitting and receiving ends and the second element is not delayed. 
     It is therefore one object of the invention to provide method and apparatus for encoding and/or decoding messages by transposing selected pairs of message elements while leaving remaining message elements untransposed. 
     Another object of the invention is to provide method and apparatus for encoding and/or decoding messages by transposing selected pairs of message elements so that one element of the pair undergoes a delay 2T and the remaining element of the pair undergoes no delay. 
     Still another object of the invention is to provide method and apparatus for encoding and/or decoding messages by transposing selected pairs of message elements so that one element of the pair undergoes a delay 2T and the remaining element of the pair undergoes no delay and wherein message elements not treated in pairs undergo a delay T, so that T = n T o  where T o  = message element length, and n = 1,2,3, . . . , n. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     The above, as well as other objects of the invention, will become apparent from the following description and drawings, in which: 
     FIG. 1 shows one manner in which message elements of a message may be transposed. 
     FIGS. 2 - 5 show waveforms of various message formats which may undergo encoding (and decoding) by the techniques and apparatus of the present invention. 
     FIG. 6 shows a circuit for carrying out the exchange of message elements in pairs, 
     FIGS. 7 and 8 are diagrammatical illustrations of the exchange of adjacent elements, 
     FIGS. 9 and 11 show circuits for obtaining control signals for the actuation of the transposition switch from cipher signals, 
     FIGS. 10 and 12 show examples of cipher signals and control signals obtained therefrom, 
     FIG. 13 shows a circuit for the transposition in pairs of non-adjacent elements with associated circuitry for obtaining the control signals, 
     FIGS. 14 and 15 are diagrammatic illustrations of the exchange in pairs of non-adjacent elements, 
     FIGS. 16 and 17 show a circuit for the repeated exchange in pairs with cipher-signal preparation and a circuit for the repeated re-exchange with cipher-signal preparation, 
     FIG. 18 is a diagrammatic illustration of the repeated exchange in pairs, 
     FIG. 19 is an illustration of the delay times which occur with repeated exchange in pairs, 
     FIG. 20 shows a circuit for permutation in accordance with a constant program, 
     FIGS. 21 and 22 are diagrammatic illustrations of permutations in accordance with a constant program, 
     FIG. 23 shows a block circuit diagram of devices for the time coding by element exchanges in pairs in conjunction with permutations in accordance with a fixed program and for the decoding by element exchanges in pairs in conjunction with permutations. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An explanation of the invention will now be given with reference to FIG. 6, which shows a simple circuit for carrying out the exchange of elements in pairs. The circuit contains a retarder R with the transit time T o , which corresponds to the length of one message element. This retarder can be connected, through the switches H 1 , H 2  (in position &#34;O&#34;, as shown), to the input line and the output line of the circuit so that one element at a time of the clear signal x is supplied to the retarder, while at the same time a stored or delayed element is extracted therefrom as output signal y. By means of a pulse of the control signal a with the duration T o , on the other hand, the switches are brought into the position designated by &#34;I&#34;, so that one element of the input signal x at a time again appears directly as an element of the output signal y, while the preceding input element continues to be stored by being fed back from the output to the input of the retarder. The position of the beginning of the element in the retarder is indicated by the variable length d. 
     In the absence of a pulse of the control signal a,  therefore, an element e 1  of the input signal x will reappear as element e 1  of the output signal y after the time T o , as shown in FIG. 7. In the course of the duration of the element e 6 , on the other hand, for example, a pulse of the control signal a appears so that this element reaches the output without delay, through the switch H 1 , (indicated in broken lines in FIG. 7), while the preceding element e 5  is fed back to the input of the retarder through the switch H 2  and therefore only reaches the output of the circuit after an additional delay time T o  or with a total delay 2T o . The passage through twice can be recognized by the position d of the initial edge of the element, which can be seen from FIG. 7. Whereas the element e 1  is merely delayed by the time T o , therefore, a delay reduced to 0 has occurred with the element e 6  and a delay increased to 2T o  with the element e 5 , so that these last two elements appear transposed in time in the output signal y. In a similar way, the pair of elements e 2 , e 3  is also transposed in time as shown in FIG. 7, while the element e 4  for example is transmitted with delay but without transposition with any other element. The same transpositions are indicated again diagrammatically in FIG. 8. It should be noted that the time zero has been advanced (i.e. shifted one &#34;frame&#34; to the left) by one time interval T o  in the signal y in order to achieve a clearer illustration. 
