ZRTP (composed of Z and Real-time Transport Protocol) is a cryptographic key-agreement protocol to negotiate the keys for encryption between two end points in a Voice over Internet Protocol (VoIP) phone telephony call based on the Real-time Transport Protocol.

It’s the standard technology for keeping an internet connection secure and safeguarding any sensitive data. It does this by making sure that any data transferred between users and sites, or between two systems remain impossible to read.
Transport Layer Security (TLS) and its predecessor, Secure Sockets Layer (SSL), are cryptographic protocols which are designed to provide communication security over the Internet. They use X.509 certificates and hence asymmetric cryptography to assure the counterparty whom they are talking with, and to exchange a symmetric key. This session key is then used to encrypt data flowing between the parties. This allows for data/message confidentiality, and message authentication codes for message integrity and as a by-product message authentication.