Security Analysis On Loiss And Other Related Stream Ciphers

Stream cipher has a simple structure. It’s encrypt and decrypt speed is faster than blockcipher. Because of these reasons, stream ciphers are widely used in military、communication、government agencies and so on which requirement for high security and fast data throughput.The stream ciphers operate only one single bit of the plaintext once time. The encryptionprocess required the key stream, and the key stream is generated by key stream generator.Since each plaintext corresponding to a random encryption key, stream cipher belongs tounconditionally secure cryptosystem in theory.Compared with the block cipher, stream cipher has many advantages, such as it is easilyimplemented by hardware、low error diffusion、more suitable for the transmission environmentwhich is required high accuracy and so on. Since stream ciphers are widely used in militaryand other high security requirements organs. Therefore, most of the stream ciphers are notwell known to most people.This paper mainly uses the Guess and the Determine cryptanalysis method to analyze thesecurity of the Loiss cryptographic algorithms from another angle. It involves the algorithmstate without initialization、after one initialization、after two initializations and after threeinitializations. According to the results of the previous rounds, we infer the security of theLoiss cipher algorithm in this case.According to the analysis results, we found that with the increase of the number rounds ofthe algorithm initialization, the complexity of Guess and Determine analysis gradually close tothe complexity of the exhaustive search. Therefore, in the finite number rounds of theinitialization state and in the later rounds initialization state, reuse this Guess and Determinecryptanalysis method will not apply to Loiss algorithm.During the analysis, the attack complexity of algorithm without initialized state isO (2²³²), it is less than the exhaustive search complexity. If the algorithm after an roundinitialization, after analysis, the attack complexity in this state is alsoO (2²³²). If thealgorithm after two rounds of initialization, after analysis, the attack complexity in this state isO (2²⁴⁰). It isO (2¹⁶)less than the exhaustive search complexity. If the algorithm after three rounds of initialization, after analysis, the attack complexity in this state isO (2²⁴⁸). It isO （2⁸）less than the exhaustive search complexity.Because Loiss cryptographic algorithm has just been raised near, there are not a lot ofattacks for Loiss cryptographic algorithm. Therefore, the results of the analysis of the papershas a certain cryptography significance to the research of the Loiss stream cipher.