Signal

Signal is a free and open-source encrypted voice calling and instant messaging application for iOS and Android. It uses advanced end-to-end encryption protocols to secure all communications to other Signal users. Signal can be used to send and receive encrypted instant messages, group messages, attachments and media messages. Users can independently verify the identity of their messaging correspondents by comparing key fingerprints out-of-band. During calls, users can check the integrity of the data channel by checking if two words match on both ends of the call.^[1]