by Joe Engelman, Seny Kamara, Tarik Moataz and Sam Zhao. Overview Signal is an end-to-end encrypted messaging app made by Open Whisper Systems. It is based on the Signal protocol which was designed by Trevor Perrin and Moxie Marlinspike. In addition to underlying the Signal App, the Signal protocol is also used by WhatsApp, Facebook Messenger and Google Allo. Why is Signal important? Together with PGP-encrypted email and OTR-based messaging, the Signal App is one of the most secure ways to communicate.