Cooling chips disables clock sensors without erasing secrets
Chypothermia: Clock Freezing for Static Side-channel Attacks
Cryptography and Security
Summary
Some attacks on computer chips freeze their clocks to steal secret information stored inside. The authors show that chilling a chip to very low temperatures disrupts the sensors that monitor these clocks and voltages, stopping the clock without alerting the system or wiping secrets. They combine this cooling trick with another method to avoid detection by temperature sensors. Their tests on real chips and a security-focused platform show this attack works on various sensor types. To defend against it, they design a heating-based sensor that resists the cooling attack and keeps secrets safe.
What this means in practice
- •For chip security engineers: Detect and mitigate attacks that disable clock and voltage sensors by cooling chips to steal secrets undetected.
- •For hardware design teams: Integrate self-heating sensors to protect hardware from cooling-based clock freeze attacks.
Authors
Fatemeh Khojasteh Dana, Mehmet Ali Cetin, Xinrui Wang, Andrew Butler, Yuval Yarom, Shahin Tajik
Abstract
Static side-channel attacks, which exploit halted-clock conditions to extract sensitive information, pose an increasing threat to chip security. To counter these attacks, various defenses have been proposed that monitor for abnormal clock behavior and trigger the clearing of sensitive data when clock anomalies are detected. In this work, we demonstrate that exposing a chip to cryogenic temperatures interferes with on-chip mixed-signal components, responsible for signal sensing and generation. Based on this observation, we develop Chypothermia, an attack that, without any electrical tampering with the system, disables the target clock sensor, the clock generation circuit, and the voltage sensors, while preserving the secret data. While effective at halting the clock, cooling is a slow process and, on its own, is often insufficient against systems equipped with temperature sensors designed to detect thermal anomalies. To bypass these protections, we combine Chypothermia with Chypnosis (Mitard et al., IEEE SP 2026) and show that, even within a moderately low-temperature operating range, this combination can halt the clock while evading detection. We implement Chypothermia on multiple FPGA/SoC platforms and demonstrate successful disabling of both soft-IP and hard-IP sensor implementations. Finally, we apply Chypothermia to the alert handler of the OpenTitan root of trust, which incorporates a state-of-the-art clock sensor, and show that the attack evades detection and prevents key zeroization. Finally, we introduce and implement an FPGA-compatible self-heating sensor as a countermeasure and demonstrate its robustness against Chypothermia.