protected function createOAuth2Service()
  {
    $auth = new OAuth2(
        [
          'clientId'          => $this->getClientId(),
          'clientSecret'      => $this->getClientSecret(),
          'authorizationUri'   => self::OAUTH2_AUTH_URL,
          'tokenCredentialUri' => self::OAUTH2_TOKEN_URI,
          'redirectUri'       => $this->getRedirectUri(),
          'issuer'            => $this->config['client_id'],
          'signingKey'        => $this->config['signing_key'],
          'signingAlgorithm'  => $this->config['signing_algorithm'],
        ]
    );

    return $auth;
  }