/**
* Derive a child key
*
* @param $sequence
* @return HierarchicalKey
* @throws \Exception
*/
public function deriveChild($sequence)
{
$chain = Buffer::int($this->getChainCode(), 32, $this->ecAdapter->getMath());
$hash = Hash::hmac('sha512', $this->getHmacSeed($sequence), $chain);
$offset = $hash->slice(0, 32);
$chain = $hash->slice(32);
$key = $this->isPrivate() ? $this->getPrivateKey() : $this->getPublicKey();
if (false === $this->ecAdapter->validatePrivateKey($offset)) {
return $this->deriveChild($sequence + 1);
}
return new HierarchicalKey($this->ecAdapter, $this->getDepth() + 1, $this->getChildFingerprint(), $sequence, $chain->getInt(), $key->tweakAdd($offset->getInt()));
}
