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The Key and IV Setup of the Stream Ciphers HC-256 and HC-128

The key and IV setup algorithms of both HC-256 and HC-128 cipher are cryptanalyzed in this paper. Both ciphers are software-efficient stream ciphers, proposed as the candidates of ECRYPT Stream Cipher Project and selected into the final portfolio. Key-schedule is one of the most important parts of designing a security cipher. The weak key initialization process is a fatal potential weakness of a cipher. The analysis results show that the master keys \emph{K} of HC-256 and HC-128 can be recovered from the states \emph{P} and \emph{Q} easily. Therefore, the key and IV setup algorithms is not perfect enough. Final, an enhanced algorithm is proposed base on subkey addition as block ciphers, to improve the security of stream ciphers.