     It should be noted that during the transposition in pairs as described, the switches H 1 , H 2  should never be actuated for longer than the duration T o  of one element, in order that no element may be stored longer than 2T o . Accordingly, immediate repetitions (for example 00110) of the switching pulses are not permitted on the control signal a. In order to extract the control signals a o  from a cipher-signal w o  following a quasi-random course, a cipher-signal addition circuit SZ o  as shown in FIG. 9 is therefore suitable. As a result of delaying each individual pulse of the cipher signal w o  by the element length T o  in the retarder V o , a blocking signal v o  results which suppresses a possible following pulse of the cipher signal in the interrupter U o . The effect of this suppression is shown by way of example in FIG. 10. The suppressed pulses are designated by underlining. A disadvantage in this case, however, is that with an uninterrupted train of three or more pulses, all the pulses except the first are cancelled. This disadvantage is avoided with the cipher-signal addition circuit SZ 1  shown in FIG. 11, in which the interrupter U 1  is actuated by the pulses of the control signal a 1  delayed in v 1 . With an uninterrupted train of a plurality of pulses of the cipher signal w 1 , only every other pulse is suppressed in this case so that the control signal a 1  indicated in FIG. 12 results for example, and meets the requirements for an exchange of elements in pairs. In FIG. 11, apart from the device PT 1  already explained for the exchange of elements in pairs, a cipher-signal generator SG is indicated, the construction and mode of operation of which may correspond to known constructions. Devices for generating cipher signals with digital circuits are described for example in the Swiss Pat. No. 361,839. 
     Depending on the nature of the clear signals x, digital or analogue stores of known construction should be used as retarders R for exchanging the elements in the pair exchanger PT. In this case, it may be a question of delay lines or balancing networks, electro-mechanical retarders (for example acoustic systems) or electromagentic stores (for example magnetic sound recording with moving medium). Electrical shift registers are particularly suitable, with which signals keyed digitally (for example as shown in FIGS. 2 and 3) can easily be stored if operated at an appropriate clock frequency. With analogue signals (for example as shown in FIGS. 4 and 5), periodic scanning and storage of the scanned values (c(t) in FIG. 5) is necessary. These scanned values can also be converted, by binary coding, into corresponding pulse groups, the storage of which is then effected with digital stores having an appropriately larger number of stages. In this case, with the pair exchanger PT 1  shown in FIG. 11, it is necessary to connect an analogue-digital converted at the input side to extract digital input signals from the clear signal x and to connect a digital-analogue converter at the output side to extract output signals y in analogue form. Delta modulation is also possible, however, instead of the binary coding. The changeover switches H 1 , H 2  may appropriately be realized by suitably controlled semiconductor switching elements, which is also true for the interrupter U 1  in the cipher-signal addition circuit SZ 1 . 
