I agree, although I'm not sure whether it's ever been implemented for public use. You get anti-spoofing naturally if you're just using secret symmetric encrypt

I agree, although I'm not sure whether it's ever been implemented for public use. You get anti-spoofing naturally if you're just using secret symmetric encryption (if your adversary doesn't know the key, they can't generate a valid signal to trick you with). To prevent spoofing against the public, you'd need to make use of public-key cryptography.