王小云，学者凯发k8国际首页主页-凯发网

md5 is one of the most widely used cryptographic hash functions nowadays. it was designed in 1992 as an improvement of md4, and its security was widely studied since then by several authors. the best known result so far was a semi free-start collision, in which the initialvalue of the hash function is replaced by a non-standard value, which is the result of the attack. in this paper we present a new powerful attack on md5 which allows us to find collisions efficiently. we used this attack to find collisions of md5 in about 15 minutes up to an hour computation time. the attack is a differential attack, which unlike most differential attacks, does not use the exclusive-or as a measure of difference, but instead uses modular integer subtraction as the measure. we call this kind of differential a modular differential. an application of this attack to md4 can find a collision in less than a fraction of a second. this attack is also applicable to other hash functions, such as ripemd and haval.

md4 is a hash function developed by rivest in 1990. it serves as the basis for most of the dedicated hash functions such as md5, shax, ripemd, and haval. in 1996, dobbertin showed how to find collisions of md4 with complexity equivalent to 220 md4 hash computations. in this paper, we present a new attack on md4 which can find a collision with probability 2-2 to 2−6, and the complexity of finding a collision doesn't exceed 28 md4 hash operations. built upon the collision search attack, we present a chosen-message pre-image attack on md4 with complexity below 28. furthermore, we show that for a weak message, we can find another message that produces the same hash value. the complexity is only a single md4 computation, and a random message is a weak message with probability 2−122. the attack on md4 can be directly applied to ripemd which has two parallel copies of md4, and the complexity of finding a collision is about 218 ripemd hash operations.

in this paper, we present new techniques for collision search in the hash function sha-0. using the new techniques, we can find collisions of the full 80-step sha-0 with complexity less than 239 hash operations.

in this paper, we present new collision search attacks on the hash function sha-1. we show that collisions of sha-1 can be found with complexity less than 269 hash operations. this is the first attack on the full 80-step sha-1 with complexity less than the 280 theoretical bound.

给出了一种hash函数haval-128的有效攻击方法. haval是由zheng等人在auscrypto'92提出的，它的轮数可以是3、4或5，输出长度为128、160、192或224比特的杂凑值本文攻击的是具有128比特杂凑值的haval算法本文的主要结论是，任给一个1024比特长的消息m，只要对m做一些适当的修改，修改后的消息m就会和另外一个消息m'以2-7的概率发生碰撞，其中m'=m △m，△m是事先选定的一个固定的明文差分另外，本文还给出了两个碰撞的实例.

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.