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Iterated attacks are comprised of iterating adversaries who can make plaintext queries, in each iteration to compute a bit, and are trying to distinguish between a random cipher and the ideal random cipher based on all bits. In EUROCRYPT '99, Vaudenay showed that a -decorrelated cipher resists to iterated attacks of order when iterations make almost no common queries. Then, he first asked what the necessary conditions are for a cipher to resist a non-adaptive iterated attack of order . Secondly, he speculated that repeating a plaintext query in different iterations does not provide any advantage to a non-adaptive distinguisher. We close here these two long-standing open problems. We show that, in order to resist non-adaptive iterated attacks of order , decorrelation of order is not sufficient. We do this by providing a counterexample consisting of a cipher decorrelated to the order and a successful non-adaptive iterated attack of order against it. Moreover, we prove that the aforementioned claim is wrong by showing that a higher probability of having a common query between different iterations can translate to a high advantage of the adversary in distinguishing from . We provide a counterintuitive example consisting of a cipher decorrelated to the order which can be broken by an iterated attack of order 1 having a high probability of common queries.
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