Security Guidance 22-002 – Hardening Signal Thursday, March 03, 2022 02:15:00 PM CET Signal is a well-known, secure, encrypted instant messaging service developed by the non-profit Signal Technology Foundation and Signal Messenger LLC. It uses standard cellular telephone numbers as identifiers and all communications between Signal users are secured with end-to end encryption.Staff of public and private organisations, including senior management, may be using Signal sometimes to quickly coordinate and exchange information on work-related matters. Signal groups may also have been set up for business continuity reasons in case corporate instant messaging tools become unavailable.The following document provides clear and pragmatic recommendations for hardening the configuration of Signal apps. If you have suggestions that could help improve it, contact us at services@cert.europa.eu. We always appreciate constructive feedback.