A Mobile Ad Hoc Network (MANET) is a infrastructure-less network interconnected with wireless links which changes dynamically. MANET lacks a well defined security mechanism, hence more vulnerable to malicious attacks. In this paper we focus on a system that uses a trust model and SHA-1 key encryption. The system is designed to detect and avoid malicious nodes in the network. The trust value is built based on the previous experiences and recommendations of other nodes in the network. The efficiency of trust system is enhanced by using SHA-1 encryption authentication mechanism when nodes enter into the network.