Variably Modified Permutation Composition

VMPC (Variably Modified Permutation Composition) for cryptography is a stream cipher similar to the well known and popular cipher RC4 designed by Ron Rivest. It was designed by Bartosz Żółtak, presented in 2004 at the Fast Software Encryption conference.

The core of the cipher is the VMPC function, a transformation of n-element permutations defined as:

for x from 0 to n-1: g(x) = VMPC(f)(x) = f(f(f(x))+1)

The function was designed such that inverting it, i.e. obtaining f from g for a 16-element permutation is about 2; for 64-element permutation, about 2; and for a 256-element permutation, about 2.P and integer value Kulesza, Kamil (2008-10-27). "On Inverting the VMPC One-Way Function" (PDF). Archived from the original (PDF) on 2015-02-09. Retrieved 9 February 2015.