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Apple Announces 'Groundbreaking' New Security Protocol for iMessage
(www.macrumors.com)
This is a most excellent place for technology news and articles.
I won’t be surprised if that doesn’t show up until iOS 18; when they announced it in November 2023 the only timeline they gave was “later next year.” This encryption has presumably been in development for a while, whereas I think they announced RCS support only as they started, to try to get ahead of regulatory issues in the EU.