Add var declaration to des.js and use strict mode

src/crypto/cipher/des.js

@@ -25,6 +25,7 @@
  * @module crypto/cipher/des
  */
 
+'use strict';
 
 function des(keys, message, encrypt, mode, iv, padding) {
   //declaring this locally speeds things up a bit
@@ -303,7 +304,7 @@ function des_createKeys(key) {
     right = temp;
 
     //now go through and perform these shifts on the left and right keys
-    for (i = 0; i < shifts.length; i++) {
+    for (var i = 0; i < shifts.length; i++) {
       //shift the keys either one or two bits to the left
       if (shifts[i]) {
         left = (left << 2) | (left >>> 26);