However I have been unable to find an NTP that uses autokey or do I need to the adopt the other option Symmetric Key and if so are there any free NTP servers that offer this?
While I understand why it would be recommended to have authenticated NTP, there's not a lot of those out there actually (AFAIK). only if you run your own NTP infrastructure (like Meinberg) they'll be hard to find (your ISP may have them)
NIST does offer this - https://www.nist.gov/pml/time-and-frequency-division/time-services/nist-authenticated-ntp-service
While I understand why it would be recommended to have authenticated NTP, there's not a lot of those out there actually (AFAIK). only if you run your own NTP infrastructure (like Meinberg) they'll be hard to find (your ISP may have them)