text: This paper establishes a fundamental theory of secure clock synchronization. Accurate clock synchronization is the backbone of systems managing power distribution, financial transactions, telecommunication operations, database services, etc. Some clock synchronization (time transfer) systems, such as the global navigation satellite systems, are based on one-way communication from a master to a slave clock. Others, such as the network transport protocol, and the IEEE 1588 precision time protocol (PTP), involve two-way communication between the master and slave. This paper shows that all one-way time transfer protocols are vulnerable to replay attacks that can potentially compromise timing information. A set of conditions for secure two-way clock synchronization is proposed and proved to be necessary and sufficient. It is shown that IEEE 1588 PTP, although a two-way synchronization protocol, is not compliant with these conditions, and is therefore insecure. Requirements for secure IEEE 1588 PTP are proposed, and a second example protocol is offered to illustrate the range of compliant systems.
  @cite_38: 
  @cite_40: Phasor Measurement Units (PMUs) provide time synchronized electrical measurements at many nodes across the power grid. This wide area time synchronization is made possible via GPS time transfer. Unfortunately, the GPS civilian signal is weak and vulnerable to signal interferences such as jamming and meaconing. As such, the GPS-based time synchronization of PMUs may be a potential point of entry for attacks on the power system. To address this concern, we present Direct Time Estimation (DTE) for the robust GPS-based time transfer to PMUs. As a direct method, Direct Time Estimation (DTE) does not rely on intermediate measurements such as independent channel range, range rates and their residuals. Instead, it generates the time and time drift solutions by operating directly on the raw signal. In this manner, the entire received signal information is utilized, leading to increased robustness in tracking. We developed and implemented DTE on our research platform — PyGNSS. To evaluate the performance of DTE, we conducted realistic simulations, such as jamming and meaconing, based on data collected from field experiments. Through the simulated attacks, we demonstrate the enhanced robustness of DTE with respect to conventional algorithms, such as scalar tracking.
  @cite_20: Global navigation satellite signals can be spoofed by false signals, but special receivers can provide defenses against such attacks. The development of good spoofing defenses requires an understanding of the possible attack modes of a spoofer and the properties of those modes that can be exploited for defense purposes. Sets of attack methods and defense methods are described in detail. An attack defense matrix is developed that documents which defense techniques are effective against the various attack techniques. Recommendations are generated to improve the offerings of commercial off-the-shelf receivers from the current situation, a complete lack of spoofing defenses, to a situation in which various levels of defense are present, some that add significant security for relatively little additional cost and others that add more security at costs that start to become appreciable.
summary: The GNSS jamming and spoofing threat has been recognized in the literature for more than a decade. A survey of the current state-of-the-art in spoofing and anti-spoofing techniques is presented in @cite_20 . Recent works on GNSS anti-spoofing techniques have specifically focused on the case of timing security. Collaborative multi-receiver @cite_38 and direct time estimation @cite_40 techniques have been proposed for robust GNSS clock synchronization.