// D.1.4 91
/**
* return a sqrt root - the routine verifies that the calculation
* returns the right value - if none exists it returns null.
*/
public override ECFieldElement sqrt()
{
// p mod 4 == 3
if (p.testBit(1))
{
// z = g^(u+1) + p, p = 4u + 3
ECFieldElement z = new Fp(p, x.modPow(p.shiftRight(2).add(ECConstants.ONE), p));
return(z.square().Equals(this) ? z : null);
}
throw new Exception("not done yet");
}