In this paper we investigate a new way for protecting block ciphers against classes of attacks (including differential and linear cryptanalysis) which is based on the notion of decorrelation distance which is fairly connected to Carter-Wegman's universal hash functions paradigm. 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 cipher families we call COCONUT and PEANUT, which implement these ideas and achieve quite reasonable performances for real-life applications.