make getSalt() abstract

lib/Crypto/AbstractCrypt.php

@@ -56,8 +56,5 @@ abstract class AbstractCrypt extends AbstractAlgorithm
      *
      * @return string The salt string.
      */
-    protected function getSalt()
+    protected abstract function getSalt();
-    {
-        return "";
-    }
 }

lib/Crypto/Crypt.php

@@ -56,4 +56,12 @@ class Crypt extends AbstractCrypt
     {
         return "Unix (Crypt)";
     }
+
+    /**
+     * Not used.
+     */
+    protected function getSalt()
+    {
+        return null;
+    }
 }