  High-precision time synchronization is a vital prerequisite for many modern
applications and technologies, including Smart Grids, Time-Sensitive Networking
(TSN), and 5G networks. Although the Precision Time Protocol (PTP) can
accomplish this requirement in trusted environments, it becomes unreliable in
the presence of specific cyber attacks. Mainly, time delay attacks pose the
highest threat to the protocol, enabling attackers to diverge targeted clocks
undetected. With the increasing danger of cyber attacks, especially against
critical infrastructure, there is a great demand for effective countermeasures
to secure both time synchronization and the applications that depend on it.
However, current solutions are not sufficiently capable of mitigating
sophisticated delay attacks. For example, they lack proper integration into the
PTP protocol, scalability, or sound evaluation with the required
microsecond-level accuracy. This work proposes an approach to detect and
counteract delay attacks against PTP based on cyclic path asymmetry
measurements over redundant paths. For that, we provide a method to find
redundant paths in arbitrary networks and show how this redundancy can be
exploited to reveal and mitigate undesirable asymmetries on the synchronization
path that cause the malicious clock divergence. Furthermore, we propose PTPsec,
a secure PTP protocol and its implementation based on the latest IEEE 1588-2019
standard. With PTPsec, we advance the conventional PTP to support reliable
delay attack detection and mitigation. We validate our approach on a hardware
testbed, which includes an attacker capable of performing static and
incremental delay attacks at a microsecond precision. Our experimental results
show that all attack scenarios can be reliably detected and mitigated with
minimal detection time.
