Using Templates to Attack Masked Montgomery Ladder Implementations of Modular Exponentiation

Since side-channel attacks turned out to be a major threat against implementations of cryptographic algorithms, many countermeasures have been proposed. Amongst them, multiplicative blinding is believed to provide a reasonable amount of security for public-key algorithms. In this article we show how template attacks can be used to extract sufficient information to recover the mask. Our practical experiments verify that one power trace suffices in order to remove such a blinding factor. In the course of our work we attacked a protected Montgomery Powering Ladder implementation on a widely used microcontroller. As a result we can state that the described attack could be a serious threat for public key algorithms implemented on devices with small word size.