Secure RTP with Asterisk

Normal asterisk servers use RTP for deliver audio over the internet. Any one can capture this RTP streams by using Packet Sniffer tools, and can listen to  the call content. So VoIP calls are not secure for confidential communications until using a secure VoIP system.

Therefore we can use advance protocol called SRTP (Secure Real-time Transport Protocol) which provide encryption for RTP stream. So no one can listen to the VoIP conversations while using SRTP. If someone try to capture the SRTP stream, it will play only a noise (even it was a empty conversation).

But it will not complete the secure VoIP system. You MUST secure the signalling (SIP) because SRTP keys are exchanged in plaintext with SDES( To do that we have to use Asterisk TLS (Transport Layer Security). It provides encryption for call signaling.

Asterisk 1.8 has native support for SRTP. So you can easily try this with a Asterisk 1.8.x server. The normal SIP phones are not support for this system. You need to find SRTP supported and TLS supported IP/soft phones for setup this. Use blink soft-phone for this ( I followed following article to setup secure VoIP system.

For SRTP :
For TLS :, ( You can find ast_tls_cert script from here :

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Brief introduction about Asterisk

Asterisk is a software that use to handle voice communications. It is created in 1999 by Mark Spencer of Digium.
Normally its installed as a server and attached telephones to make calls to one another, and to connect to other telephone services including the public switched telephone network (PSTN) and Voice over Internet Protocol (VoIP) services.
IP PBX systems, VoIP gateways and conference servers are same of application that we can develop using Asterisk. It is used by small businesses, large businesses, call centers, carriers and governments worldwide. I’m not going to describe about these application but if you are not familiar with this application, then you can simply search them on Wikipedia.
It is a free opensource software and originally design for Linux. But now Asterisk also runs on a variety of different operating systems including NetBSD, OpenBSD, FreeBSD, Mac OS X, and Solaris. And it is available for Microsoft Windows as AsteriskWin32. But still it is most stable on Linux and recommended OSs are Debian, Fedora, CentOS. And also other Linux OSs like Gentoo, SuSE, Mandrake and Slackware.
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