Patent GB-1356970-A (GB)
Title: Method of and apparatus for enciphering information by time- interchange of information elements
IPC: H04K1/06
Filed: 1971-05-28
Abstract: 1356970 Speech secrecy systems PATELHOLD PATENTVERWERTUNGS & ELEKTRO-HOLDING AG 28 May 1971 [7 July 1970] 18146/71 Heading H4R A speech signal in analogue or digitally encoded form is divided into segments of equal  length and interchanged in their positions in time under the control of signals generated by a cipher signal. The cipher signal is used to derive control signals determining the position of feed in, or take out, of the elements to or from an information store and a further store is used in the derivation of control signals to ensure that no speech segment is omitted from the transmission and no speech segment is transmitted more than once. As described the segments can be arranged in frames and the segments re-arranged within those frames, or each segment can be re-arranged to occupy a position within a predetermined delay from its original position i.e. a "sliding window" system. The individual segments may also have their polarity inverted, or be masked by adding a digital or analogue cipher signal to increase the difficulty of interception. Fig. 4 shows a system for interchanging elements within a frame and includes stores NS1 and NS2, each of which stores a frame of 6 speech segments each of which is represented by a series of analogue pulses. Elements of incoming speech are stored in one register, NS2, while the previously stored elements in the other register NS1 are read out, in modified order controlled by the cipher generator SG. To prevent repetitions or omissions the circuit ZE includes a store BS which is initially filled at the same time as the information store, NS1, is filled. The cipher generator SG produces signals controlling, via an adder AD and code converter CW, the generation of hunting pulses h to control the emptying of one of the storage locations in the control store BS and feed out of the corresponding speech element from NS1. Should the selected control store location in BS already be emptied a pulse k is generated and applied to adder AD to modify the hunting pulse generation until a filled storage location is encountered, ensuring that the elements are extracted in unbroken and unrepeated extraction. After all the storage locations are emptied the function of the information stores NS1 and NS2 are interchanged. An alternative scrambler, Fig. 5, working on a "sliding window" system has the input speech segments fed into storage locations in NS under the control of the cipher code converter SW which provides switching signals a 2 , a 3  and a 4  having probabilities selected in accordance with values derived in the Specification. In order to prevent elements not being transmitted, or being repeated, a control store BR is provided stepped at the same rate as the information store NS, and interconnected with the switching circuits S1 to S4 to prevent a loading pulse c being directed into an already filled location. An overriding switch S1 is provided to direct a loading pulse into the last storage stage if the stage is not already filled, while the other stages are filled only if that stage is selected by the cipher input, and is not already filled. Other embodiments described, Figs. 7 and 9 (not shown), use magnetic disc stores on a common shaft, each disc carrying the signals representing one speech element with additional control storage registers to allow for progressive switching of the information on the discs to make it equivalent to a moving tape storage system.