Abstract—Recently, the idea of design of dynamic symmetric cryptosystems is proposed. According to this idea, the property of cipher system is varied related to secret key. But unfortunately this cipher is not safer than whatever is claimed. In this paper, two attacks on the proposed design are investigated. The first attack is a partial key recovery which for a (k,k,m) q- cascaded convolutional transducers is determined a fragment of ciphertext without calculating master key with complexity O(k 6). In addition, a weak key on this system is announced that one can recover longer fragment of plaintexts with complexity O(2×k 6)
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