Editing Substitution–permutation network (section)

{{Short description|Cipher design construction}}
[[Image:SubstitutionPermutationNetwork-en.svg|thumb|360px|right|A sketch of a substitution–permutation network with 3 rounds, encrypting a plaintext block of 16 bits into a ciphertext block of 16 bits. The S-boxes are the ''S<sub>i</sub>'', the P-boxes are the same ''P'', and the round keys are the ''K<sub>i</sub>''.]]
In [[cryptography]], an '''SP-network''', or '''substitution–permutation network''' ('''SPN'''), is a series of linked mathematical operations used in [[block cipher]] algorithms such as [[Advanced Encryption Standard|AES (Rijndael)]], [[3-Way]], [[Kalyna (cipher)| Kalyna]], [[Kuznyechik]], [[PRESENT (cipher)|PRESENT]], [[Secure and Fast Encryption Routine|SAFER]], [[SHARK]], and [[Square (cipher)|Square]].

Such a network takes a block of the [[plaintext]] and the [[key (cryptography)|key]] as inputs, and applies several alternating ''rounds'' or ''layers'' of [[substitution box]]es (S-boxes) and [[permutation box]]es (P-boxes) to produce the [[ciphertext]] block. The S-boxes and P-boxes transform {{nobr|(sub-)blocks}} of input [[bit]]s into output bits.  It is common for these transformations to be operations that are efficient to perform in hardware, such as [[exclusive disjunction|exclusive or]] (XOR) and [[bitwise rotation]]. The key is introduced in each round, usually in the form of "[[round key]]s" derived from it. (In some designs, the [[S-box]]es themselves depend on the key.)

[[Decryption]] is done by simply reversing the process (using the inverses of the S-boxes and P-boxes and applying the round keys in reversed order).