Research On Some Topics Of Modern Cryptosystem

With informatization degree unceasing being enhanced in human society, information security has become a serious problem for protecting the world information activities and national security. Because modern cryptography theory and technology is the core content of information security, the in-depth research on some key issues of modern cryptosystem will further push forward the development of information security framework.Some topics of modern cryptosystem are studied in this thesis, including analysis and construction of the forward-secure proxy blind signature, the design of verifiable multi-secret sharing, the study of the flexible key escrow scheme, analysis and research of algebraic attacks on stream ciphers, the algorithm of computing the probability distribution of XOR difference between mod 2" sum and XOR etc. Main contributions of our work are as follows:1. We firstly propose a new generalized forward-secure proxy blind signature scheme, namely, although the attacker get the key in some period, the attacker cannot forge the proxy key and proxy signature in the past periods. With analyzing, we find our scheme has many other secure characters, such as, unforgeability, distinguishability, identifiability, strong blindness. Then, combin-ing the mechanism of the hash chain with the forward-secure proxy blind signature, we propose a new strong forward-secure proxy blind signature scheme, namely, although the attacker get the key in some period, the attacker cannot forge the signature in the past and future periods.2. A verifiable multi-secret sharing scheme based on LUC cryptosystem is proposed. In the recovery phase of our scheme, only the secret that we need can be recovered, while the other secret is unknown completely, so it is a multi-secret sharing schemes in its true sense. With analyzing, we find our scheme has perfectness, verifiability, optimal information ratio, low realization cost.3. The two representative key escrow schemes are compared and analyzed systematically. Then a new key escrow scheme based on ElGamal cryptosystem was proposed, which can set the number and right of escrow agents flexibly. With analyzing, we find our scheme can prevent subliminal channel attack and partial escrow agents internal attack. It also solves the problem of "once monitor, monitor forever".4. The basic theory and realizing methods of algebraic attacks on stream ciphers are presented. Then the algebraic attacks on stream ciphers with linear feedback shift register and the efficient techniques to decrease the degree of the nonlinear equations system are described in detail. After analyzing the complexity of whole procedures in algebraic attacks, we summarize the direction and future works of this research filed in the end.5. An efficient algorithm to calculate the probability distribution of XOR difference between single-mod 2" sum and XOR is presented. The complexity of our algorithm is O((n-1)/2), which is lower than the complexity of Maximov's result. For the case of multi-mod2" sum, we give a recursive formula to calculate the XOR difference between multi-mod 2" sum and XOR.