Cryptography with Cellular Automata

This abstract discusses a stream cipher based on a simple one-dimensional cellular automaton. The cellular automaton consists of a circular register with N cells, each having value ai equal to 0 or 1. The values are updated synchronously in discrete time steps according to the rule ai? = ai-1 XOR (ai OR ai+1), (1a) or, equivalently, ai? = (ai-1 + ai + ai+1 + aiai+1) mod 2. (1b) The initial state of the register is used as a seed or key. The values a(i) attained by a particular cell through time can then serve as a random sequence. Ciphertext C can be obtained from binary plaintext P as usual according to Ci = Pi XOR a(i); the plaintext can be recovered by repeating the same operation, but only if the sequence a(i) is known.