Searching for cost functions

Boolean function design is at the heart of cryptography, and is the subject of a great deal of theoretical research. We have use a simulated annealing approach to find functions with particular desirable cryptographic properties; for functions of a small number of variables, results with properties as good as (and sometimes better than) the best so far have been achieved. The success of this approach is very sensitive to the cost function chosen; here we investigate this property, and describe a meta-search approach to finding the most effective cost function for this class of problems.