Signal is a common open-source messenger that the developers present as private.

Note for those who respect their privacy:
* The signal application though open source, but earlier was noticed for backdoors in this code, after which there were some changes in the open source signal code. And we should not forget that we don't know for sure the exact code of signal server.
* Signal asks for a phone number. This is questionable, cause why would a private messenger need a phone number? Yes, we can