Towards side-channel secure block ciphers

Since the late 90's side-channel attacks have been a threat for cryptographic implementations. They use observations of physical features of a device while it computes cryptographic algorithms. These leakages can give information about the key. A common countermeasure against such attacks is masking. The main idea of masking is to randomize the internal state. In consequence an adversary must combine of several leakages to mount the attack. Masking schemes have an impact on the efficiency of the implementations. In this thesis, we explore different solutions to improve the efficiency of masking implementations. Firstly, we explore solutions for masking implementations for the AES. We investigate the use of amortization technique to reduce the cost of polynomial masking. We also propose a new way to combine Boolean secure multiplications to reduce the cost of masking. Next, we focus on the assumptions used for proofs of masking. We show that the use of low entropy masking schemes can be risky. We also show how to convert proofs in the probing model into proofs in the transition based model. Then, we propose several easier-to-build strategies for masking block ciphers. We present three block ciphers. Finally, we focus on leakage resilience and compare the security that can be obtained by using either leakage-resilient construction, masking or their combination.