Periodic Properties of Counter Assisted Stream Ciphers
This paper analyses periodic properties of counter assisted stream ciphers. In particular, we analyze constructions where the counter system also has the purpose of providing additional complexity. We then apply the results to the recently proposed stream cipher Rabbit, and increase the lower bound on the internal state period length from 2158 to 2215. With reasonable assumptions we illustrate that the period length of Rabbit is at least the period of the counter system, i.e. at least 2256-1. The investigations are related to a “mod 3” characteristic of Rabbit. Attacks based on this characteristic are discussed and found infeasible.
KeywordsStream cipher period counter diversity degeneracy Rabbit
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