     The effectiveness of the time coding is increased by transposition in pairs, of elements which are not immediately adjacent. In FIG. 13 a device PT 3  is shown for the transposition in pairs of two elements at a time, the beginnings of which have a mutual spacing of three element lengths T o , and corresponding element trains are illustrated in FIGS. 14 and 15 to explain the operation by way of example. When the switches H 3 , H 4  are in the normal position shown, the elements of the output signal y appear delayed by 3T o  in comparison with the input signal x, if the delay of the retarder R 3  likewise amounts to 3T o . This is the case, for example, with the element e 2  (see FIG. 14), because said switches are in the normal position shown both during the supply and also during the extraction of this element. Although the element e 3  is likewise supplied to the retarder through the switch H 3 , nevertheless after a first passage through this retarder, it is again fed back to the input of the retarder through the switch H 4 , because at this time, this switch is brought into the operative position (not shown) by a pulse of the control signal a 3 . At the same time, an element e 6  of the input signal x is conveyed, without delay to the output through the switch H 3  which is likewise actuated (indicated in broken lines in FIG. 14). Only three element lengths later does the stored element e 3  finally appear through the switch H 4  restored to the normal position, in the output signal y. In a similar manner, the elements e 1 , e 4  and e 7 , e 10  for example are also transposed, while e 5  and e 8  are passed on with simple delay without being transposed. This process is illustrated again, with the associated control signals, in FIG. 15. The advancing of the time zero (i.e., the shifting left of the time frame) should again be noted in this simplified illustration. As a result of operation with control pulses having the uniform length T o  , the effect is achieved that a plurality of elements of corresponding length always travel through the retarder. 
     In order to avoid a further feedback of all elements which have already been delayed twice, care must be taken to ensure that no further pulse follows a pulse of the control signal a 3  with the spacing 3T o . For this reason there is provided in the cipher-signal addition circuit SZ 3 , a blocking switch U 3  which is actuated by the pulses of the control signal a 3  delayed by three element lengths T o  in V 3 , so that any following inadmissible control pulses are eliminated. Here, too, the cipher signals w 3 , from which the control signals a 3  are obtained by suppression of inadmissible pulses, are taken from a cipher-signal generator SG. 
     In order to further increase the effectiveness of a time coding, the interconnection of a plurality of pair-exchange process circuits is advisable so that an increase in the possible displacements of each element comes about. In FIG. 16, a device ZT can be seen in which a first transposition in pairs is effected of elements of the clear signal x through the retarder R 3  and the switches H3, H4, as a result of which a signal y results, the elements of which may have additional displacements by 3T o  or 6T o  as in FIGS. 13 and 15. A second transposition in pairs is then effected through the retarder R 1  and the switches H 1  and H 2  with smaller displacements similar to FIGS. 6 and 8. The cipher-signal addition circuit SZ is also equipped with retarders V 3  and V 1  respectively, corresponding to FIGS. 13 and 11 respectively, in accordance with the unequal displacement times. This cascade connection of two transposition processes in pairs produces, from a clear signal x, the element numbers of which are designated by n(x) in FIG. 18, first the intermediate signal y, of which the element numbers n(y) are likewise given in FIG. 18, and finally, as a result of further element exchange in pairs, the output signal z with the element numbers n(z). Whereas displacements of 0 and +3 element lengths occur in the intermediate signal, the second exchange produces displacements of 0, +T o , +2T o , +3T o , +4T o  can appear in the output signal z in comparison with a mid position of the elements. In view of the fact that even this mid position has a displacement of 4T o , because negative displacements in time are impossible, the output elements of the time coding device ZT therefore appear with delays of O, T o , 2T o  3T o , . . . to 8T o  in comparison with the input elements. The delays occurring in the example shown are given in FIG. 19 as integral multiples r(n) of the element length T o  over the element numbers n of the input signal x. It can be seen that a very effective mixing of all the elements of the message comes about already as a result of pair exchanging twice. This process could be extended by one or more further pair exchanges. In this case, it is advisable to avoid the same storage times for the various exchange processes. The number of possible displacements becomes particularly high if the storage times are graduated in accordance with a ternary system, in that retarders are used having transit times of T o , 3T o , 9T o  . . . =3 i  T o  (i = a whole number), because thus all total delays mT o  between 0 and (3 k   +1  - 1)T.sub. o are possible (m = a whole number, k = total number of the pair transposition devices). 
