We explore the security in context-aware computing with focus on user authentication. We propose an improved user security authentication protocol for context-aware computing, which uses a zeroknowledge- based protocol for identification and cryptographic calculations, and uses a context-aware system for verifying and protecting the user?s location by the way that access control policy for context information based on IPv6, the mist-link and the password. We present the details of the prototyped implementation, and then we analyze the security of this protocol via active attacks and passive attacks, finally we discuss the tradeoff between usability and security.