telegram with secret chats
If you are a Pidgin user, there is a telegram-purple plugin: https://github.com/majn/telegram-purple
It is provided for Ubuntu by the webupd8 repository:
sudo add-apt-repository ppa:nilarimogard/webupd8
sudo apt-get update
sudo apt-get install telegram-purple
telegram-cli, although not the easiest to use, supports secret chats:
sudo snap install telegram-cli
but the pidgin plugin is easier from https://www.ubuntuupdates.org/ppa/webupd8?dist=bionic