Bisimulation as a technique could be well invested for proving authenticity and secrecy properties of cryptographic protocols to gain the legality of protocol optimization. In this paper, we will do some changes in the spi-calculus after the original work of M.Abadi and A.Gordan. Then we will introduce evade bisimulation following Abadi and Gordan's framed bisimulation proposal, in which a convenient proof technique is presented. It will impose minimality requirements on the environment and

more »

... ct the limit beyond which the bisimilarity is kept valid and furthermore it will avoid quantification over contexts. Also, it will give a solution for input transitions for the case of finite processes. Based on the revised spi-calculus would be used to prove that evade bisimilarity, an equivalence relation, is decidable for main security properties: Authenticity and Secrecy.