# ZRTP
> A cryptographic key-exchange protocol for securing VoIP calls end-to-end

ZRTP negotiates encryption keys directly between VoIP endpoints for true end-to-end security.

## What is ZRTP?
ZRTP is a cryptographic key-agreement protocol designed specifically for VoIP. Created by Phil Zimmermann (of PGP fame), ZRTP negotiates encryption keys directly between the two call endpoints using a Diffie-Hellman exchange, then uses those keys to encrypt the [SRTP](/voip-glossary/srtp/) media stream.

## ZRTP vs. SDES
Unlike SDES (which passes keys in [SIP](/voip-glossary/sip/) signaling and relies on [TLS](/voip-glossary/tls/) for protection), ZRTP negotiates keys in-band on the media path. This provides true end-to-end encryption even if the signaling servers are compromised. Users verify security by reading a short authentication string (SAS) to each other.

## Related Terms
- [SRTP](/voip-glossary/srtp/)
- [TLS](/voip-glossary/tls/)