Threema 2.0 for desktop (beta), which was audited by nostr:nprofile1qy2hwumn8ghj7un9d3shjtnddaehgu3wwp6kyqpq387j52vwz47dtdlleqdh46zjnt266vnpz8ylzndqvapap3cw3qhsmc7wke earlier this year, recently passed a bug bounty challenge and is now open source: https://threema.ch/bp/threema-desktop-open-source