Authenticating time and frequency signals

I describe a protocol that can be used to authenticate digital time signals transmitted between time servers and client systems. This authentication is useful to protect the data from being modified en route and to guarantee that the messages really originated from a bona fide server operated by a national timing center. The protocol is based on the standard Network Time Protocol, which is widely used for synchronizing computers on the Internet. The authenticated version of the protocol can be realized using either packet-switched networks (such as the Internet) or standard dial-up telephone circuits. The implementation using dial-up telephone circuits can provide authenticated messages with an accuracy of about 1 ms, but this version requires dedicated modems and telephone lines for its implementation. The Internet version can provide equivalent accuracy in principle, but is vulnerable to a number of attacks that can degrade the accuracy and that are difficult to detect. Either method is suitable for applications that only require a timing accuracy on the order of 1 s. The protocol will be generally available, but will be most useful for systems that are involved in time-sensitive commercial or financial transactions