Patent ID: 7206408

Claim:
A countermeasure method in a first electronic component communicating with a second electronic component using a cryptographic algorithm A with a secret key K[i] of size k, said method comprising the step of changing K[i] between successive uses of A, i being the number of executions of the algorithm A, wherein the secret key K[i] for the next iteration i of the algorithm A is calculated in the second electronic component by means of the formula: K[i]=f [( K[i− 1])], where f is a function taking a key as an input and returning a key as an output, and wherein said first electronic component performs a calculation shortcut enabling it to calculate K[i] from K[0], for i>1, without having to generate the i−1 keys K[1], . . . , K[i−1] separating K[0] and K[i].