Template Attack Assisted Linear Cryptanalysis on Outer Rounds Protected DES Implementations

Abstract

Abstract In practice, when the security of a block cipher implementation is considered, the leakages related to the outer rounds encryptions can be used by side channel attacks (SCA) to recover the secret key. Therefore, the outer rounds of block cipher implementations should be protected. However, in order to lower the implementation price, the inner rounds of block cipher implementations may be unprotected. In light of this, the security of an outer rounds protected DES implementation is considered. In detail, template attack (TA), which is information theoretically the strongest SCA style, can be used to obtain the inner round output. Then, linear cryptanalysis (LC) can be used to recover the secret key. Finally, the optimal key enumeration algorithm can be used to optimize the efficiency of TA assisted LC. We evaluate the efficiency of TA assisted LC in simulated scenarios where a three outer rounds protected DES implementation is targeted. The evaluation results show that when 800 correct samples are available and the number of key enumeration is $2^{10}$, the efficiency of TA assisted LC can reach 83% of success rate. Overall, an efficient combination attack style that can be used to accurately evaluate the security of an outer rounds protected DES implementation is proposed.