Method and device for preventing unauthorized access to a computer system

In a method and a device for preventing unauthorised access to a computer system comprising at least one processing unit (1) or like, a disk unit (3), a display (5), and a keyboard (7), use is made of a floppy disk (9). Information in the form of a personal identification code containing the name of an authorised user and an access code encrypted by means of a one-way encrypting function, and an authenticity code formed by a system access code and at least the personal identification code are recorded on the floppy disk. After insertion of the floppy into the disk unit, the computer system is adapted, in starting-up, to read at least the personal identification code and the authenticity code; to check the authenticity code by means of the system access code and the information recorded on the floppy disk and used to form the authenticity code; to request, via the display, the user to enter his name and access code in non-encrypted form via the keyboards; to encrypt the access code entered by the user; to check the read name against the entered name and, respectively, the read, encrypted access code against the entered access code which is then encrypted by the computer system; and of give the user access to the computer system merely in case of a positive result of all three checks.