Wikipedia
TextSecure
TextSecure was a free and open-source encrypted messaging application for Android that was first released in May 2010. It was developed by Open Whisper Systems and used end-to-end encryption to secure the transmission of instant messages, group messages, attachments and media messages to other TextSecure users. In November 2015, TextSecure was merged with an encrypted voice calling application called RedPhone and was renamed as Signal.
TextSecure was the first application to use the Double Ratchet Algorithm, which has since been implemented into WhatsApp and other applications.