Decrypt multiple blocks with cipher block chaining

Generalize the decryption function to use the initialization vector and decrypt multiple four-word sequences using cipher block chaining. Add a utility function to convert ASCII bytes to strings to clean up the test cases a bit.

@@ -52,9 +52,7 @@ var AES, decrypt;
  * @param key {Array} The key as an array of 4, 6 or 8 words.
  */
 AES = function (key) {
-  if (!this._tables[0][0][0]) {
-    this._precompute();
-  }
+  this._precompute();
   
   var i, j, tmp,
     encKey, decKey,
@@ -212,27 +210,40 @@ AES.prototype = {
   }
 };
 
-decrypt = function(encrypted, key) {
+decrypt = function(encrypted, key, initVector) {
   var
-    view = new DataView(encrypted.buffer),
-    decrypted = new AES(key)
-      // convert big-endian input to platform byte order for decryption
-      .decrypt(new Uint32Array([
-        view.getUint32(0),
-        view.getUint32(4),
-        view.getUint32(8),
-        view.getUint32(12)
-      ])),
-    bytes = new Uint8Array(encrypted.byteLength);
+    encryptedView = new DataView(encrypted.buffer),
+    platformEndian = new Uint32Array(encrypted.byteLength / 4),
+    decipher = new AES(key),
+    decrypted = new Uint8Array(encrypted.byteLength),
+    decryptedView = new DataView(decrypted.buffer),
+    decryptedBlock,
+    word, byte;
+
+  // convert big-endian input to platform byte order for decryption
+  for (byte = 0; byte < encrypted.byteLength; byte += 4) {
+    platformEndian[byte >>> 2] = encryptedView.getUint32(byte);
+  }
+  // decrypt four word sequences, applying cipher-block chaining (CBC)
+  // to each decrypted block
+  for (word = 0; word < platformEndian.length; word += 4) {
+    // decrypt the block
+    decryptedBlock = decipher.decrypt(platformEndian.subarray(word, word + 4));
 
-  // convert platform byte order back to big-endian for unpadding
-  view = new DataView(bytes.buffer);
-  view.setUint32(0, decrypted[0]);
-  view.setUint32(4, decrypted[1]);
-  view.setUint32(8, decrypted[2]);
-  view.setUint32(12, decrypted[3]);
+    // XOR with the IV, and restore network byte-order to obtain the
+    // plaintext
+    byte = word << 2;
+    decryptedView.setUint32(byte, decryptedBlock[0] ^ initVector[0]);
+    decryptedView.setUint32(byte + 4, decryptedBlock[1] ^ initVector[1]);
+    decryptedView.setUint32(byte + 8, decryptedBlock[2] ^ initVector[2]);
+    decryptedView.setUint32(byte + 12, decryptedBlock[3] ^ initVector[3]);
+
+    // setup the IV for the next round
+    initVector = platformEndian.subarray(word, word + 4);
+  }
 
-  return unpad(bytes);
+  // remove any padding
+  return unpad(decrypted);
 };
 
 // exports

@@ -23,37 +23,53 @@
 
 // see docs/hlse.md for instructions on how test data was generated
 
+var stringFromBytes = function(bytes) {
+  var result = '', i;
+
+  for (i = 0; i < bytes.length; i++) {
+    result += String.fromCharCode(bytes[i]);
+  }
+  return result;
+};
+
 module('Decryption');
 
-test('decrypts using AES-128 CBC with PKCS7', function() {
-  // the string "howdy folks" with key and initialization
-  // vector 
+test('decrypts a single AES-128 with PKCS7 block', function() {
   var
     key =  [0, 0, 0, 0],
     initVector = key,
+    // the string "howdy folks" encrypted
     encrypted = new Uint8Array([
       0xce, 0x90, 0x97, 0xd0,
       0x08, 0x46, 0x4d, 0x18,
       0x4f, 0xae, 0x01, 0x1c,
-      0x82, 0xa8, 0xf0, 0x67]),
-    length = 'howdy folks'.length,
-    plaintext = new Uint8Array(length),
-    i;
-
-  i = length;
-  while (i--) {
-    plaintext[i] = 'howdy folks'.charCodeAt(i);
-  }
+      0x82, 0xa8, 0xf0, 0x67]);
+
+  deepEqual('howdy folks',
+            stringFromBytes(videojs.hls.decrypt(encrypted, key, initVector)),
+            'decrypted with a byte array key');
+});
+
+test('decrypts multiple AES-128 blocks with CBC', function() {
+  var
+    key = [0, 0, 0, 0],
+    initVector = key,
+    // the string "0123456789abcdef01234" encrypted
+    encrypted = new Uint8Array([
+      0x14, 0xf5, 0xfe, 0x74,
+      0x69, 0x66, 0xf2, 0x92,
+      0x65, 0x1c, 0x22, 0x88,
+      0xbb, 0xff, 0x46, 0x09,
 
-  // decrypt works on the sjcl example site
-  // correct output: [1752135524, 2032166511, 1818981125, 84215045]
+      0x0b, 0xde, 0x5e, 0x71,
+      0x77, 0x87, 0xeb, 0x84,
+      0xa9, 0x54, 0xc2, 0x45,
+      0xe9, 0x4e, 0x29, 0xb3
+    ]);
 
-  deepEqual(plaintext,
-        new Uint8Array(videojs.hls.decrypt(encrypted, key, initVector)),
-        'decrypted with a numeric key');
-  deepEqual(plaintext,
-        new Uint8Array(videojs.hls.decrypt(encrypted, key, initVector)),
-        'decrypted with a byte array key');
+  deepEqual('0123456789abcdef01234',
+            stringFromBytes(videojs.hls.decrypt(encrypted, key, initVector)),
+            'decrypted multiple blocks');
 });
 
 })(window, window.videojs);