some pseudo code
function powermod(base, exponent, modulus) {
if (base < 1 || exponent < 0 || modulus < 1)
return -1
result = 1;
while (exponent > 0) {
if ((exponent % 2) == 1) {
result = (result * base) % modulus;
}
base = (base * base) % modulus;
exponent = floor(exponent / 2);
}
return result;
}
and call
powermod(45, x, 257)