public static UInt128 PowModP_r(UInt128 a, UInt128 b)
{
UInt128 half_b = new UInt128(b);
if (b._high == 0 && b._low == 1)
{
return(new UInt128(a));
}
half_b.Rshift();
UInt128 t = PowModP_r(a, half_b);
t = MulModP(t, t);
if (b.IsOdd())
{
t = MulModP(t, a);
}
return(t);
}
public static UInt128 MulModP(UInt128 _a, UInt128 _b)
{
UInt128 r = new UInt128(0, 0);
UInt128 a = new UInt128(_a);
UInt128 b = new UInt128(_b);
while (!b.IsEmpty())
{
if (b.IsOdd())
{
UInt128 t = sub(P, a);
if (Compare(r, t) >= 0)
{
r = sub(r, t);
}
else
{
r = Add(r, a);
}
}
UInt128 double_a = new UInt128(a);
double_a.lshift();
UInt128 P_a = sub(P, a);
if (Compare(a, P_a) >= 0)
{
a = Add(double_a, INVERT_P);
}
else
{
a = double_a;
}
b.Rshift();
}
return(r);
}