diff --git a/lib/HashAlgorithm/HashAlgorithm.php b/lib/HashAlgorithm/HashAlgorithm.php
new file mode 100644
index 0000000..5c074ef
--- /dev/null
+++ b/lib/HashAlgorithm/HashAlgorithm.php
@@ -0,0 +1,57 @@
+<?php
+/**
+ * Nextcloud - user_sql
+ * Copyright (C) 2012-2018 Andreas Böhler <dev (at) aboehler (dot) at>
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as
+ * published by the Free Software Foundation, either version 3 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+namespace OCA\user_sql\HashAlgorithm;
+
+/**
+ * Interface which defines all function required by a hash algorithm.
+ * Please note that this interface must be implemented by every hash function supported in this app.
+ * @author Marcin Łojewski <dev@mlojewski.me>
+ */
+interface HashAlgorithm
+{
+    /**
+     * Used by reflection to get the class instance.
+     * @return HashAlgorithm
+     */
+    public static function getInstance();
+
+    /**
+     * Get the hash algorithm name.
+     * This name is visible in the admin panel.
+     * @return string
+     */
+    public function getVisibleName();
+
+    /**
+     * Hash given password.
+     * This value is stored in the database, when the password is changed.
+     * @param String $password The new password.
+     * @return boolean True if the password was hashed successfully, false otherwise.
+     */
+    public function getPasswordHash($password);
+
+    /**
+     * Check password given by the user against hash stored in the database.
+     * @param String $password Password given by the user.
+     * @param String $dbHash Password hash stored in the database.
+     * @return boolean True if the password is correct, false otherwise.
+     */
+    public function checkPassword($password, $dbHash);
+}