Well, iMessage is end-to-end encrypted and works smoothly on both mobile and desktop. Caveats apply, of course: supports OS X and iOS only, is not open source a

Well, iMessage is end-to-end encrypted and works smoothly on both mobile and desktop. Caveats apply, of course: supports OS X and iOS only, is not open source and its protocol is not publicly available for audit. OP’s post indicates they need support for Android mobile and Linux desktop, so it won’t fit their bill, but it’s a viable option for some.