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Closed source, so E2E has to be taken on faith.

Strictly better than Skype in some ways, but privacy can't be taken to be one of them.

According to the table here https://wire.com/privacy/ it's open source

Further to the point, here are their GitHub repos: https://github.com/wireapp

A-ha! Normally I'm used to these apps proclaiming their Free-ness on the front webpage, but it didn't jump out at here. Much thanks for the linK!