Somewhat topical: in case you want authenticated access to NIST NTP servers, you need to send a letter to NIST using the US mail or FAX machine (e-mail is not acceptable).

NIST will reply with a key number and a key value. The reply will be by US mail only, e-mail will never be used.

The office that normally receives US mail and FAX messages currently has limited access, which may result in significant delays in processing requests

https://www.nist.gov/pml/time-and-frequency-division/time-se...

(things you discover when you implement fedramp)