Enhancing the security of block ciphers with the aid of parallel substitution box construction

When considering block cipher designs, one feature that is seemingly not related to their robustness of a design is algorithmic variability, i.e. the ability to effect changes on a design that essentially leave its structure unchanged while they modify its functional characteristics. This feature, however, is related to robustness as there are situations where a specific algorithm is either suspected to be under cryptanalytic attack or it is not considered secure any more due to a discovered weakness. The easiest action would be to change the characteristics of the algorithm in a way that obscures the cryptanalytic attack or that eliminates the cipher's weaknesses. Our focus is on this kind of changes, using as a specific case the CAST-128 cipher The changes we consider refer to the algorithm's substitution boxes and since the creation of good substitution boxes is a highly time consuming process, we also provide a parallel algorithm for completing this task quickly.