Bricklayer Attack: A Side-Channel Analysis on the ChaCha Quarter Round

ChaCha is a family of stream ciphers that are very efficient on constrainted platforms. In this paper, we present electromagnetic sidechannel analyses for two different software implementations of ChaCha20 on a 32-bit architecture: one compiled and another one directly written in assembly. On the device under test, practical experiments show that they have different levels of resistance to side-channel attacks. For the most leakage-resilient implementation, an analysis of the whole quarter round is required. To overcome this complication, we introduce an optimized attack based on a divide-and-conquer strategy named bricklayer attack.