Patent ID: 7284122

Claim:
A cryptographic method of providing protection against fraud in transactions between an application ( 1 ) and an electronic chip ( 2 ) of a user, the method comprising the following steps: determining a non-linear pseudo random function f known to the application ( 1 ) and implanted ( 4 ) in the electronic chip ( 2 ), allocating to the electronic chip ( 2 ) a first secret key K known only to the electronic chip ( 2 ) and to the application ( 1 ) and kept secret ( 5 ) in the electronic chip ( 2 ), on each authentication of the electronic chip ( 2 ), generating a variable input word R referred to as a random seed, computing ( 15 , 16 ) by both the electronic chip ( 2 ) and the application ( 1 ) a certificate (Sp, S) which is the result of applying the non-linear pseudo random function f to a list of arguments (e 1 , e 2 ) comprising at least the seed R and the secret key K, allocating to the electronic chip ( 2 ) a second secret key K′ known only to the electronic chip ( 2 ) and to the application ( 1 ) and kept secret ( 6 ) in the electronic chip ( 2 ), on each authentication of the electronic chip ( 2 ), determining ( 17 , 18 ) a mask M computed from at least a portion of the secret key K′, masking ( 19 ) the value of the certificate (Sp) by means of the mask M to provide a second layer of security such that only the masked value of the certificate (Spm) is made available to the application ( 1 ), and using the application ( 1 ) to verify the masked value of the certificate (Spm) computed by the electronic chip ( 2 ).