Improved Brute-Force Search Strategies for Single-Trace and Few-Traces Template Attacks on the DES Round Keys
We present an improved search strategy for a template attack on the secret DES key of a widely–used smart card, which is based on a Common–Criteria certified chip. We use the logarithm of the probability function as returned by the template attack itself, averaged over all 28 template positions along the rings representing the C and D Registers of the DES key schedule, as the sorting criteria for the key candidates. For weak keys — which in this attack model have a minimal rest entropy of only two bits — we find that on average only 37.75 bits need to be recovered by brute force when using only a single trace in the Exploitation Phase. This effort goes down to just a few bits for a single DES key when using only a few traces in Exploitation Phase.