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Apple has seemingly found a way to block Android’s new iMessage app
(www.theverge.com)
This is a most excellent place for technology news and articles.
It is usually easy to detect a specific client. Like even if you ignore the keys there are dozens of little details like the TLS fingerprint of whatever library they use not matching iOS. Things that are easy to miss and sometimes hard to bypass. Then there are heuristics on how it is used is likely unique.