"These results must be false": A usability evaluation of constant-time analysis tools

Cryptography secures our online interactions, transactions, and trust. To achieve this goal, not only do the cryptographic primitives and protocols need to be secure in theory, they also need to be securely implemented by cryptographic library developers in practice. However, implementing cryptographic algorithms securely is challenging, even for skilled professionals, which can lead to vulnerable implementations, especially to side-channel attacks. For timing attacks, a severe class of side-channel attacks, there exist a multitude of tools that are supposed to help cryptographic library developers assess whether their code is vulnerable to timing attacks. Previous work has established that despite an interest in writing constant-time code, cryptographic library developers do not routinely use these tools due to their general lack of usability. However, the precise factors affecting the usability of these tools remain unexplored. While many of the tools are developed in an academic context, we believe that it is worth exploring the factors that contribute to or hinder their effective use by cryptographic library developers [61] . To assess what contributes to and detracts from usability of tools that verify constant-timeness (CT), we conducted a two-part usability study with 24 (post) graduate student participants on 6 tools across diverse tasks that approximate real-world use cases for cryptographic library developers. We find that all studied tools are affected by similar usability issues to varying degrees, with no tool excelling in usability, and usability issues preventing their effective use. Based on our results, we recommend that effective tools for verifying CT need usable documentation, simple installation, easy to adapt examples, clear output corresponding to CT violations, and minimal noninvasive code markup. We contribute first steps to achieving these with limited academic resources, with our documentation, examples, and installation scripts 1 . 1. Timing attacks. Since Kocher's introduction of sidechannel vulnerabilities in 1996 [68], these threats have persisted despite significant efforts to address them. Considering the vast range of side-channel attacks, we will highlight a few pivotal moments with a focus on timing attacks. Kocher's seminal work highlighted vulnerabilities in asymmetric cryptographic algorithms like RSA and DSS through "Timing Attacks", emphasizing the potential for exploitation based on secret-dependent operation times. In 2002, Tsunoo et al. [104, 105] expanded timing attacks to symmetric cryptography, noting vulnerabilities in MISTY1, DES, and suggesting AES being vulnerable to cache-timing attacks. Independent work by Bernstein [13] and Osvik et al. [87] confirmed these AES vulnerabilities. In 2003, Brumley and Boneh [28] revealed that these attacks could be conducted remotely via network timings. Subsequent vulnerabilities were discovered in the SSL/TLS libraries [3, 27, 29, 44] and on hardwareassisted defenses, such as Yarom et al.'s "CacheBleed" [125]. Kaufman et al. [66] also warned of persistent vulnerabilities post-compilation. Despite these vulnerabilities and an emphasis on fixing them, side channels remain common in numerous platforms [19-21, 46-48, 79, 108, 109]. Some Common Criteria certified devices, despite their countermeasures, were found vulnerable [62] . Moreover, even recent post-quantum cryptographic efforts are affected [26, 54, 88, 89, 103, 113] . 2. Constant-time Analysis. In this paper, we focus on investigating usability aspects of tools that evaluate timing leakages 2