Cryptology ePrint Archive: Report 2001/046

The simple ideal cipher system

Boris Ryabko

Abstract: We address the problem of how to construct ideal cipher systems when the length of a key is much less than the length of an encrypted message. We suggest a new secret key cipher system in which firstly the message is transformed into two parts in such a way that the biggest part consists of independent and equiprobable letters. Secondly the relatively small second part is enciphered wholly by the Vernam cipher whereas only few bits from the biggest part are enciphered. This transformation is based on the fast version of the Elias construction of an unbiased random sequence.

The time required for encoding and decoding and the memory size of the encoder and decoder are presented as functions of the ratio of the key length and the message length. The suggested scheme can be applied to sources with unknown statistics.

Category / Keywords: secret-key cryptography / ideal cipher system,fast algorithms,Shannon entropy

Date: received 4 Jun 2001

Contact author: ryabko at neic nsk su

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Version: 20010607:112333 (All versions of this report)

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