Template Attacks with a Power Model

This article analyses some properties of the template attack. Examples come from attacks against an unprotected ASIC implementation of DES. The principal components analysis (PCA) is used to represent the templates in two dimensions. We give a physical interpretation of the templates PCA eigenvalues and eigenvectors. We show that the S-boxes are not the target of template attacks. We point out that the efficiency of template attacks on unprotected implementations can be unleashed by using a power model. The most suitable power-model happens to be linked to the key schedule. This casts a new light on key schedule requirements for SCA resistance against a “template” attacker. The results are tailored for DES, because this symmetric block cipher is emblematic and is still promised a long life. Its key schedule is also remarkably simple, with cryptanalytic weaknesses, that paradoxically turn out to be a strength against SCA.