ZRTP

Zimmermann Real-time Transport Protocol

A cryptographic key-agreement protocol designed to enable secure voice and video communication over the Real-time Transport Protocol (RTP). Developed by Phil Zimmermann, the creator of Pretty Good Privacy (PGP), ZRTP is notable for performing encryption key negotiation directly within the media stream, rather than relying on a signaling protocol for key exchange.

This in-band key exchange approach means ZRTP does not depend on a particular VoIP or messaging platform and can provide end-to-end encryption even when the signaling path is untrusted. It supports Perfect Forward Secrecy (PFS) by generating ephemeral session keys and includes a Short Authentication String (SAS) feature that allows users to verbally compare a short code to verify the session’s authenticity.

ZRTP is commonly used in secure VoIP systems, encrypted video conferencing tools, and other real-time communication applications where independent, decentralized encryption is preferred.