Product cipher

A product cipher is a cipher that combines two or more transformations in a way meant to make the result more secure, and more resistant to cryptanalysis, than any one of the components alone. It typically chains simple operations such as substitution through an S-box, permutation through a P-box, and modular arithmetic. The idea is credited to Claude Shannon, who presented it in his foundational paper Communication Theory of Secrecy Systems. When all of the constituent round functions share the same structure, the design is called an iterative cipher and the repetitions are called rounds. A product cipher that uses only substitutions and permutations is called an SP-network, and Feistel ciphers are an important class of product ciphers. IBM used the alternating substitution and permutation approach in the Lucifer cipher system, and it became the standard pattern for national encryption standards such as the Data Encryption Standard and the Advanced Encryption Standard.