Substitution-Permutation Networks (SPN) are often used to construct an iterative block cipher. In an iterative block cipher, an input plaintext block goes through several rounds. In each round, separate round keys are used. In a Substitution-Permutation Network (SPN), an alternative layer of substitution box (S-box) and permutation box (P-box) is applied in each round. A different round key is XOR’ed in each round and the final round produces the ciphertext.

The S-box substitutes a block of input bits with another block of bits. The substitution is such that if one bit in the input block changes, that will change about half of the bits in the output block. This property is called the avalanche effect. Also, a good S-box ensures that every bit in the output block depends on each bit in the input block.