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U.S. officials urge Americans to use encrypted apps amid unprecedented cyberattack
(www.nbcnews.com)
This is a most excellent place for technology news and articles.
You have the key, not the provider. They are explicit about this in the implementation.
They can only read the messages before encryption if they are backdooring all android phones in an act of global sabotage. Pretty high consequences for soke low stakes data.
I mean, Google does, with Play Services.
I'm pretty sure the key is stored on the device, which is backed up to Google. I cannot say for sure if they do or don't backup your keyring, but I feel better not using it.