Online Ciphers from Tweakable Blockciphers

Phillip Rogaway and Haibin Zhang

Online ciphers are deterministic length-preserving permutations EK :  ({0,1}n )+→({0, 1}n )+ where the i-th block of ciphertext depends only on the first i blocks of plaintext. Definitions, constructions, and applications for these objects were first given by Bellare, Boldyreva, Knudsen, and Namprempre.We simplify and generalize their work, showing that online ciphers are rather trivially constructed from tweakable blockciphers, a notion of Liskov, Rivest, and Wagner. We go on to show how to define and achieve online ciphers for settings in which messages need not be a multiple of n bits.

Proceedings of CT-RSA 2011

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