Terrorists have long used existing commercial and public communications infrastructure to send commands and plan operations. Telegram provides a secret chat feature that is end to end encrypted. There are problems with this encryption, but it hasnt been publicly broken. Even if Telegrams encryption is solid, there are serious problems with the safe operational use of the program. The safest way to use Telegram would be to use a clean burner phone to communicate with another phone regularly. There is no way to opportunistically encrypt an existing session, instead users must get select a New Secret Chat and then start chatting.”]
Source: https://medium.com/@thegrugq/operational-telegram-cbbaadb9013a