Over hiaten in de beveiliging tegen foutaanvallen en gecombineerde aanvallen en het efficiënte ontwerp van tegenmaatregelen

Cryptographic algorithms are designed to be secure against cryptanalytic attacks targeting their mathematical structure, yet their implementations in real-world devices, such as smart cards, remain susceptible to physical attacks. Such attacks may exploit unintentional leakages arising from the physical characteristics of devices, such as fluctuations in power consumption or electromagnetic emissions, in what are known as side-channel attacks. Alternatively, they may also deliberately disrupt computations to produce exploitable erroneous outputs, referred to as fault attacks. Combined attacks, which pair the capabilities of side-channel attacks and fault attacks, represent an even more powerful threat. To analyze and understand these attacks, researchers rely on theoretical adversary models that systematically abstract the capabilities of adversaries and guide the design of countermeasures. Despite increasing research efforts, existing approaches to model physical attack adversaries in theory, particularly for fault and combined attacks, often fail to reflect the full intrinsic nature of practical attack scenarios. Consequently, many countermeasures that are provably secure under these models either fail under realistic attack conditions, due to overly weak assumptions, or incur excessive implementation costs, due to overly strong ones. Similarly, the lack of standardized evaluation methodologies results in unnoticed vulnerabilities in countermeasures. Addressing these fundamental challenges in the field forms the first research direction of this thesis. We introduce a parameterized adversary model that more accurately reflects practical adversarial capabilities. Using this model, we reveal discrepancies between theoretical abstractions and real-world adversaries. We further demonstrate how insufficient evaluations can lead to overlooked weaknesses by breaking two state-of-the-art countermeasures, RS-Mask and CAPA. These findings underscore the immediate need for more realistic adversary abstractions and systematic evaluation frameworks. The second research direction focuses on the design of efficient protection mechanisms against fault and combined attacks. Traditional approaches often rely on intermediate error detection or correction mechanisms, which increase hardware cost and complexity. For combined attacks, achieving security is even more challenging: existing schemes typically assume that fine-grained fault control is unavoidable, making intermediate error correction mechanisms the common state-of-the-art solution. This thesis challenges this assumption. We first introduce two novel fault countermeasure frameworks that avoid intermediate checks or corrections by enforcing fault propagation to the output. The first, based on the notion of \emph{stability}, ensures that any injected fault reaches the output, enabling detection through a single final error check. The second, MAC-stability, extends this idea by integrating message authentication code (MAC) tags as redundancy, improving security and efficiency trade-off over prior MAC-based approaches. Following the same principles, we further introduce combined stability, a novel security notion for combined attacks. Remarkably, combined stability achieves security against combined attacks using only a single error detection mechanism at the end of the computation, challenging the long-standing belief that costly intermediate checks and corrections are necessary. Taken together, these contributions advance the state of the art in three ways. First, they highlight fundamental limitations in existing adversary models and evaluation frameworks. Second, they introduce novel countermeasure frameworks for efficiently protecting against fault attacks, relying on a single error detection at the end of the computation. Third, they establish a new security notion for combined attacks that, for the first time, extends this single error detection approach to the more challenging combined attacks context. In conclusion, this thesis bridges the gap between theoretical abstractions and practical adversarial capabilities. It not only deepens our understanding of fault and combined attacks, but also contributes to the development of protection mechanisms, introducing a new direction focusing on efficiency through a single error check, particularly for combined attacks.