ZRTP is a voice over IP (VoIP) encryption extension for the Real-Time Transport Protocol (RTP). The "Z" in "ZRTP" is the first letter of the surname of Phil Zimmermann, an engineer best known for inventing Pretty Good Privacy (PGP). ZRTP is a part of a software developer's kit (SDK) for an encryption program Zimmerman created called Zfone. It allows the user to encrypt any Session Initiation Protocol (SIP) VoIP voice stream, as long as the other person on the call is also using Zfone.
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