--- /dev/null
+<?php
+
+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);
+
+}