diff --git a/lib/Payone/ClientApi/Service/GenerateHash.php b/lib/Payone/ClientApi/Service/GenerateHash.php
index 1ad60ae6..aa85c86e 100644
--- a/lib/Payone/ClientApi/Service/GenerateHash.php
+++ b/lib/Payone/ClientApi/Service/GenerateHash.php
@@ -103,8 +103,7 @@ public function generate(Payone_ClientApi_Request_Interface $request, $securityK
             $hashString .= $requestData[$key];
         }
 
-        $hashString .= $securityKey;
-        $hash = md5($hashString);
+        $hash = hash_hmac('sha384', $hashString, $securityKey);
         return $hash;
     }