Fast exponentiation method obtained by folding the exponent in half

On average, the square-and-multiply method requires 1.5 k multiplications to evaluate /spl chi//sup E/, where k is the bit length of E. Recently, Wu and Chang (see ibid., vol. 31, no. 20, p. 1738-9, 1995) proposed an efficient method for computing /spl chi//sup E/, which needs 156 k/128 multiplications. By folding the exponent E in half and combining Wu and Chang's method, we present a more efficient method for evaluating /spl chi//sup E/. On average, our new method needs only 149 k/128+3 multiplications.