Patent ID: 6973190

Claim:
A method adapted to protect a smart card implementing a cryptographic process involving calculation of a modular exponentiation of a quantity (x), said modular exponentiation using a secret exponent (d), comprising breaking down said secret exponent (d) into unpredictable values (d1, d2, . . . , dk), wherein k is reater than 2, and at least one of said (k−1) values has a length at least equal to 64 bits, the sum of which is equal to said secret exponent (d) including: deriving (k−1) unpredictable values (d1, d2, . . . , dk-1), using a random generator; obtaining a final unpredictable value (dk) from the difference between the secret exponent (d) and the (k−1) unpredictable values (d1, d2, . . . , dk-1), creating k intermediate results by performing modular exponentiation on the quantity (x) using the k unpredictable values (d1, d2, . . . , dk−1, dk); and calculating a final results based on the k intermediate results, equal to the modular exponentiation of the quantity (x) using the secret exponent (d).