Method of protecting an electronic chip against fraud

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 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 logic function g to a list of arguments (e1, e2) comprising at least the random seed R and the secret key K,
• which method is characterized in that it further comprises the following steps:
• determining a non-linear 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 function f to a list of arguments (e5, e6) 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).