diff --git a/composer.json b/composer.json
index 6acc8c4..d2c15b3 100644
--- a/composer.json
+++ b/composer.json
@@ -18,6 +18,9 @@
   "require-dev": {
     "phpunit/phpunit": "~4.0"
   },
+  "suggest": {
+    "ext-mcrypt": "required if you use encryption cast5"
+  },
   "autoload": {
     "classmap": ["lib/"]
   }