protected internal override ECPoint DecompressPoint(
int yTilde,
BigInteger X1)
{
ECFieldElement x = FromBigInteger(X1);
ECFieldElement alpha = x.Multiply(x.Square().Add(a)).Add(b);
ECFieldElement beta = alpha.Sqrt();
//
// if we can't find a sqrt we haven't got a point on the
// curve - run!
//
if (beta == null)
{
throw new ArithmeticException("Invalid point compression");
}
BigInteger betaValue = beta.ToBigInteger();
int bit0 = betaValue.TestBit(0) ? 1 : 0;
if (bit0 != yTilde)
{
// Use the other root
beta = FromBigInteger(q.Subtract(betaValue));
}
return(new FpPoint(this, x, beta, true));
}
