Advances in the Side-Channel Analysis of Symmetric Cryptography

Side-Channel Analysis (SCA) is an implementation attack where an adversary exploits unintentional outputs of a cryptographic module to reveal secret information. Unintentional outputs, also called side-channel outputs, include power consumption, electromagnetic radiation, execution time, photonic emissions, acoustic waves and many more. The real threat of SCA lies in the ability to mount attacks over small parts of the key and to aggregate information over many different traces. The cryptographic community acknowledges that SCA can break any security module if the adequate protection is not implemented. In this dissertation, we propose several advances in side-channel attacks and countermeasures. We focus on symmetric cryptographic primitives, namely: block-ciphers and hashing functions.

In the first part, we focus on improving side-channel attacks. First, we propose a new method to profile highly parallel cryptographic modules. Profiling, in the context of SCA, characterizes the power consumption of a fully-controlled module to extract power signatures. Then, the power signatures are used to attack a similar module. Parallel designs show excessive algorithmic-noise in the power trace. Hence, we propose a novel attack that takes design parallelism into consideration, which results in a more powerful attack. Also, we propose the first comprehensive SCA of the new secure hashing function mbox{SHA-3}. Although the main application of mbox{SHA-3} is hashing, there are other keyed applications including Message Authentication Codes (MACs), where protection against SCA is required. We study the SCA properties of all the operations involved in mbox{SHA-3}. We also study the effect of changing the key-length on the difficulty of mounting attacks. Indeed, changing the key-length changes the attack methodology. Hence, we propose complete attacks against five different case studies, and propose a systematic algorithm to choose an attack methodology based on the key-length.

In the second part, we propose different techniques for protection against SCA. Indeed, the threat of SCA can be mitigated if the secret key changes before every execution. Although many contributions, in the domain of leakage resilient cryptography, tried to achieve this goal, the proposed solutions were inefficient and required very high implementation cost. Hence, we highlight a generic framework for efficient leakage resiliency through lightweight key-updating. Then, we propose two complete solutions for protecting AES modes of operation. One uses a dedicated circuit for key-updating, while the other uses the underlying AES block cipher itself. The first one requires small area (for the additional circuit) but achieves negligible performance overhead. The second one has no area overhead but requires small performance overhead. Also, we address the problem of executing all the applications of hashing functions, e.g. the unkeyed application of regular hashing and the keyed application of generating MACs, on the same core. We observe that, running unkeyed application on an SCA-protected core will involve a huge loss of performance (3x to 4x). Hence, we propose a novel SCA-protected core for hashing. Our core has no overhead in unkeyed applications, and negligible overhead in keyed ones.

Our research provides a better understanding of side-channel analysis and supports the cryptographic community with lightweight and efficient countermeasures.