Variably Modified Permutation Composition: Difference between revisions

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{{Short description|Stream cipher}}

{{context|date=October 2009}}

'''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 ZoltakŻółtak, presented in 2004 at the [[Fast Software Encryption]] conference. VMPC is a modification of the [[RC4]] cipher.<ref name=maximov>{{cite documentjournal |title=Two Linear Distinguishing Attacks on VMPC and RC4A and Weakness of RC4 Family of Stream Ciphers (Corrected) |author=Alexander Maximov |journal=Cryptology ePrint Archive |date=2007-02-22 |url=https://eprint.iacr.org/2007/070 }} (originally presented at FSE 2006 confernececonference)</ref>

'''VMPC''' ('''Variably Modified Permutation Composition''') is a [[stream cipher]] similar to the

well known and popular cipher [[RC4]] designed by [[Ron Rivest]]. It was designed by Bartosz Zoltak, presented in 2004 at the [[Fast Software Encryption]] conference. VMPC is a modification of the [[RC4]] cipher.<ref name=maximov>{{cite document |title=Two Linear Distinguishing Attacks on VMPC and RC4A and Weakness of RC4 Family of Stream Ciphers (Corrected) |author=Alexander Maximov |date=2007-02-22 |url=https://eprint.iacr.org/2007/070 }} (originally presented at FSE 2006 confernece)</ref>

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

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The function was designed such that inverting it, i.e. obtaining {{mono|f}} from {{mono|g}}, would be a complex problem. According to computer simulations the average number of operations required to recover {{mono|f}} from {{mono|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}}

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".<ref name="Kulesza2006">{{cite documentweb|last1=Kulesza|first1=Kamil|date= 2008-10-27|title=On Inverting the VMPC One-Way Function|url=http://www-old.newton.ac.uk/preprints/NI06009.pdf|accessdateaccess-date=9 February 2015}}</ref>

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

<source lang="text" line>

<span style="color: green;">''All arithmetic is performed modulo 256.''</span>

ni := 0

Repeat steps 3-8 L times:

'''while''' GeneratingOutput:

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

Outputj P:= S[j (P+ S[P[si]]+1) mod 256 ]

Temp = P[n]

P'''output''' S[nS[S[j]] =+ P[s1]

swap S[i] and S[j] <span style="color: green;">(''b := S[j]; S[j] := S[i]; S[i] := b)'')</span>

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

n = (n + 1) mod 256

i := i + 1

</source>

'''endwhile'''

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