Although power LEDs have been integrated in various devices that perform cryptographic operations for decades, the cryptanalysis risk they pose has not yet been investigated. In this paper, we present optical cryptanalysis, a new form of cryptanalytic side-channel attack, in which secret keys are extracted by using a photodiode to measure the light emitted by a device's power LED and analyzing subtle fluctuations in the light intensity during cryptographic operations. We analyze the optical leakage of power LEDs of various consumer devices and the factors that affect the optical SNR. We then demonstrate end-to-end optical cryptanalytic attacks against a range of consumer devices (smartphone, smartcard, and Raspberry Pi, along with their USB peripherals) and recover secret keys (RSA, ECDSA, SIKE) from prior and recent versions of popular cryptographic libraries (GnuPG, Libgcrypt, PQCrypto-SIDH) from a maximum distance of 25 meters.

Timing attacks are among the most devastating side-channel attacks, allowing remote attackers to retrieve secret material, including cryptographic keys, with relative ease. In principle, “these attacks are not that hard to mitigate the basic intuition, captured by the constant-time criterion, is that control-flow and memory accesses should be independent from secrets. Furthermore, there is a broad range of tools for automatically checking adherence to this intuition. Yet, these attacks still plague popular cryptographic libraries twenty-five years after their discovery, reflecting a dangerous gap between academic research and cryptographic engineering. This gap can potentially undermine the emerging shift towards high-assurance, formally verified cryptographic libraries. However, the causes for this gap remain uninvestigated. To understand the causes of this gap, we conducted a survey with 44 developers of 27 prominent open-source cryptographic libraries. The goal of the survey was to analyze if and how the developers ensure that their code executes in constant time. Our main findings are that developers are aware of timing attacks and of their potentially dramatic consequences and yet often prioritize other issues over the perceived huge investment of time and resources currently needed to make their code resistant to timing attacks. Based on the survey, we identify several shortcomings in existing analysis tools for constant-time, and issue recommendations that can make writing constant-time libraries less difficult. Our recommendations can inform future development of analysis tools, security-aware compilers, and cryptographic libraries, not only for constant-timeness, but in the broader context of side-channel attacks, in particular for micro-architectural side-channel attacks, which are a younger topic and too recent as focus for this survey.