     A device which as shown in FIG. 17, corresponds largely to the transposition device at the transmitting end, serves for the re-exchange of the message elements at the receiving end. From the coded signal z* received, which coincides with z, as a result of a first re-exchange with the retarder R* 1  and the switches H* 1 , H* 2 , an intermediate signal y* is again formed which coincides with y and (apart from the delay of the transmission channel) is delayed by 2T o  in comparison with y, because the untransposed elements are subjected to a delay of T o  at the transmitting end and at the receiving end. With the transposition of the elements e 5  and e 6  shown in FIG. 7, re-exchange of these elements comes about when a following analogue transposition device receives a control pulse a at the moment the element e 5  is received, so that this element is not further delayed, while the preceding element e 6  is delayed by 2T o  and so comes back into the original position in relation to e 5 . Accordingly, the control pulses a* 1  of the first re-exchange with the switches H* 1 , H* 2  must be displaced by T o  in comparison with the control pulses a 1  of the exchange shown in FIG. 16 with the switches H 1 , H 2 , in the device also shown in FIG. 17. This displacement is achieved by an additional delay T o  of the cipher signal w* 1  at the receiving end (FIG. 17). In this case, it is assumed that the cipher-signal generator SG* at the receiving end is synchronized with the cipher-signal generator SG at the transmitting end by auxiliary signals u and u* transmitted separately, for example by the method described in the Swiss Pat. No. 361,839. In the case of element exchange in pairs with displacement by three element lengths as shown in FIGS. 13 and 14, it should be noted that an element e 3  which is displaced by six element lengths in the exchange process at the transmitting end (see FIG. 14), must not be further delayed during the re-exchange at the receiving end, while the element e 6  which is not delayed at the transmitting end has to be delayed by six element lengths at the receiving end. The control pulse for the re-exchange at the receiving end must therefore coincide with the element e 3  received; that is to say the control of the re-exchange must be delayed by 3T o  in comparison with the control at the transmitting end, if no additional delays have to be taken into consideration. In the transmission system as shown in FIGS. 16, 17, however, as already explained, there is a difference in time of 2T o  between the signals y and y*, so that the control signal a* 3  for the re-exchange in pairs in the retarder R* 3 , the transit time of which amounts to 3T o , must be delayed altogether by 3T O  + 2T O  = 5T o  in comparison with the control signal R 3  for the exchange in pairs in R* 3 . The retarder W* 3  is provided in the cipher-signal addition circuit SZ* at the receiving end to ensure this delay time (FIG. 17). 
     The effectiveness of an enciphering by exchanging elements in pairs is also increased by additional permutation of the elements in accordance with a fixed program. A device ZT o , which is suitable for this, may contain two retarders R 1 , R 2  with an identical transit time, as shown in FIG. 20. Individual elements of the input signal y 1  can be supplied to these retarders through the switches A 1  and B 1  respectively, while the extraction of elements to form the output signal y 2  is possible through the switches A 2  and B 2  respectively. When the switches are not actuated, however, the retarder output is connected back to its input in each case. Finally direct passing-on of elements of the input signal y 1  to the output of the device is possible through the further switches C 1 , C 2 . The switches A 1 , A 2  are always actuated simultaneously, likewise the switches B 1 , B 2  and C 1 , C 2 , for example in accordance with the periodic program S given at the top in FIG. 21 (the switches not recited in a time interval being in the normal position in each case). The elements of the input signal y 1  are numbered consecutively with the numbers given below the switch program S in FIG. 21. The switching through by the switch C is indicated diagrammatically underneath (DC). The element No. 3 is passed on directly through the switch C to the output so that this element appears without delay in the output signal y 2  (FIG. 21 bottom). The element No. 5 on the other hand, passes through the simultaneously actuated switch A 1  to the retarder R 1  (the delay in R 1  is illustrated symbolically in the next line &#34;VR 1  &#34;), and immediately after being delayed only once, it is conveyed to the output through A 2 . The input element No. 4, which reaches the retarder R 2  through the switch B 1  (see next line &#34;VR 2  &#34;), on the other hand, is fed back from the output of the retarder to the input thereof through the switches B 1 , B 2  which alternate in the normal position after this input; it is only extracted therefrom again after passing through three times and added to the output signal y 2 , as soon as the switches B are actuated again. On the assumption that the transit time of a retarder R coincides with the element length T o , such storage and switching-over finally leads to an output signal y 2  with elements transposed in time, as can be seen from the resulting numbering shown at the bottom of FIG. 21. 
