Substitution/Permutation Network
A Substitution/Permutation network, also known as a SP network, is a class of block cipher which comprises several rounds, each round being a substitution, a permutation, and the addition of key material. The figure below shows a Substitution/Permutation network with three rounds.
The four keys K1, K2, K3, and K4, are derived from a single key. This series of keys is called a key schedule and the derived keys are called round keys.
Note that the machinery for decryption
is almost identical to that for encryption.
The process of decryption is just the reverse of the one for encryption
but the inverses of the S-boxes must be used.
As for the key addition, for any given key, k, the key addition function, fk,
must be an involution.
References
L. R. Knudsen, M. J. B. Robshaw, The Block Cipher Companion. Springer, 2011.