Method for performing cryptographic operations on data in a processing device comprising an encryption procedure, said method comprising generating a not repeating, in particular random, initialization vector by an initialization vector generator, in particular random generator, obtaining at least a unique key (K 0; K 0,K 1), supplying said at least a unique key (K 0; K 0,K 1) as encryption key to a pair of output feedback encryption branches (31a, 31b), each encryption branch (31a, 31b) performing a first encryption operation invoking an encryption block cipher performing an encryption operation using said at least a unique key (K 0; K 0,K 1) as encryption key and receiving a respective first (CNa) and second (CNb) constant as input to be encrypted, the outputs (Oa 0, Ob 0) of said first encryption operations on each encryption branch (31a, 31b) and a first portion (P 0) of a plaintext being the inputs of a first XOR function (X 0) producing a first portion of cipher text (C 0), performing at least one subsequent encryption invoking an encryption block cipher using said at least a unique key (K 0; K 0,K 1) as encryption key and receiving the outputs (Oa 0, Ob 0) of the previous encryptions as input to be encrypted, the outputs (Oa 1, Ob 1) of said subsequent encryptions and a subsequent portion (P 1) of a plaintext being the inputs of a subsequent XOR function (X 1) producing a subsequent portion of cipher text (C 1), characterized in that said having the outputs (Oa 0, Ob 0) of the first encryption operations on each encryption branch (31a, 31b) and a first portion (P 0) of a plaintext being the inputs of a first XOR function (X 0) producing a first portion of cipher text (C 0) is implemented by first performing a first XOR between the outputs (Oa 0, Ob 0) of the first encryption operations on each encryption branch (31a, 31b) then on the result of such first XOR operation is performed a XOR with the first portion P 0 of the plaintext, producing said first portion of cipher text (C 0), the subsequent XOR functions (X 1) being also so implemented to produce subsequent portions of cipher text (C 1). said method comprising generating a not repeating, in particular random, initialization vector by an initialization vector generator, in particular random generator, obtaining at least a unique key (K 0; K 0,K 1), supplying said at least a unique key (K 0; K 0,K 1) as encryption key to a pair of output feedback encryption branches (31a, 31b), each encryption branch (31a, 31b) performing a first encryption operation invoking an encryption block cipher performing an encryption operation using said at least a unique key (K 0; K 0,K 1) as encryption key and receiving a respective first (CNa) and second (CNb) constant as input to be encrypted, the outputs (Oa 0, Ob 0) of said first encryption operations on each encryption branch (31a, 31b) and a first portion (P 0) of a plaintext being the inputs of a first XOR function (X 0) producing a first portion of cipher text (C 0), performing at least one subsequent encryption invoking an encryption block cipher using said at least a unique key (K 0; K 0,K 1) as encryption key and receiving the outputs (Oa 0, Ob 0) of the previous encryptions as input to be encrypted, the outputs (Oa 1, Ob 1) of said subsequent encryptions and a subsequent portion (P 1) of a plaintext being the inputs of a subsequent XOR function (X 1) producing a subsequent portion of cipher text (C 1), characterized in that said having the outputs (Oa 0, Ob 0) of the first encryption operations on each encryption branch (31a, 31b) and a first portion (P 0) of a plaintext being the inputs of a first XOR function (X 0) producing a first portion of cipher text (C 0) is implemented by first performing a first XOR between the outputs (Oa 0, Ob 0) of the first encryption operations on each encryption branch (31a, 31b) then on the result of such first XOR operation is performed a XOR with the first portion P 0 of the plaintext, producing said first portion of cipher text (C 0), the subsequent XOR functions (X 1) being also so implemented to produce subsequent portions of cipher text (C 1).