Industrial control system (ICS) networks used in critical infrastructures such as the power grid present a unique set of security challenges. The distributed networks are difficult to physically secure, legacy equipment can make cryptography and regular patches virtually impossible, and compromises can result in catastrophic physical damage. To address these concerns, this research proposes two device type fingerprinting methods designed to augment existing intrusion detection methods in the ICS environment. The first method measures data response processing times and takes advantage of the static and lowlatency nature of dedicated ICS networks to develop accurate fingerprints, while the second method uses the physical operation times to develop a unique signature for each device type. Additionally, the physical fingerprinting method is extended to develop a completely new class of fingerprint generation that requires neither prior access to the network nor an example target device. Fingerprint classification accuracy is evaluated using a combination of a real world five month dataset from a live power substation and controlled lab experiments. Finally, simple forgery attempts are launched against the methods to investigate their strength under attack. Permission to freely reproduce all or part of this paper for noncommercial purposes is granted provided that copies bear this notice and the full citation on the first page. Reproduction for commercial purposes is strictly prohibited without the prior written consent of the Internet Society, the first-named author (for reproduction of an entire paper only), and the author's employer if the paper was prepared within the scope of employment.

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