Implementation of an Instant Messaging System with Focus on Protection of User Presence

Instant messaging (IM) systems provide its users with the information of which of their contacts are currently online. This presence information supplements text communication of IM systems and is an additional value compared to other synchronous communication media. Unauthorized users could illegally generate online logs of users by exploiting their presence information. Public IM systems lack the reliable means to protect user presence and force the user to trust in the central registry server. In this paper, we propose an IM system, which is explicitly designed to protect user presence without the need of a trusted central registry. We present a Java implementation based on the anonymous communication network Tor [1], the cryptographic suite Bouncy Castle [2], and the distributed hash table OpenDHT [3].