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Nothing Phone builds a blue bubble iMessage bridge while Google and Apple fight over RCS
(www.androidcentral.com)
This is a most excellent place for technology news and articles.
They'd need to control the app on both phones in order to control what it's encrypting/decrypting. Their system only works because they've got a device in the middle separately decrypting/re-encrypting each message. Google's Messages app can't read iMessages; Apple's Messages app can't read Google's proprietary encrypted RCS messages.
Of course if you want universally cross-platform messaging, complete with full-resolution photos and available with end-to-end encryption, there's this crazy new technology called "email." I feel like there's a missed opportunity for making setting up S/MIME easier.