Abstract: Encryption and decryption of information of a message is performed by partitioning a plaintext message into blocks of binary digits and by further partitioning said blocks into subblocks which are interpreted as elements in a Galois-field. A plaintext matrix (M) of said elements is multiplied by a first key matrix (A) of a group over said Galois-field, the resulting product M.multidot.A) being multiplied by a second key matrix (B) of the same group over said Galois-field. The final product (B.multidot.M.multidot.A) thus received constitutes the encrypted message block (K). Decryption is performed by multiplying the transmitted product (B.multidot.M.multidot.A) by inverse key matrices (A.sup.-1, B.sup.-1) generated by the same keys (a, b) as used for decryption and taken in the proper order. (FIG.