NTS: Authenticated Time at Meta

This post details Meta's implementation and open-sourcing of Network Time Security (NTS) for its public time service. It explains the security vulnerabilities of unauthenticated NTP, particularly in the context of modern certificate validation and token expiry requirements. The post describes the two-phase NTS process (NTS-KE and Authenticated NTP), Meta's stateless cookie derivation mechanism using HKDF-SHA256 and epoch day, and the use of Chrony for client configuration. It highlights how NTS prevents man-in-the-middle attacks that could manipulate system clocks to validate expired certificates or extend credential lifetimes, and discusses limitations such as delay attacks and bootstrapping. The open-sourcing of the protocol, server, and client library is a key contribution.