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VMPC (Variably Modified Permutation Composition) is a stream cipher technology designed by Bartosz Zoltak, presented in 2004 at the Fast Software Encryption conference. VMPC is a modification of the RC4 cipher.〔 (originally presented at FSE 2006 confernece)〕 The core of the cipher is the VMPC function, a transformation of n-element permutations 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. 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: 1. n = 0 2. Repeat steps 3-6 L times: 3. s = P 4. Output = P 5. Temp = P() P() = P() P() = 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). ==References== 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Variably Modified Permutation Composition」の詳細全文を読む スポンサード リンク
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