Patent ID: 7165177

Claim:
A method of protecting an electronic chip ( 1 ) of a user against fraud in transactions between an application ( 2 ) and the electronic chip ( 1 ), the method comprising the following steps: determining ( 3 ) a combinatorial logic function g known to the application ( 2 ) and implanted ( 4 ) in the electronic chip ( 1 ), allocating to the electronic chip ( 1 ) a first secret key K, known only to the electronic chip ( 1 ) and to the application ( 2 ), and kept secret ( 13 ) in the electronic chip ( 1 ), on each authentication of the electronic chip ( 1 ), generating a variable input word R referred to as a seed, computing ( 16 , 17 ) by both the electronic chip ( 1 ) and the application ( 2 ) a certificate (Sp, S) which is the result of applying the combinatorial logic function g to a list of arguments (e 1 , e 2 ) comprising at least the random seed R and the secret key K, determining a non-linear pseudo-random function f known to the application ( 2 ) and implanted in the electronic chip ( 1 ), allocating to the electronic chip ( 1 ) a second secret key K′ known only to the electronic chip ( 1 ) and to the application ( 2 ) and kept secret ( 13 ) in the electronic chip ( 1 ), on each authentication of the electronic chip ( 1 ), determining ( 18 , 19 ) a mask M by applying the non-linear pseudo-random function f to a list of arguments (e 5 , e 6 ) comprising at least a portion of the secret key K′, masking ( 20 ) the value of the certificate (Sp) by means of the mask M to make available to the application ( 2 ) only the masked value of the certificate (Spm), and using the application ( 2 ) to verify the masked value of the certificate (Spm) computed by the electronic chip ( 1 ).