The invention concerns a process to protect a vulnerable software working on a data processing system against its unauthorized usage using a processing and memorizing unit. The process comprises creating a protected software by choosing in the source of the vulnerable software at least one algorithmic processing and by producing the source of the protected software so that during the execution of the protected software appear several distinct steps, namely 1) the placing of at least one operand at the processing and memorizing unit's disposal 2) the carrying out by the processing and memorizing unit, of the algorithmic processing's functionality on at least said operand 3) and possibly, the placing at the data processing system's disposal of at least one result by the processing and memorizing unit.