Add Common Encryption support to BMFF, including subsample decryption.

media/crypto/aes_decryptor.cc

Index: media/crypto/aes_decryptor.cc
diff --git a/media/crypto/aes_decryptor.cc b/media/crypto/aes_decryptor.cc
index 129bc330131585c2a77a66dc7ab8b96fb96c76e6..963f23f00c6662e962129d5477be45877bc260f7 100644
--- a/media/crypto/aes_decryptor.cc
+++ b/media/crypto/aes_decryptor.cc
@@ -16,9 +16,6 @@
 
 namespace media {
 
-// TODO(xhwang): Get real IV from frames.
-static const char kInitialCounter[] = "0000000000000000";
-
 uint32 AesDecryptor::next_session_id_ = 1;
 
 // Decrypt |input| using |key|.
@@ -29,27 +26,88 @@ static scoped_refptr<DecoderBuffer> DecryptData(const DecoderBuffer& input,
   CHECK(input.GetDataSize());
   CHECK(key);
 

[fgalligan1, 2012/06/27 00:28:36]

Maybe add a TODO to support a key change?

[strobe_, 2012/06/27 02:01:21]

Not sure what a key change means in this context.

+  base::StringPiece iv(
+      reinterpret_cast<const char*>(input.GetDecryptConfig()->iv()),
+      input.GetDecryptConfig()->iv_size());
+
   // Initialize encryption data.
   // The IV must be exactly as long as the cipher block size.
   crypto::Encryptor encryptor;
-  if (!encryptor.Init(key, crypto::Encryptor::CBC, kInitialCounter)) {
-    DVLOG(1) << "Could not initialize encryptor.";
-    return NULL;
+  if (input.GetDecryptConfig()->use_cbc()) {
+    if (!encryptor.Init(key, crypto::Encryptor::CBC, iv)) {
+      DVLOG(1) << "Could not initialize encryptor.";
+      return NULL;
+    }
+  } else {
+    if (!(encryptor.Init(key, crypto::Encryptor::CTR, base::StringPiece()) &&

[ddorwin, 2012/06/26 06:09:19]

Suggest: !foo || !bar

[strobe_, 2012/06/27 02:01:21]

Done.

+          encryptor.SetCounter(iv))) {
+      DVLOG(1) << "Could not initialize encryptor.";
+      return NULL;
+    }
   }
 
-  std::string decrypted_text;
-  base::StringPiece encrypted_text(
-      reinterpret_cast<const char*>(input.GetData()),
-      input.GetDataSize());
-  if (!encryptor.Decrypt(encrypted_text, &decrypted_text)) {
-    DVLOG(1) << "Could not decrypt data.";
-    return NULL;
-  }
+  if (input.GetDecryptConfig()->subsample_count()) {

[ddorwin, 2012/06/26 06:09:19]

Extract function for this case please. Then test it well!

[strobe_, 2012/06/27 02:01:21]

Will do in next patch set.

+    int size = 0;

[ddorwin, 2012/06/26 06:09:19]

maybe total_subsample_size

[strobe_, 2012/06/27 02:01:21]

Done.

+    for (int i = 0; i < input.GetDecryptConfig()->subsample_count(); i++)

[ddorwin, 2012/06/26 06:09:19]

This would be easier to read if you had aliases for subsamples and
subsample_count.

[strobe_, 2012/06/27 02:01:21]

Done.

+      size += input.GetDecryptConfig()->subsamples()[i].cypher_bytes;
+    if (size > input.GetDataSize()) {

[ddorwin, 2012/06/26 06:09:19]

The input only contains encrypted bytes? Shouldn't the unencrypted bytes be
counted too?
Should it ever be anything other than == once we've counted all?

[strobe_, 2012/06/27 02:01:21]

Input contains both encrypted and unencrypted bytes. Did change to equality
test.

[ddorwin, 2012/07/03 21:03:46]

It's still >, but I'm not sure you meant to/can change it.