     Mutual displacements of the elements by greater times are possible with an increased transit time of the registers R. With a delay time 3T o  of the registers R 1  and R 2 , the displacements which can be seen from FIG. 22 result, as the switch control is effected in accordance with program S given across the top of FIG. 22. The element No. 2 for example is transmitted directly through switches C 1 , C 2  while the element No. 3 is delayed by three element lengths in the retarder R 1 . The element No. 5, on the other hand, after being fed back twice, is subjected to a delay of 9T o  in the retarder R 2 . The element No. 4 is subjected to a delay of 6T o  in the same retarder and the element No. 1 is actually delayed by 12T o  in R 1 . Because of the periodic repetition of the switching-over program, the elements No. 1, 6, 11 . . . are delayed by the same amounts, likewise the elements 2, 7, 12 . . . and the elements 3, 8, 13 . . . and so on. Further possibilities for carrying out the periodically repeated transposition are provided, for example, by increasing the delay times of R 1  and R 2  to 4T o  or even greater amounts, or by using three or more retarders which are connected to the inputs and outputs of the device in a similar manner by switches actuated in pairs. 
     An interconnection of the device ZT o , which has been explained, for the periodically repeatd permutation of message elements, with devices PT 1  and PT 2  for the exchange of such elements in pairs, is shown in FIG. 23. The control-signal additions for obtaining the control signals a 1  and a 2  from the cipher signals w 1  and w 2  are designated by circuits SZ 1  and SZ 2 . A further control-signal addition circuit SZ o  serves to produce the periodically repeated control signals a o  for the actuation of the switches A, B, C of the permutation device ZT o . The corresponding devices at the receiving end for reversing the transpositions and the signals appearing in the course of this are shown in FIG. 23 using the same symbols. An additional asterisk (for example y* 2 ) serves to make a distinction from the devices and signals at the transmitting end. The transit times of the retarders contained in PT 1  and PT 2  are preferably selected unequal in order to obtain, once again, as great a multiplicity as possible of the element displacements which can be achieved. 
     The interconnection described, between devices for exchanging elements in pairs and a device for permutating elements in accordance with a fixed program, leads to resulting transpositions of the message elements which are still very difficult to take in at a glance even with knowledge of the fixed permutations. In particular, the fact should be noted that the number of possible displacements of elements is considerably greater than with simple exchange of elements in pairs and that the total expenditure necessary remains comparatively low because even with the permutations, operation involves optimum utilization of all signal stores. 
     Supplementing the exchange of elements in pairs by an additional time coding of known type is, of course, also possible. In this case, too, the individual transposition operations at the receiving end must be provided in reverse sequence compared with the transmitting end. There is also the possibility, however, of an effective amplification of the exchange of elements in pairs according to the invention by enciphering processes of another kind, such as additional splitting up of the elements into individual frequency bands which are transmitted in a transposed frequency position. In particular, there is also the possibility of a division into two or more frequency bands, which are each subjected, independently of one another and in accordance with a different program, to a time coding by exchange of elements in pairs. Thus apart from at least two devices for the exchange of elements in pairs, separate filters for dividing the message into at least two sub-bands are necessary for carrying out such enciphering. 
     The effectiveness of the exchange of elements in pairs can also be increased by interconnecting two or more devices for the exchange of elements in pairs, working with different lengths of element. The element lengths are preferably in an integral ratio to one another so that at least some of the element dividing points are common to the longer and shorter elements. 
     Instead of a direct transmission of the coded signals from the device at the transmitting end to that at the receiving end, provision may also be made for recording the coded signals at the transmitting end, for example a sound-tape recording. This recording can then be played back again at a later time and be supplied to the deciphering device at the receiving end to recover the original clear signals. 
     Although this invention has been described with respect to its preferred embodiment, it should be understood that many variations and modifications will now be obvious to those skilled in the art and, therefore, it is preferred that the invention be limited not by the specific disclosure herin but only by the appended claims.