SRTP

Secure Real-time Transport Protocol (SRTP) is an extension of RTP that adds confidentiality, message authentication, and replay protection to voice and video streams. It encrypts the media payload so that even if packets are intercepted, the content cannot be understood.

SRTP protects the media plane while TLS protects the signaling plane (SIP messages). Together, they provide end-to-end encryption for VoIP calls. For maximum security, ZRTP can negotiate keys without relying on the signaling path.