A method for carrying out modular arithmetic computations involving multiplication operations by utilizing a non-reduced and extended Montgomery multiplication between a first A and a second B integer values, in which the number of iterations required is greater than the number of bits n of an odd modulo value N. The method comprises storing n+2 bit values in an accumulating device (S) capable of, of adding n+2-bit values (X) to it content, and of dividing its content by 2. Whenever desired, the content of the accumulating device is set to zero value. At least s(>n+1) iterations of the following steps are performed, while in each iteration choosing one bit, in sequence, from the value of said first integer value A, starting from its least significant bit: adding to the content of the accumulating device S the product of the selected bit and said second integer value B; adding to the resulting content the product of its current least significant bit and N; dividing the result by 2; and obtaining a non-reduced and extended Montgomery multiplication result by repeating these steps s-1 additional times while in each time using the previous result (S).