Keystream generators for secure data transmission, the keystream generators being operated in counter mode, against repeated or improper generation of an already generated keystream and to protect the data transmission against repeated use of a keystream, so-called reuse are provided. The keystream generator is operated, with respect to realization options, selectively in one of two operating modes, an encryption operating mode and a decryption operating mode. In the encryption operating mode, a keystream generated on the basis of a first control data set is used to encrypt data, in particular payload data, to form cipher-data, the product of ciphered data or payload data. In the decryption operating mode, a keystream generated on the basis of a second control data set is used to decrypt the cipher-data. The keystream is output only if the generation of the keystream from the encryption of a counter value of the keystream generator operated in counter mode with a block cipher key is error-free.