Let y be the output of an SPN (Substitution Permutation Network) Cipher on input x, where pS and pP.

Let y be the output of an SPN (Substitution Permutation Network) Cipher on input x, where pS and pP are the substitution and permutation transformation of the ciphers. Thus, y = SP N(x, pS, pP ,(K1 , . . . , KNr+1)) where (K1 , . . . , KNr+1) is the key schedule. Find a substitution p * S and p * P such that: x = SP N(y, p* S, p* P ,(K1 , . . . , KNr+1))