class CsrfTokenGenerator

class CsrfTokenGenerator {
    
    /**
     * The private key service.
     *
     * @var \Drupal\Core\PrivateKey
     */
    protected $privateKey;
    
    /**
     * The session metadata bag.
     *
     * @var \Drupal\Core\Session\MetadataBag
     */
    protected $sessionMetadata;
    
    /**
     * Constructs the token generator.
     *
     * @param \Drupal\Core\PrivateKey $private_key
     *   The private key service.
     * @param \Drupal\Core\Session\MetadataBag $session_metadata
     *   The session metadata bag.
     */
    public function __construct(PrivateKey $private_key, MetadataBag $session_metadata) {
        $this->privateKey = $private_key;
        $this->sessionMetadata = $session_metadata;
    }
    
    /**
     * Generates a token based on $value, the user session, and the private key.
     *
     * The generated token is based on the session of the current user. Normally,
     * anonymous users do not have a session, so the generated token will be
     * different on every page request. To generate a token for users without a
     * session, manually start a session prior to calling this function.
     *
     * @param string $value
     *   (optional) An additional value to base the token on.
     *
     * @return string
     *   A 43-character URL-safe token for validation, based on the token seed,
     *   the hash salt provided by Settings::getHashSalt(), and the
     *   'drupal_private_key' configuration variable.
     *
     * @see \Drupal\Core\Site\Settings::getHashSalt()
     * @see \Symfony\Component\HttpFoundation\Session\SessionInterface::start()
     */
    public function get($value = '') {
        $seed = $this->sessionMetadata
            ->getCsrfTokenSeed();
        if (empty($seed)) {
            $seed = Crypt::randomBytesBase64();
            $this->sessionMetadata
                ->setCsrfTokenSeed($seed);
        }
        return $this->computeToken($seed, $value);
    }
    
    /**
     * Validates a token based on $value, the user session, and the private key.
     *
     * @param string $token
     *   The token to be validated.
     * @param string $value
     *   (optional) An additional value to base the token on.
     *
     * @return bool
     *   TRUE for a valid token, FALSE for an invalid token.
     */
    public function validate($token, $value = '') {
        $seed = $this->sessionMetadata
            ->getCsrfTokenSeed();
        if (empty($seed)) {
            return FALSE;
        }
        return hash_equals($this->computeToken($seed, $value), $token);
    }
    
    /**
     * Generates a token based on $value, the token seed, and the private key.
     *
     * @param string $seed
     *   The per-session token seed.
     * @param string $value
     *   (optional) An additional value to base the token on.
     *
     * @return string
     *   A 43-character URL-safe token for validation, based on the token seed,
     *   the hash salt provided by Settings::getHashSalt(), and the site private
     *   key.
     *
     * @see \Drupal\Core\Site\Settings::getHashSalt()
     */
    protected function computeToken($seed, $value = '') {
        return Crypt::hmacBase64($value, $seed . $this->privateKey
            ->get() . Settings::getHashSalt());
    }

}