Yes something like that has been done, the solution uses asymmetric cryptography instead of symmetric and it protects the entire communication stream instead of just opening the gates - it's called Wireguard.
I jest, but you're describing authentication protocol which uses the UDP destination port field to carry each character of the "one time password" in a roundabout way. You could simply send 1 packet containing the password to the daemon.
LOL'd at Wireguard, but I really miss Plan 9-style networking, where you can basically just scope VPNs to a subprocess rather than having to use some system wide network filter with split tunneling or whatever. Wireguard has huge usability issues due to that.
Reading the fwknop README is such a breath of fresh air, thank you for that treat. I hate AI-generated documentation so much.
I jest, but you're describing authentication protocol which uses the UDP destination port field to carry each character of the "one time password" in a roundabout way. You could simply send 1 packet containing the password to the daemon.
I've never seen it used in the wild but apparently, it exists: https://github.com/mrash/fwknop