Efficient homophonic coding

Homophonic coding, or homophonic substitution, is referred to as a technique that contributes to reliability of the secret key cipher systems. Its main goal is to convert the plaintext into a sequence of completely random (equiprobable and independent) code letters. In solving this problem three characteristics are to be considered: (i) redundancy, defined as the difference between the mean codeword length and the source entropy, (ii) an average number of random bits used in encoding, and (iii) complexity of the encoder and decoder, measured by memory size (in bits) and computation time (in bit operations). A class of homophonic codes is suggested for which both the redundancy and the average number of random bits can be made as small as required with nonexponential growth of memory size and roughly logarithmic growth of computation time.