Variably Modified Permutation Composition

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VMPC ("Variably Modified Permutation Composition") is encryption technology designed by Bartosz Zoltak, publicly presented in 2004 at an international cryptography conference Fast Software Encryption in Delhi, India.

The core of the technology is the VMPC one-way function, applied in an encryption algorithm - the VMPC stream cipher. The cipher is efficient in software implementations and appears to offer better security - of both the encryption process and the key scheduling algorithm, than the still very popular RC4 stream cipher.

The best currently known attack against VMPC is a distinguishing attack by Alexander Maximov - it can distinguish the keystream 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 distinguishing attacks requiring less than 2^30 observations but still RC4 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.