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Phone Push Notifications: A Double-Edged Sword for User Privacy and Law Enforcement
(www.theregister.com)
This is a most excellent place for technology news and articles.
Signal doesn't encrypt notifications from what I understand. It uses Google/Apples notification system like everything else. But the notification only says "Hey, wake up!". Then the Signal app goes and retrieves the message from Signal's servers. That retrieval will be encrypted, but it's outside the push notification system at the point.