International Association
for Cryptologic Research

The Network Time Protocol (NTP) is used by many network-connected devices to synchronize device time with remote servers. Many security features depend on the device knowing the current time, for example in deciding whether a certificate is still valid. Currently, most services implement NTP without authentication, and the authentication mechanisms available in the standard have not been formally analyzed, require a pre-shared key, or are known to have cryptographic weaknesses. In this paper we design an authenticated version of NTP, called ANTP, a generic construction which protects against desynchronization attacks. To make ANTP suitable for large-scale deployments, it is designed to minimize server-side public-key operations and requires no server-side state. Additionally, ANTP ensures that authenticity does not degrade accuracy. Authentication adds no latency to server responses, by using the fact that authentication information can arrive in a separate, subsequent message. We define a novel provable security framework involving adversary control of time, and use the framework to analyze ANTP. The framework may also be used to analyze other secure time synchronization protocols.