added a test to verify that deepcopy for precompute works

deepcopy aesTables rather than using a reference to it

@@ -103,7 +103,7 @@ const precompute = function() {
   return _tables;
 };
 
-let tables;
+let aesTables;
 
 /**
  * Schedule out an AES key for both encryption and decryption. This
@@ -126,10 +126,10 @@ class AES {
     *
     * @private
     */
-    if (!tables) {
-      tables = precompute();
+    if (!aesTables) {
+      aesTables = precompute();
     }
-    this._tables = tables;
+    this._tables = JSON.parse(JSON.stringify(aesTables));
     let i;
     let j;
     let tmp;

@@ -53,6 +53,44 @@ QUnit.test('decrypts multiple AES-128 blocks with CBC', function() {
                   'decrypted multiple blocks');
 });
 
+QUnit.test(
+'verify that the deepcopy works by doing two decrypts in the same test',
+function() {
+  let key = new Uint32Array([0, 0, 0, 0]);
+  let initVector = key;
+  // the string "howdy folks" encrypted
+  let pkcs7Block = new Uint8Array([
+    0xce, 0x90, 0x97, 0xd0,
+    0x08, 0x46, 0x4d, 0x18,
+    0x4f, 0xae, 0x01, 0x1c,
+    0x82, 0xa8, 0xf0, 0x67
+  ]);
+
+  QUnit.deepEqual('howdy folks',
+                  stringFromBytes(unpad(decrypt(pkcs7Block, key, initVector))),
+                  'decrypted with a byte array key'
+  );
+
+// the string "0123456789abcdef01234" encrypted
+  let cbcBlocks = new Uint8Array([
+    0x14, 0xf5, 0xfe, 0x74,
+    0x69, 0x66, 0xf2, 0x92,
+    0x65, 0x1c, 0x22, 0x88,
+    0xbb, 0xff, 0x46, 0x09,
+
+    0x0b, 0xde, 0x5e, 0x71,
+    0x77, 0x87, 0xeb, 0x84,
+    0xa9, 0x54, 0xc2, 0x45,
+    0xe9, 0x4e, 0x29, 0xb3
+  ]);
+
+  QUnit.deepEqual('0123456789abcdef01234',
+                  stringFromBytes(unpad(decrypt(cbcBlocks, key, initVector))),
+                  'decrypted multiple blocks');
+
+});
+
+
 QUnit.module('Incremental Processing', {
   beforeEach() {
     this.clock = sinon.useFakeTimers();