In cryptography, a product cipher is a popular type of block cipher that works by executing in sequence a number of simple transformations such as substitution, permutation, and modular arithmetic. Product ciphers usually consist of iterations of several rounds of the same algorithm. While the individual operations are not themselves secure, it is hoped that a sufficiently long chain would imbue the cipher with sufficient confusion and diffusion properties as to make it resistant to cryptanalysis. The concept of product ciphers is due to Claude Shannon, who presented the idea in his foundational paper, Communication Theory of Secrecy Systems.
A product cipher that uses only substitutions and permutations is called a SP-network. Feistel ciphers are an important class of product ciphers.