A security system for preventing unauthorized use of a computer device. An extractable security piece includes an extractable main private key and a main PC public key. A PC security area which is a non-extractable part of the computer device includes a PC private key and an extractable main public key, which, together with the keys of the extractable security piece, constitute a Public Key Infrastructure. The extractable security piece and the PC security area include processing means for mutual authentication of the extractable security piece and the PC security area after the extractable security piece, which had been previously removed, has been reinserted in the computer device, thereby enabling the authorized user to access data stored in the computer device.