本文介绍了串空间模型中的基本概念和定理，并首次利用串空间理论，从机密性和认证性两个方面，对改进的NSSK 协议[1]进行了分析。分析结果表明改进的NSSK 协议是安全的。关键词：安全协议;串空间;NSSK 协议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 modifiedNSSK protocol.Key words: Security Protocol ; Strand Space ; NSSK Protocol