No polynomial classical algorithms can distinguish between the 3-round Feistel cipher with internal permutations and a random permutation. It means that the 3-round Feistel cipher with internal permutations is secure against any chosen plaintext attack on the classical computer. This paper shows that there exists a polynomial quantum algorithm for distinguishing them. Hence, the 3-round Feistel cipher with internal permutations may be insecure against a chosen plaintext attack on a quantum computer. This distinguishing problem is an instance that can be efficiently solved by exploiting the quantum parallelism. The proposed algorithm is the first application of Simon's algorithm to cryptographic analysis.
This paper investigates the practical security of RC4 in broadcast setting where the same plaintext is encrypted with different user keys. We introduce several new biases in the initial (1st to 257th) bytes of the RC4 keystream, which are substantially stronger than known biases. Combining the new biases with the known ones, a cumulative list of strong biases in the first 257 bytes of the RC4 keystream is constructed. We demonstrate a plaintext recovery attack using our strong bias set of initial bytes by the means of a computer experiment. Almost all of the first 257 bytes of the plaintext can be recovered, with probability more than 0.8, using only 2 32 ciphertexts encrypted by randomly-chosen keys. We also propose an efficient method to extract later bytes of the plaintext, after the 258th byte. The proposed method exploits our bias set of first 257 bytes in conjunction with the digraph repetition bias proposed by Mantin in EUROCRYPT 2005, and sequentially recovers the later bytes of the plaintext after recovering the first 257 bytes. Once the possible candidates for the first 257 bytes are obtained by our bias set, the later bytes can be recovered from about 2 34 ciphertexts with probability close to 1.
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