Patent GB-1487089-A (GB)
Title: Secret signalling systems
IPC: G09C1/00
Filed: 1974-12-11
Abstract: 1487089 Secret signalling systems INTERNATIONAL BUSINESS MACHINES CORP 11 Dec 1974 [21 Dec 1973] 53560/74 Heading H4P A secret signalling system operates on binary data in "group substitution" mode, such that to cypher binary input data, the data bits are subdivided into groups, each group containing n bits and defining a word in a particular position in a base list of the different n bit words, and each such group is substituted by a group from the same position in any one of the factorial 2<SP>n</SP> sequences of groups of n bits which are possible, freely determined in response to a first key. The substituted groups are decyphered using a complementary second key related to the first such that the double substitution restores the original data. In one embodiment (Fig. 4), raw input data applied at M is divided by circuits SE into groups of two bits A1, A2 appearing alternately on two output lines. Each such bit may be "1" or "0"; there are four possible binary 2 bit words containing 1 and 0 and 24 (factorial 2<SP>2</SP>) possible sequences of the four possible words (Fig. 1, not shown). The 24 possible sequences are stored in a memory 50 and any particular sequence can be selected by the cyphering key control CC. The bits from a selected sequence are fed to one of logic blocks 51 or 52. These blocks incorporate arrangements of AND and OR gates (Fig. 5, not shown) such that, when the binary data group from the input, A1A2 is applied, binary bits B1B2 are delivered which represents the binary pair corresponding to A1A2 in the particular sequence selected by the key CC. At the receiver an identical system can be used, but a different key will usually be necessary to restore the original data (Fig. 2, not shown, indicates corresponding key sequences). To reduce the amount of storage required possible versions of separate sequences of the first and second bits may be held respectively in separate stores which are addressed in turn by the key CC (Fig. 6, not shown). An appropriate correspondence table will then be needed in CC, but nevertheless the storage needed is much reduced. For sequential operation it is explained that only one such smaller store is needed, being addressed in two passes. The description outlines similar systems for use when the input data is divided into three bit groups (Figs. 9 and 10, not shown) and suggests how to extend the arrangement to N bit groups. The key need not remain the same throughout a message-the groups of bits may be formed into packs of varying length each of which is cyphered using a different key, selected pseudo randomly. A pseudo random generator for selecting the key CC may be of the type wherein bits of arbitrary value held in a store are read out in sequence through logic gates by successively accessing the addresses of the store (Figs. 11-13, not shown). The store used may be part of that used in the cyphering arrangement. For example (Fig. 15) words stored in a part 80b of the store 80 are read out into logic 81, having been accessed by counter 82 operating from clock Cl. The pseudo random words are transferred to system 84 and when gate G is operated in response to a change key command applied to the key control, the pseudo random word is used to determine the key for addressing the cypher store 80a, corresponding to store 50 in Fig. 4. The data at M is cyphered as explained above with reference to Fig. 4 to appear at M<SP>1</SP>, and may be further cyphered by being combined with the pseudo random sequence from the register 83 to provide the doubly cyphered output at M<SP>11</SP>. This system may be enlarged to three or n bit groups (Fig. 16, not shown).