public function exists($user, $key)
    {
        if (!is_string($user)) {
            throw new \InvalidArgumentException();
        }

        if ($key instanceof \fpoirotte\Pssht\PublicKeyInterface) {
            $key = $this->getIdentifier($key);
        }
        if (!is_string($key)) {
            throw new \InvalidArgumentException();
        }

        return isset($this->keys[$user][$key]);
    }