protected ECPoint decompressPoint(int yTilde, BigInteger X1) {
ECFieldElement x = fromBigInteger(X1);
ECFieldElement alpha = x.square().add(a).multiply(x).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 RuntimeException("Invalid point compression");
}
if (beta.testBitZero() != (yTilde == 1)) {
// Use the other root
beta = beta.negate();
}
return new ECPoint.Fp(this, x, beta, true);
}
