VMPC meaning in General ?

Answer: What is Variably Modified Permutation Composition mean?

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

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, would be a complex problem. According to computer simulations the average number of operations required to recover f from g for a 16-element permutation is about 211; for 64-element permutation, about 253; and for a 256-element permutation, about 2260.

In 2006 at Cambridge University, Kamil Kulesza investigated the problem of inverting VMPC and concluded "results indicate that VMPC is not a good candidate for a cryptographic one-way function".

The VMPC function is used in an encryption algorithm – the VMPC stream cipher. The algorithm allows for efficient in software implementations; to encrypt L bytes of plaintext do:

All arithmetic is performed modulo 256.i := 0while GeneratingOutput: a := S[i] j := S[j + a] output S[S[S[j]] + 1] swap S[i] and S[j] (b := S[j]; S[i] := b; S[j] := a)) i := i + 1endwhile

Where 256-element permutation P and integer value s are obtained from the encryption password using the VMPC-KSA (Key Scheduling Algorithm).

reference

Full Form	Category
Variably Modified Permutation Composition	General
Ventral Medial Prefrontal Cortex	Medical