Phil Zimmermann, the creator of PGP, has released a free VOIP encryption software for Windows, Mac OSX and Linux called Zfone that allows filtering, encrypting and decrypting of all VoIP media packets from any VOIP client such as Skype or Gizmo as they pass through your computer. Both the caller and the recipient need to have the software installed in order to encrypt and decrypt the calls. The keys are negotiated between the two parties using an algorithm known as the Diffie-Hellman algorithm. Due to the nature of Public-Key Infrastructure (PKI), even though the VOIP packets can be intercepted using a network trace, it would be next to impossible to decrypt the contents of the packets in a human readable form.

ZRTP is a protocol that defines how VoIP phones talk to each other in an encrypted way while Zfone is a program that for end users that utilizes the ZTRP protocol. They both use strong cryptographic algorithms to negotiate cryptographic keys between two parties without the participation of any phone company or any middleman.

The keys are strings of bits without which you cannot decrypt the conversation. These keys are automatically created at the start of the call, and destroyed at the end. Only the two parties know the keys, and the phone company isn’t in a position to give out the keys to a third party.

After you have installed Zfone, you should open it before launching your VoIP client software, and leave it running all the time, in the background so that it can encrypt/decrypt VOIP traffic in real time. Zfone will secure any VoIP call you make to another Zfone client while it’s running, and will tell you the that status of the security of the call in the graphical user interface.