private static RawKeyBytes deriveChildKeyBytes(DeterministicKey parent, ChildNumber childNumber)
throws HDDerivationException {
byte[] parentPublicKey = HDUtils.getBytes(parent.getPubPoint());
assert parentPublicKey.length == 33 : parentPublicKey.length;
ByteBuffer data = ByteBuffer.allocate(37);
if (childNumber.isPrivateDerivation()) {
data.put(parent.getPrivKeyBytes33());
} else {
data.put(parentPublicKey);
}
data.putInt(childNumber.getI());
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 = HDUtils.toBigInteger(il);
assertLessThanN(ilInt, "Illegal derived key: I_L >= n");
byte[] keyBytes;
final BigInteger privAsFieldElement = parent.getPrivAsFieldElement();
if (privAsFieldElement != null) {
BigInteger ki = privAsFieldElement.add(ilInt).mod(HDUtils.getEcParams().getN());
assertNonZero(ki, "Illegal derived key: derived private key equals 0.");
keyBytes = ki.toByteArray();
} else {
checkArgument(
!childNumber.isPrivateDerivation(),
"Can't use private derivation with public keys only.");
ECPoint Ki = HDUtils.getEcParams().getG().multiply(ilInt).add(parent.getPubPoint());
checkArgument(
!Ki.equals(HDUtils.getCurve().getInfinity()),
"Illegal derived key: derived public key equals infinity.");
keyBytes = HDUtils.toCompressed(Ki.getEncoded());
}
return new RawKeyBytes(keyBytes, chainCode);
}
