

So this may still be possible
This article seems to be saying that’s it’s not only possible, it’s being actively (and I would assume widely) exploited on current versions of Android. Google is supposed to catch any abuses of listed exceptions, but they are either missing a bunch or letting them intentionally slide through. Either way, apps being able to see other apps is a big security risk that IMO only the user should be able to explicitly allow, and on a case-by-case basis.
Where is kryptkey? I don’t see it on F-droid.
I do see KryptEy, which sounds like what you’re talking about. What’s important to mention is that it’s a proof-of-concept app that hasn’t been updated in two years. It may still function fine and be secure, but I wouldn’t necessarily assume that, especially when we are talking about high stakes like avoiding government intrusion into chats. Has anyone ever checked or audited it?