diff --git a/src/BaseUrlSigner.php b/src/BaseUrlSigner.php
index eace93c..9e8217d 100644
--- a/src/BaseUrlSigner.php
+++ b/src/BaseUrlSigner.php
@@ -9,6 +9,13 @@
 
 abstract class BaseUrlSigner implements UrlSigner
 {
+    /**
+     * The key that is used to generate secure signatures.
+     *
+     * @var string
+     */
+    protected $signatureKey;
+
     /**
      * The URL's query parameter name for the expiration.
      *