The Strand Space Model of NSSK Protocol and Its Analysis ZHOU Qing-lei1,2 , WANG Feng2 , ZHAO Dong-ming2 ( 1. School of Information Engineering ,PLA Information Engineering University ,Zhengzhou Henan China 450000 ; 2. School of Information Engineering , Zhengzhou University ,Zhengzhou Henan China 450052) Abstract: The basic notion and theorems of strand spaces model are introduced. It is the first time to prove the modified version of NSSK protocol with strand space theory from the aspects of both confidentiality and authentication . The analysis proved the correctness of the modified NSSK protocol. Key words: Security Protocol ; Strand Space ; NSSK Protocol