Unpredictability, and a related notion, complexity, are of major concern in the theory of dynamical systems. The most interesting unpredictability and complexity arise from simply built deterministic dynamical systems whose large-time behavior bears little evident connection with the equations which define them. One way to clarify the meaning of unpredictability in the context of dynamical systems is it compare it with similar notions in a neighboring scientific discipline. Here the neighboring discipline is cryptology. Cryptology is the study of making and breaking codes meant to protect information from unintended use. Our bridge between dynamical systems and cryptology consists of making codes based on dynamical systems and using methods of cryptology to evaluate them.
In this article introductory sections present some rudiments of cryptology, and quickly survey previous attempts to use dynamical systems in this domain. Then a detailed specification of a particular cryptosystem based on cellular automata is built up with reference to the cryptanalytic attacks it is meant to defend against. A final discussion section speculates on the impact the new ideas embodied in this cryptosystem might have on basic questions in both cryptology and dynamical systems theory.