/* a=1/a mod 2^256. This is very fast! */
public virtual void InvMod2m()
{
int i;
BIG U = new BIG(0);
BIG b = new BIG(0);
BIG c = new BIG(0);
U.Inc(InvMod256(LastBits(8)));
for (i = 8; i < BIGBITS; i <<= 1)
{
U.Norm();
b.Copy(this);
b.Mod2m(i);
BIG t1 = SMul(U, b);
t1.Shr(i);
c.Copy(this);
c.Shr(i);
c.Mod2m(i);
BIG t2 = SMul(U, c);
t2.Mod2m(i);
t1.Add(t2);
t1.Norm();
b = SMul(t1, U);
t1.Copy(b);
t1.Mod2m(i);
t2.One();
t2.Shl(i);
t1.RSub(t2);
t1.Norm();
t1.Shl(i);
U.Add(t1);
}
U.Mod2m(BIGBITS);
Copy(U);
Norm();
}