Patent ID: 8600046

Claim:
A method of coding a secret formed by a numerical value d, in a secret-key or public-key cryptography scheme, in which the secret is subdivided into a plurality of a determined number N of elements d i of rank i, i being a rank varying between 1 and N, to which a composition law is successively applied to obtain the numerical value d of said secret, said composition law being formed by a distributive arithmetic or logic operation, endowed with a neutral element, said method being carried out by a security component of a cryptographic device, the security component comprising a bus with a read-write protected access, said method furthermore comprising: calculating a first image T N of said secret by iterative application of the composition law term by term between a first image T i-1 of rank i−1, T o being a predefined arbitrary value, T i being a first current image of rank i, and a composition product according to said composition law of the element d i of rank i and a random value R i of rank i, chosen from among a first set of a plurality of N random values [ T i =T i-1 ( d i R i )] i=1 i=N →T N ; calculating a first numerical value S 1 by successively applying said composition law to said random values R i of said first set of N random values for i varying between 1 and N: S 1 = π i = 1 ⊗ N ⁢ R i ; calculating a second numerical value S 2 , the composition product according to said composition law applied successively to random values A j of a second set of N−1 random values S 2 = π j = 1 ⊗ N - 1 ⁢ A j ; calculating a second image T′ of said secret by applying an inverse composition law to said first image T N of said secret and to said second numerical value S 2 , so as to produce an intermediate image T x of said secret, and then by applying said inverse composition law to said intermediate image T x and to said first numerical value S 1 , so as to produce said second image T′ of said secret T x =T N S 2 T′=T x S 1 allocating to each of the first N−1 successive elements d i of said plurality of elements [d i ] the random value A j of corresponding rank of said second set of at least one random value and to the element d N of rank N the numerical value of said second image T′; delivering on the bus with read-write protected access solely the elements d i successively, under the supervision of said cryptographic device.