Telegram is one of the more secure messaging systems of today. Telegram messages are heavily encrypted and can self-destruct so that no future hacker can ever read them. With Telegram, you can SECURELY and SAFELY encrypt personal and business secret communications, send documents of any type, coordinate groups of up to 5000 members, destroy your sent messages with a timer, and much, much more. It is available for Android, Apple iOS (iPad, iPhone, and iPod Touch), Windows, Macintosh, Linux, and there is a web-based version that should run on Chromebooks or any other computer that can open a web browser and connect to the Internet.
Telegram is free forever. No ads. No subscription fees.