International Journal of Internet and Computer Security. ISSN 0974-2247 Volume 3, Number 1 (2012), pp. 15-24 © Research India Publications http://www.ripublication.com Security Mechanism In Session Initiation Protocol Harish C. Sharma*, Pradeep Semwal** *Lecturer, Department of Computer Science, SGRR-ITS, Patel Nagar, Dehradun, Uttarakhand, India **Senior Lecturer, Department of Computer Science, SGRR-ITS, Patel Nagar, Dehradun, India ABSTRACT VoIP enables peoples to communicates using a high speed Internet connection for all video, voice, and data communications, VoIP uses Session Initiation Protocol (SIP) defined by IETF. The SIP is a Signaling protocol for the Internet Telephony. SIP security begins with basic IP and VoIP specific security. SIP is the future protocol for VoIP. The paper looks at different Security Mechanism for the Session Initiation Protocol (SIP). The different security mechanism includes HTTP digest, IPSec, S/MIME and TLS. Keywords: Security, SIP, TLS, S/MIME, IPSec. 1. Introduction Voice over Inter Protocol (also called VoIP, IP Telephony, Internet Telephony, and Broadband Phone ) is the routing of voice conversations over the Internet or through any other IP-based network. VoIP allows one to use a single high speed internet connection for all voice, video, and data communications. The process of VoIP is dependent on signaling and media transport. A signaling protocol, such as SIP performs the legwork: locating users call parameters, modifications and building or ending a session. A session can be voice, video or instant messaging and is described by SDP. SIP is a text based protocol, using UTF-8 encoding, very much like HTTP or SMTP that can establish in IP network. SIP thus enables service providers to integrate basic IP Telephony services with web, Email, and chat services. In short, SIP promises to enrich consumer’s live through a host of innovative services, such as audio, videoconferencing, interactive gaming, and voice-enrich e-commerce. The paper describes different security mechanism available with the SIP protocol such as HTTP digest, S/MIME, TLS and IPSec.