> It's not illegal and is a core part of modern radio protocols.

LOL, yes it's for most practical purposes on hobby ham bands.

If you wanna be pedantic about it, the FCC rules for ham operations doesn't mention encryption, it mentions encoding instead [0]. So yes, if you encrypt or encode without obscuring the meaning (by publishing your encoding spec or encryption keys) then yes you can "encrypt". digital mode radio works that way.

[0] https://www.ecfr.gov/current/title-47/chapter-I/subchapter-D...