/e/
@BurungHantu @one @win10ltsc
It does use the open NaCl library and has had a recent audit. https://threema.ch/en/blog/posts/audit19e
If by "not free" you mean you most pay for the app, it's a one time cost of a coffee. I'm willing to support their efforts through paying for the app.
@supernova @one @win10ltsc Threema is closed source and not free at all.
Some reddit discussions: https://www.reddit.com/r/privacytoolsIO/search?q=threema&restrict_sr=on&sort=relevance&t=all