This document describes ANTP, an authentication protocol designed to be built over the Network Time Protocol operating in client/server mode. ANTP’s design meets the requirements of NTP and the Security Requirements of Time Protocols in Packet-Switched Networks, a TICTOC Working Draft. In particular, the server does not need to keep per-client state, and the authentication steps does not degrade timestamp accuracy when compared to unauthenticated NTP. This specification is meant to accompany a paper describing ANTP and analyzing its security (“Authenticated Network Time Synchronization” by Dowling, Stebila and Zaverucha).