Abstract: A system for encrypting and decrypting digital data wherein the data is divided in packets of N blocks X(1) . . . X(N) of 2.sup.m bits, comprises an encryption device and a decryption device. The encryption device reverses the sequence of the blocks X(1) . . . X(N) before a XOR operation and next an encryption operation by means of an encryption algorithm E is carried out on each block of a packet. Thereby the following encrypted blocks Y(1) . . . Y(N) are formed: Y(1)=E ?X(N)+IV!, Y(i)=E ?X(N-i+1)+Y(i-1)! for i>1 and i.ltoreq.N. The encrypted blocks Y(1) . . . Y(N) are transferred by a sender in reversed sequence Y(N) . . . Y(1) to a receiver. The decryption device at the receiver obtains the original blocks X(1) . . . X(N) by carrying out a decryption operation by means of a decryption algorithm D and next a XOR operation on each block Y(N) . . . Y(l) received. Thereby the original blocks are obtained as follows: X(i)=D ?Y(N-i+1)!+Y(N-i) for i=1, 2, . . . , N-1; X(N)=D ?Y(1)!+IV.