In cryptography, a product cipher combines two or more transformations in a manner intending that the resulting cipher is more secure than the individual components to make it resistant to cryptanalysis.
[1] The product cipher combines a sequence of simple transformations such as substitution (S-box), permutation (P-box), and modular arithmetic.
The concept of product ciphers is due to Claude Shannon, who presented the idea in his foundational paper, Communication Theory of Secrecy Systems.
[2] For transformation involving reasonable number of n message symbols, both of the foregoing cipher systems (the S-box and P-box) are by themselves wanting.
Shannon suggested using a combination of S-box and P-box transformation—a product cipher.