[nominatronic]

There's no problem in principle with using public key cryptography to sign messages sent by ham radio. So authentication is possible although it's not common in practice. It's encrypting the content of messages that's not permitted.

[kawfey]

Legally speaking, it's "messages encoded for the purpose of obscuring their meaning" [0] that's illegal, which is fraught with conflicting interpretations and loopholes.

[0] https://www.ecfr.gov/cgi-bin/text-idx?SID=f9e42063714bbba9b7...