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Google Messages just flicked the switch on end-to-end encryption for all RCS and group chats
(chromeunboxed.com)
This is a most excellent place for technology news and articles.
With a closed source app? Of course you can. How is anyone supposed to know what keys you use for encryption? Doesn't even need to be a remote one - just the key generation be reproducible by the developer.
I don't know if you're understanding that that's his point.
If Google can reproduce the key it's not fully "end to end" unless one of the "end"s is Google.