In this paper, a single sign-on system has been designed and implemented. The purpose of the proposed system is that a user logs in once using a single password and gets authenticated access to all the servers that he is authorized to use without sending any passwords over the network. The proposed system involves the use of digital signature and cryptographic authentication protocols. For its implementation, a public-key infrastructure is also designed and implemented. To view the behavior of the proposed system, message sequence charts and state transition diagrams have been