Not all end-to-end encrypted messaging is completely private.
One good way to ensure that you can trust your end-to-end encryption is to use an open protocol that allows you to use whatever client you want, which means that the system isn't controlled by just one entity.

Why do we care about encryption? Why was it a big deal, at least in theory, when Mark Zuckerberg announced earlier this year that Facebook would move to end-to-end encryption on all three of its messaging platforms? We don’t just support encryption for its own sake. We fight for it because...
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