A method of protecting a microcomputer system against manipulation of data stored in a storage assembly of the microcomputer system, particularly for protecting a program stored in the storage assembly, is described. In the method, the data is stored in marked or encrypted form in the storage assembly with the aid of an asymmetrical encryption method. In order to minimize the misuse of private keys which fall into the hands of unauthorized persons, a smart card protected by a personal identification number, on which the private key and an encryption algorithm for the asymmetrical encryption method are stored, is used for marking or encrypting the data.