Audio content (also known as the RTP stream) is perhaps the most valuable part of a VoIP conversation. This makes it one of the most important parts for VoIP security. Encryption of the RTP stream ensures that the actual content of phone calls cannot be listened in on, recorded, or otherwise illegally obtained. There are multiple ways to achieve this security.
At its core, the theme for encryption algorithms requires that both sides involved in the encryption agree on a method and an encryption algorithm for encrypting and decrypting the data being transmitted and received. In other words, you can't use an encryption method that isn't supported by both sides. In addition, encryption algorithms are based on key exchanges, generally at the beginning of a call. These key exchanges are similar to exchanging passwords by both parties, but in an electronic and often automated way.
There are two popular forms of encryption generally used when encrypting audio and media streams. These...