Scanning the Cycle: Timing-based Authentication on PLCs

TL;DR

This paper introduces a timing-based authentication method for PLCs that passively captures scan cycle timing to detect spoofing and replay attacks, enhancing security in industrial control systems without disrupting real-time operations.

Contribution

It presents a novel passive timing fingerprinting technique using scan cycle analysis and a watermarking approach for replay attack detection in PLCs.

Findings

PLCs can be distinguished by scan cycle timing characteristics.

The method effectively detects spoofing and replay attacks.

Validated on water treatment and smart grid testbeds.

Abstract

Programmable Logic Controllers (PLCs) are a core component of an Industrial Control System (ICS). However, if a PLC is compromised or the commands sent across a network from the PLCs are spoofed, consequences could be catastrophic. In this work, a novel technique to authenticate PLCs is proposed that aims at raising the bar against powerful attackers while being compatible with real-time systems. The proposed technique captures timing information for each controller in a non-invasive manner. It is argued that Scan Cycle is a unique feature of a PLC that can be approximated passively by observing network traffic. An attacker that spoofs commands issued by the PLCs would deviate from such fingerprints. To detect replay attacks a PLC Watermarking technique is proposed. PLC Watermarking models the relationship between the scan cycle and the control logic by modeling the input/output as a function of request/response messages of a PLC. The proposed technique is validated on an operational water treatment plant (SWaT) and smart grid (EPIC) testbed. Results from experiments indicate that PLCs can be distinguished based on their scan cycle timing characteristics.