Data security system

A data security system to prevent unauthorised access to data which is held on a data medium, preferably a disc-shaped data medium such as a CD-ROM, a diskette or a hard disk, with a code marking (4), which is on the data medium itself or an additional data medium, is in the form of a code card (3) which identifies the user, and can only be read using a special auxiliary device, with a supplementary device (9), which is connected to the device (8) to read the data from the data medium, to read the code marking (4), with a coincidence device (14), to control the data flow between the data medium and a data processing device (15) when the data is read, and with an additional data pattern which is assigned to the code marking (4) and, for its part, is assigned to a recording area for protected data. The data flow for reading protected data is enabled by the coincidence device (14) if the code marking (4) agrees with the data pattern, or with information which is contained in it.