For a secure encryption of original data the original data are first of all encrypted using an encryption key or an encryption algorithm. The thus obtained data are then again decrypted using a decryption algorithm and a decryption key in order to obtain decrypted data. These data are again used together with the original data in order to calculate an auxiliary key. The decrypted data are then encrypted using the calculated auxiliary key in order to obtain output data. In case of a DFA attack no output of the device is suppressed, but the output result is encrypted using the auxiliary key which deviates from the original encryption key in case of the DFA attack so that an attacker cannot use the output data anymore and the DFA attack is useless.