In this paper, we propose a methodology based on genetic programming to automatically generate hardware designs of substitution boxes necessary for many cryptosystems such as DES encryption system. We aim at evolving minimal hardware specifications, which minimise both space (i.e. required gate number), response time (i.e. encryption and decryption time) and dissipated power. We compare our results against existing and well-known designs, which were produced by human designers using conventional methods.