public function createChain($ECPublicKey_theirRatchetKey, $ECKeyPair_ourRatchetKey)
{
$sharedSecret = Curve::calculateAgreement($ECPublicKey_theirRatchetKey, $ECKeyPair_ourRatchetKey->getPrivateKey());
$derivedSecretBytes = $this->kdf->deriveSecrets($sharedSecret, 'WhisperRatchet', DerivedRootSecrets::SIZE, $this->key);
$derivedSecrets = new DerivedRootSecrets($derivedSecretBytes);
$newRootKey = new self($this->kdf, $derivedSecrets->getRootKey());
$newChainKey = new ChainKey($this->kdf, $derivedSecrets->getChainKey(), 0);
return [$newRootKey, $newChainKey];
}
