Computer communications have been playing a vital role in the world economy. Government organizations, large companies and banks are using those networks in trading their data. This imposed a challenge due to the increasing need for protecting the sensitive data traded over those networks. This research presents a mechanism to protect computers communication over open un-trusted networks, primarily, that mechanism relies on securing communication authentication. In order to do that, the communication protection mechanism modifies the Challenge-Handshake Authentication Protocol (CHAP) which is responsible for the authentication of communication of layer two High-level Data Link Control (HDLC) protocol.