In this presentation we investigate a new way of protecting block ciphers against classes of attacks (including differential and linear crypt-analysis) which is based on the notion of decorrelation which is fairly connected to Carter-Wegman's notion of universal functions. This defines a simple and friendly combinatorial measurement which enables to quantify the security. We show that we can mix provable protections and heuristic protections. We finally propose two new block ciphers family we call COCONUT and PEANUT, which implement these ideas and achieve quite reasonable performances for real-life applications.