In Eurocrypt'05, Wang et al. presented new techniques to find collisions of Hash function MD4. The techniques are not only efficient to search for collisions, but also applicable to explore the secondpreimage of MD4. About the second-preimage attack, they showed that a random message was a weak message with probability 2−122 and it only needed a one-time MD4 computation to find the second-preimage corresponding to the weak message. A weak message means that there exits a more efficient attack than the brute force attack to find its secondpreimage. In this paper, we find another new collision differential path which can be used to find the second-preimage for more weak messages. For any random message, it is a weak message with probability 2−56, and it can be converted into a weak message by message modification techniques with about 227 MD4 computations. Furthermore, the original message is close to the resulting message (weak message), i. e, the Hamming weight of the difference for two messages is about 44.

Serpent is one of the five AES finalists. In our paper, we give solne differentials about Serpent, two of the differentials are a 5-round differential with the probability of 1/207 and a 6-round diffierential with the probability of 1/207. The best known differential before our paper is a 5-round differential with the probability of 1/207 given in [9]. Additionally, we provide all the possible best differentials for some cases about Serpent. From these best differentials, we eonclude that the 16-round best differential is not higher than 1/207 and that the 17-round differential is less than 1/2128.

In this paper, we use the a 12-round differential (5-16 rounds) to analyze the reduced Skipjack variants starting from the first round. The analysis result is that, breaking 1-21 rounds Skipjack variant needs about 217 chosen plaintexts and 264 encryptions, breaking 1-24 variant needs about 246 chosen plaintexts and 272 encryptions, and 1-26 variants needs about 246 chosen plaintexts and 260 encryptions.

In this paper, we present another tree-structure signature scheme based on discrete logarithm problem modulo p, where p is a large prime. The basic signing algorithm is the original ELGmal signature scheme. The scheme attains ideal security, i. e, finding existential forgeries under adaptively chosen message attacks is equivalent to solving the discrete logarithm of any random integer y∈Z*p. The scheme is also efficient, it can be implemented almost as efficiently as the original ELGamal signature scheme. We can regard the scheme as an application of ELGamal signature scheme in tree-structure signature schemes.