Abstract: The device (12) comprises nine encryption stages (61.1, 61.2, 69), the first eight of which are constructed identically. It further comprises a key subblock generation unit (63), an input unit (21), and an output unit (79). It serves for the block-by-block encryption of a plaintext (X) proceeding from a message source (11) into a ciphertext (Y) to be delivered on a transmission line (13), wherein a secret key block (Z) is inputted beforehand via a secure channel 17. The encryption is effected in a step-by-step and parallel manner for four subblocks (X.sub.1 -X.sub.4 ; W.sub.11 -W.sub.14 ; W.sub.21 -W.sub.24 ; W.sub.81 -W.sub.84 ; Y.sub.1 -Y.sub.4). Every encryption stage (61.1, 61.2, 69) comprises four first inputs (25-28; 35-38), six and four second inputs (29, 30, 32, 33, 49, 52; 129, 130, 132, 133), respectively, and four outputs (75-78). A total of fifty-two key subblocks (Z.sub.1 -Z.sub.52) which are formed from the key block (Z) are connected to the second inputs.