A Simple Power Analysis Attack on the Serpent Key Schedule

We describe an SPA attack on an 8-bit smart card implementation of the Serpent block cipher. Our attack uses measurements taken during an on-the-fly key expansion together with linearity in the cipher’s key schedule algorithm to drastically reduce the search time for an initial key. An implementation finds 256-bit keys in 3.736 ms on average. Our work shows that linearity in key schedule design and other cryptographic applications should be carefully evaluated for susceptibility to side-channel attacks and that search algorithm design can greatly speed up side-channel attacks.