A cryptographic method of protection against fraud in transactions between an application and an electronic chip of a user. Both the electronic chip and the application compute a certificate (Sp, S) which is the result of applying a non-linear function f to a list of arguments (e.sub.1, e.sub.2) comprising at least a seed R and a secret key KO. A second secret key K' which is known only to the electronic chip and to the application is allocated to and kept secret in the electronic chip. Upon each authentication of the electronic chip, a mask M is determined by computing it from at least a portion of the secret key K'. The value of the certificate (Sp) is masked by means of the mask M to make available to the application only the masked value of the certificate (Spm). The application is used to verify the masked value of the certificate (Spm) computed by the electronic chip.