Wang and Yu proposed a block cipher scheme based on dynamic sequences generatedby multiple chaotic systems, which overcomes the problem of periodical degradation onrandom sequences due to computational precision. Their scheme has a feature that aplaintext is encrypted by a keystream created from several one-dimensional chaotic maps. However, this feature results in some weaknesses of the encryption algorithm. We showthree kinds of attacks in this paper, through which one can recover the plaintext froma given ciphertext without the secret key. We also present an improvement on theirscheme, which prevents the three attacks mentioned above. Security of the enhancedcipher is presented and analyzed, which shows that our improved scheme is secure underthe current attacks.
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