By arranging a redundancy means and a control means upstream from an encryption means which encrypts and decrypts the data to be stored in an external memory, the integrity of data may be ensured when the generation of redundancy information is realized by the redundancy means, and when the generation of a syndrome bit vector indicating any alteration of the data is implemented by the control means. What is preferred is a control matrix constructed from idempotent, thinly populated, circulant square sub-matrices only. By arranging redundancy and control means upstream from the encryption/decryption means, what is achieved is that both errors in the encrypted data and errors of the non-encrypted data may be proven, provided that they have occurred in the data path between the redundancy/control means and the encryption/decryption means.