Abstract: The degrees of completeness and avalanche effect and strict avalanche criterion for SHA-1 with increased number of steps were statistically analyzed. In order to improve the performance of collision resistance and nonlinear diffusion for SHA-1，the original algorithm was improved for its design defects and vulnerability indicated in the field of the current cryptology. The improved algorithm with mix function applied inverse message expansions sequence and inscrted Integer tent maps at the first round of mix function, to accelerate differential diffusion, to alter the original linked variables passing method,to correct the inner design architecture defects of the algorithm. The test and analysis results proved the reforming algorithm improved the degrees of nonlinear diffusion and enhanced the security of the algorithm.

Key words: Hash function, SHA-1, Mcssagc expansions, Tent map, Diffusion