1. A digital processing method for determining A mod N using a calculating engine having two inputs x and y and which produces an output x y −mk mod N, where n is the number of bits in the binary representation of N, where k is the size of the words processed by said engine in bits, and where m is the smallest integer for which mk&gE;n&plus;2, said method comprising the steps of: operating said engine with inputs 1 and A 0 to produce a first result, where A 0 is the low order bits in the representation of A as A 1 2 mk &plus;A 0 ; adding said first result to A 1 to produce a second result, where the addition is modulo N; and operating said engine with inputs being said second result and 2 &plus;2mk mod N, whereby the output of said engine is A mod N.