diff --git a/gatekeeper/1.0/types.hal b/gatekeeper/1.0/types.hal
index 8c184ee..59076e0 100644
--- a/gatekeeper/1.0/types.hal
+++ b/gatekeeper/1.0/types.hal
@@ -37,6 +37,6 @@
      * otherwise unused (0)
      */
     uint32_t timeout;
-    /* optional crypto blob */
+    /* optional crypto blob. Opaque to Android system. */
     vec<uint8_t> data;
 };