private static RawKeyBytes deriveChildKeyBytesFromPublic(
DeterministicKey parent, ChildNumber childNumber) throws HDDerivationException {
checkArgument(!childNumber.isHardened(), "Can't use private derivation with public keys only.");
byte[] parentPublicKey = ECKey.compressPoint(parent.getPubKeyPoint()).getEncoded();
assert parentPublicKey.length == 33 : parentPublicKey.length;
ByteBuffer data = ByteBuffer.allocate(37);
data.put(parentPublicKey);
data.putInt(childNumber.i());
byte[] i = HDUtils.hmacSha512(parent.getChainCode(), data.array());
assert i.length == 64 : i.length;
byte[] il = Arrays.copyOfRange(i, 0, 32);
byte[] chainCode = Arrays.copyOfRange(i, 32, 64);
BigInteger ilInt = new BigInteger(1, il);
assertLessThanN(ilInt, "Illegal derived key: I_L >= n");
ECPoint Ki = ECKey.CURVE.getG().multiply(ilInt).add(parent.getPubKeyPoint());
assertNonInfinity(Ki, "Illegal derived key: derived public key equals infinity.");
return new RawKeyBytes(Ki.getEncoded(true), chainCode);
}
