The invention relates to an authentication system for a user possessing a means (3) for authentication at an authentication entity (1), said authentication means including a means (11) for storing at least one status variable and a single-use access-code generator (2) actuated upon a request of the user, said access code including at least one unpredictable portion and being transmitted to the authentication entity for validation, said validation authenticating the user at the authentication entity, characterised in that the status variable is updated in a random manner by and upon the initiative of the authentication means systematically and before any generation of a new access code, in that the generator is suitable for computing the access code using the status variable once it is updated, and in that the authentication entity is adapted for modifying the value of at least one internal status variable during the at least partial validation of the access code by using information previously known by the authentication entity as well as the unpredictable portion conveyed by the transmitted access code.