Moxie Marlinspike leaves Twitter and founds Open Whisper Systems (OWS) as a collaborative open source project for the continued development of TextSecure and RedPhone. TextSecure and RedPhone are released as free and open-source software under the GPLv3 license. Marlinspike could help the then-startup improve its security.' Whisper Systems is acquired by Twitter, 'primarily so that Mr.

Moxie Marlinspike and Stuart Anderson ( Whisper Systems) launch TextSecure and RedPhone on Android. The Android version also optionally functions as an SMS app, but this functionality will be removed in 2023. As of May 2021, it was downloaded more than 105 million times. As of January 2022, the platform had approximately 40 million monthly active users. The non-profit Signal Foundation was launched in February 2018 with initial funding of $50 million from Brian Acton. The client software includes mechanisms by which users can independently verify the identity of their contacts and the integrity of the data channel. It secures all communications with end-to-end encryption. Signal uses mobile telephone numbers as identifier for users. Registration for desktop use requires an iOS or Android device. Signal is also distributed for iOS and desktop programs for Windows, macOS, and Linux. The official Android app generally uses the proprietary Google Play Services, although it is designed to work without them. Its mobile clients are published under the GPL-3.0-only license, while the desktop client and server are published under the AGPL-3.0-only license. Signal's software is free and open-source. It is developed by the non-profit Signal Foundation and its subsidiary, Signal Messenger LLC.

The application uses a centralized computing architecture, and is cross-platform software. Communication may be one-to-one between users, or for group messaging. The instant messaging function includes sending text, voice notes, images, videos, and other files.

Signal is an encrypted messaging service for instant messaging, voice, and video calls.

Single private message