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.

翻译：高精度时间同步是现代众多应用与技术（包括智能电网、时间敏感网络（TSN）及5G网络）的关键前提。尽管精密时间协议（PTP）能够在可信环境中满足这一要求，但在特定网络攻击下其可靠性会降低。其中，时间延迟攻击对该协议构成最大威胁，可使攻击者在未被察觉的情况下偏移受控时钟。随着网络攻击（尤其是针对关键基础设施的攻击）日益严峻，亟需有效的防御措施来保护时间同步及其依赖的应用。然而，现有方案在应对复杂延迟攻击时能力不足，例如缺乏与PTP协议的紧密集成、可扩展性不足，或在微秒级精度要求下缺乏有效的评估。本文提出一种基于冗余路径循环路径不对称性测量的方法，以检测并对抗针对PTP的延迟攻击。我们首先提供在任意网络中寻找冗余路径的方法，并展示如何利用这种冗余性揭示并消除同步路径上导致恶意时钟偏差的不良不对称性。此外，我们提出PTPsec——一种基于最新IEEE 1588-2019标准的安全PTP协议及其实现。通过PTPsec，我们改进传统PTP以支持可靠的延迟攻击检测与缓解。我们在硬件测试平台上验证了该方法，该平台包含能够以微秒精度执行静态和增量延迟攻击的攻击者。实验结果表明，所有攻击场景均可被可靠检测并缓解，且检测时间极短。