Since the intranet is a combination of open internet technologies and private information systems, various technologies for information security are essentially needed. On recent, a lot of firewall systems are being constructed to be secure the informations from external networks such as Internet in many private companies. Even though internal attacks are more frequently happened than external ones in the intranet environments, there are quite few researches on secure intranet and the internal threats are underestimated so far. In this paper, we study the security threats for each service in the intranet and propose the security models appropriate to the intranet environments by using several cryptographic tools and protocols. Furthermore, we implement the proposed security models in Java applications through computer simulation.