Patent ID: 7085378

Claim:
A countermeasure method in an electronic component using a secret key cryptographic algorithm for calculating an encoded message from an input message, of the type in which sixteen calculation rounds are employed where each round supplies an output data item from an input data item, and the output data item is manipulated by critical instructions in at least the first three and last three rounds, said method including the following steps: forming a group comprising at least the first three rounds and another group comprising at least the last three rounds, in each of these groups selectively applying a first sequence that uses a first manipulating means for said critical instructions in each round or a second sequence that uses other manipulating means for said critical instructions at least in certain rounds, said first and second sequences being such that they supply the same result at the output from the last round in each group for the same given input message, and selecting the sequence to be executed in the groups as a function of a statistical half probability distribution, in order to make the data manipulated by said critical instructions unpredictable.