Cryptanalysis And Improvement Of Two Cryptographic Protocols

With the improvement of science and technology, the cryptolography has achieved immensedevelopment in recent years and has involved in every field of the whole society. The cryptographictechnique is an important subject as well as the most efficient and available method to safeguard thecomputer system. Having been widely used in encryption of communications, identityauthentication, data transmission, digital signature and so on, the cryptographic technique is ofparamount important system. It prevents the system from being intruded or destroyed by the illegalusers and guards the security of the communication data in a potential unsafe channel. The bitcommitment schemes are important basic protocols in cryptology as well as one of the mostessential protocols in secure multiparty computation. The bit commitment scheme has been widelyconsidered as a useful building block in the design of more elaborate cryptographic protocols, suchas zero-knowledge proof schemes, mental poker games and lottery schemes. However, newthreatens are approaching the traditional cryptosystem because of the improvement of the quantumcomputation. A lot of researches and explorations have been widely studied in order to build asecure bit commitment scheme which can resist the quantum attacks. In this paper, we study a bitcommitment scheme and its generalizations based on the twisted conjugacy search problem thatattain higher security by generalizing the platform of our scheme. Another work in this paper isabout the threshold proxy signature schemes. As a hot topic, the threshold proxy signature has manyuseful properties, however, the general threshold proxy signature schemes are confronted withvarious safety potentials. In this paper we take a specific threshold proxy signature scheme as anexample, give the cryptanalysis of the scheme and propose a new scheme as well. The main workand research results lie in the following:Chapter 3 proposes a basic bit commitment scheme based on the twisted conjugacy searchproblem and generalizes the basic scheme to the biased bit commitment scheme and the bit stringcommitment scheme later, gives the prove of correctness and security respectively. At last wegeneralize the platform to the semi-group of all n×nmatrices over truncated q-variablepolynomials overZ pand set up the parameters of the platform.Chapter 4 introduces the Pomykala-Barabasz threshold proxy signature scheme briefly, givestwo new attacks on the scheme and proposes the improved scheme based on several weakness ofthe Pomykala-Barabasz’s scheme. At the end we give the cryptanalysis of the improved scheme,point out that our scheme has the basic properties of threshold proxy signature scheme and can resist the known attacks.