The device (12) comprises nine coding stages (61.1, 61.2, 69), the first eight of which are of identical construction. It also comprises a coding partial block generation unit (63), an input unit (21) and an output unit (79). It is used for the blockwise coding of a plain text (X) from an information source (11) into a coded text (Y) to be taken to a transmission line (13), whereby a secret code block (Z) is previously introduced via a secure channel (17). Coding takes place stepwise and in parallel for four partial blocks (X1 - X4; W11 - W14; W21 - W24; W81 - W84; Y1 - Y4) at a time. To this end, each coding stage (61.1, 61.2, 60) has four first inputs (25 - 28; 35 - 38), six or four second inputs (29, 30, 32, 33, 49, 52; 129, 130, 132, 133) and four outputs (75 - 78).; At the second inputs there are a total of fifty-two coding partial blocks (Z1 - Z52) formed from the code block (Z). The device (12) may also be used unchanged for decoding an incoming cipher text (Y). For this purpose it is merely necessary to apply other code partial blocks to the second inputs.