Drupal 8.0.0 beta 12. More info: https://www.drupal.org/node/2514176

core/modules/user/src/UserAuthInterface.php

@@ -0,0 +1,27 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\user\UserAuthInterface.
+ */
+
+namespace Drupal\user;
+
+/**
+ * An interface for validating user authentication credentials.
+ */
+interface UserAuthInterface {
+
+  /**
+   * Validates user authentication credentials.
+   *
+   * @param string $username
+   *   The user name to authenticate.
+   * @param string $password
+   *   A plain-text password, such as trimmed text from form values.
+   * @return int|bool
+   *   The user's uid on success, or FALSE on failure to authenticate.
+   */
+  public function authenticate($username, $password);
+
+}