diff --git a/src/crypto/cipher/aes.js b/src/crypto/cipher/aes.js
index 2a13e946..3892aa08 100644
--- a/src/crypto/cipher/aes.js
+++ b/src/crypto/cipher/aes.js
@@ -442,9 +442,11 @@ function keyExpansion(key) {
       }
     }
   }
-  this.rounds = rounds;
-  this.rk = keySched;
-  return this;
+
+  return {
+    rounds: rounds,
+    rk: keySched
+  };
 }
 
 function AESencrypt(block, ctx) {