Variably Modified Permutation Composition: Difference between revisions

Revision as of 11:15, 13 March 2016

Revision as of 04:34, 22 March 2016

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The core of the cipher is the VMPC function, a transformation of n-element [[permutation]]s defined as:

'''for''' x '''from''' 0 '''do''' 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.{{Citation needed|date=September 2015}}

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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:

<source lang="text" line>

1. n = 0

2. Repeat steps 3-6 L times:

3. s = P[ (s + P[n]) mod 256 ]

4. Output = P[ (P[P[s]]+1) mod 256 ]

5.Swap(P[n], P[s]) # Temp = P[n]; P[n] = P[s]; P[s] = Temp

n = (n + P[n]1) =mod P[s]256

</source>

P[s] = Temp

6. n = (n + 1) mod 256

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