[strobe_, 2012/07/13 00:47:07]

Yes, you're correct, the comment got out of sync with the code. Hopefully
variable names make that clearer.

+      DVLOG(1) << "Subsample encrypted size larger than input size";
+      return NULL;
+    }
 
-  // TODO(xhwang): Find a way to avoid this data copy.
-  return DecoderBuffer::CopyFrom(
-      reinterpret_cast<const uint8*>(decrypted_text.data()),
-      decrypted_text.size());
+    scoped_array<uint8> encrypted_bytes(new uint8[size]);

[ddorwin, 2012/06/26 06:09:19]

Please explain what this code does. Copies encrypted bytes to be back-to-back
then decrypts them and copies them back?

[strobe_, 2012/06/27 02:01:21]

Precisely. Comment added.

+    int in_pos = 0, out_pos = 0;
+    for (int i = 0; i < input.GetDecryptConfig()->subsample_count(); i++) {
+      SubsampleEntry subsample = input.GetDecryptConfig()->subsamples()[i];
+      in_pos += subsample.clear_bytes;
+      if (in_pos + subsample.cypher_bytes > input.GetDataSize()) {
+        DVLOG(1) << "Subsample total size larger than input size";
+        return NULL;
+      }
+      memcpy(encrypted_bytes.get() + out_pos, input.GetData() + in_pos,
+             subsample.cypher_bytes);
+      in_pos += subsample.cypher_bytes;
+      out_pos += subsample.cypher_bytes;
+    }
+
+    std::string decrypted_text;
+    base::StringPiece encrypted_text(
+      reinterpret_cast<const char*>(encrypted_bytes.get()), size);
+    if (!encryptor.Decrypt(encrypted_text, &decrypted_text)) {
+      DVLOG(1) << "Could not decrypt data.";
+      return NULL;
+    }
+    encrypted_bytes.reset();

[ddorwin, 2012/06/26 06:09:19]

Why explicitly here? And while encrypted_text is still valid and could still be
referenced.

[strobe_, 2012/06/27 02:01:21]

Removed. (Original reason was to minimize number of live objects on
memory-constrained devices, but prefer safety over optimization.)

+
+    scoped_refptr<DecoderBuffer> output = DecoderBuffer::CopyFrom(

[ddorwin, 2012/06/26 06:09:19]

We've copied at least twice now. Are there any other ways we could do this?

[strobe_, 2012/06/27 02:01:21]

I didn't find any good ways without modifying Encryptor, but I'll take another
pass.

One desperate strategy would be (if I understand CTR correctly) to decrypt a
buffer of zeros of equal size to the encrypted bytes, and then manually XOR the
bytes in place, but this seems unwise.

+        input.GetData(), input.GetDataSize());
+    in_pos = 0;
+    out_pos = 0;
+    for (int i = 0; i < input.GetDecryptConfig()->subsample_count(); i++) {
+      SubsampleEntry subsample = input.GetDecryptConfig()->subsamples()[i];
+      out_pos += subsample.clear_bytes;
+      memcpy(output->GetWritableData() + out_pos,
+             reinterpret_cast<const uint8*>(decrypted_text.data()) + in_pos,
+             subsample.cypher_bytes);
+      in_pos += subsample.cypher_bytes;
+      out_pos += subsample.cypher_bytes;
+    }
+    return output;
+  } else {
+    std::string decrypted_text;
+    base::StringPiece encrypted_text(
+        reinterpret_cast<const char*>(input.GetData()),
+        input.GetDataSize());
+    if (!encryptor.Decrypt(encrypted_text, &decrypted_text)) {
+      DVLOG(1) << "Could not decrypt data.";
+      return NULL;
+    }
+
+    // TODO(xhwang): Find a way to avoid this data copy.
+    return DecoderBuffer::CopyFrom(
+        reinterpret_cast<const uint8*>(decrypted_text.data()),
+        decrypted_text.size());
+  }
 }
 
 AesDecryptor::AesDecryptor(DecryptorClient* client)