private BigInteger derivePrivateKeyDownwards(
DeterministicKey cursor, byte[] parentalPrivateKeyBytes) {
DeterministicKey downCursor =
new DeterministicKey(
cursor.childNumberPath,
cursor.chainCode,
cursor.pub,
new BigInteger(1, parentalPrivateKeyBytes),
cursor.parent);
// Now we have to rederive the keys along the path back to ourselves. That path can be found by
// just truncating
// our path with the length of the parents path.
ImmutableList<ChildNumber> path =
childNumberPath.subList(cursor.getPath().size(), childNumberPath.size());
for (ChildNumber num : path) {
downCursor = HDKeyDerivation.deriveChildKey(downCursor, num);
}
// downCursor is now the same key as us, but with private key bytes.
checkState(downCursor.pub.equals(pub));
return checkNotNull(downCursor.priv);
}
/**
* Derives a child at the given index using hardened derivation. Note: <code>index</code> is not
* the "i" value. If you want the softened derivation, then use instead <code>
* HDKeyDerivation.deriveChildKey(this, new ChildNumber(child, false))</code>.
*/
public DeterministicKey derive(int child) {
return HDKeyDerivation.deriveChildKey(this, new ChildNumber(child, true));
}