A New Cryptanalytic Technique on Bit-Oriented Stream Ciphers and Application to ACORN V3

In this paper, a new insight into the link between bit-oriented and word-oriented stream ciphers from a cryptanalytic perspective is given. That is, for a given bit-oriented stream cipher with a key size of n bits, it can be transformed to a word-oriented stream cipher with a key size of \(n-m\) bits, where a word consists of \(2^m\) bits, the bitwise XOR and table lookup operations replace the bit XOR and AND operations respectively. Based on the new insight, a new cryptanalytic technique on bit-oriented stream ciphers is proposed. The new attack combines the guessing strategy with algebraic technique, and aims at slightly improving the exhaustive key search. To demonstrate the effectiveness of transformation attack, we apply it to the bit-oriented stream cipher ACORN v3.