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Missing Linux native, but that might be too much to ask. Thanks for the link. I'll give it a try.

Do you know if it's open source or at least gone through an audit?

The crypto modules are indeed open source. https://github.com/wireapp

I don't know whether they have been formally audited.

That would be the source of some crypto code, not necessarily the code the app itself uses. Since the app is closed, there's no way to verify.