Information Theory And Encryption

EE 62 December 1987 INFORMATION THEORY AND ENCRYPTION by B.P. McArdle Anyone who is, or becomes, involved in encryption operations and cryptosystems must wonder about their connection with Information Theory. In this article, Brian McArdle briefly explains the areas of overlap and difference. Consider a channel where a message xi drawn from a set {xi ,x2,x3, ,xii) of n possible messages, as illustrated in fig- ure 1, is transmitted between sender A and receiver B. The message could be just a letter from an alphabet of n letters or a symbol. However, it is infor- mation of some type and is ex- changed between A and B. The electronic representation of xi could be a particular waveform or a set of binary digits (bits) etc. For example, the English alphabet of 26 letters requires a set of 5 bits to represent a letter and since 25=32 there are 6 redundant combinations. For the present the method of signalling is not being con- sidered. If each xi has pro- bability Pr(xi)=pi of being ch...