Signal

An instant messaging, voice calling and video calling[66] application for iOS and Android. It uses end-to-end encryption protocols to secure all communications to other Signal users.

Signal can be used to send end-to-end encrypted group messages, attachments and media messages to other Signal users. All calls are made over a Wi-Fi or data connection and are free of charge, including long distance and international.

Signal has a built-in mechanism for verifying that no man-in-the-middle attack has occurred. Signal Messenger has set up dozens of servers to handle the encrypted calls in more than 10 countries around the world to minimize latency.

The clients are published under the GPLv3 license.