- Variably modified permutation composition
Variably Modified Permutation Composition (VMPC) is an encryption technology designed by Bartosz Zoltak, publicly presented in 2004 at an international cryptography conference for Fast Software Encryption in Delhi, India.
The core of the technology is the VMPC one-way function, which is probably the simplest currently known function which cannot be inverted. The function is applied in an encryption algorithm known as the VMPC stream cipher. The cipher is efficient in software implementations and appears to offer better security than the still very popular RC4 stream cipher, for both the encryption process and the Key Scheduling Algorithm.
The best currently known attack against VMPC is a dinstinguishing attact by Alexander Maximov - it can distinguish the keystram generated by VMPC from a random data-stream after observing about 2^54 bytes (approximately 18 million gigabytes). Distinguishing attacks however do not cause a direct threat to the secrecy of the encrypted data or the cryptographic key. RC4 has known distinguishig attacks requiring less than 2^30 observations but still RC4 is considered secure and is often applied by software developers. VMPC can be seen as an improved successor the original RC4, but the RC4 is still much more popular.
* [http://www.vmpcfunction.com/ VMPC Homepage]
* [http://www.springerlink.com/link.asp?id=bdb6y010ybg8vr0l AlexanderMaximov, Two Linear Distinguishing Attacks on VMPC and RC4A andWeakness of RC4 Family of Stream Ciphers, FSE 2005]
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