A method of automatically controlling fraud in an electronic transaction system. When a user initiates a session in the electronic transaction system, an element is generated and stored in a database in association with information identifying the user. Each time during the session the user commands the execution of an operation, an equation is determined that is satisfied by the element stored in the database. When a sufficient given number of operations has been effected, the system of equations consisting of the equations determined as above is solved to deduce the element. By consulting the database, the corresponding information identifying the user is deduced from the element obtained in this way.