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..8910d278945e5bd8d149c7003705e450428c5ff1 100644
--- a/media/crypto/aes_decryptor.cc
+++ b/media/crypto/aes_decryptor.cc
@@ -16,11 +16,24 @@
 
 namespace media {
 
-// TODO(xhwang): Get real IV from frames.
-static const char kInitialCounter[] = "0000000000000000";
-
 uint32 AesDecryptor::next_session_id_ = 1;
 
+static void CopySubsamples(const std::vector<SubsampleEntry>& subsamples,
+                           const bool src_contains_clear_bytes,

[ddorwin, 2012/07/17 01:14:21]

Consider an enum since it's not clear that false means dst_contains... See
http://www.chromium.org/developers/coding-style#Code_Formatting.

[strobe_, 2012/07/19 02:43:35]

Done.

+                           const uint8* src, uint8* dst) {

[ddorwin, 2012/07/17 01:14:21]

same line only for call sites, not defn/decl.

[strobe_, 2012/07/19 02:43:35]

Done.

+  for (size_t i = 0; i < subsamples.size(); i++) {
+    const SubsampleEntry& subsample = subsamples[i];
+    if (src_contains_clear_bytes) {
+      src += subsample.clear_bytes;
+    } else {
+      dst += subsample.clear_bytes;
+    }
+    memcpy(dst, src, subsample.cypher_bytes);

[ddorwin, 2012/07/17 01:14:21]

Should we verify we don't exceed either buffer size before copying?

[strobe_, 2012/07/19 02:43:35]

Since this is a static internal helper function, it might be okay to rely on the
checks done by the caller. WDYT?

+    src += subsample.cypher_bytes;
+    dst += subsample.cypher_bytes;
+  }
+}
+
 // Decrypt |input| using |key|.
 // Return a DecoderBuffer with the decrypted data if decryption succeeded.
 // Return NULL if decryption failed.
@@ -31,10 +44,61 @@ static scoped_refptr<DecoderBuffer> DecryptData(const DecoderBuffer& input,
 
   // Initialize encryption data.
   // The IV must be exactly as long as the cipher block size.
+  const std::string& iv = input.GetDecryptConfig()->iv();
   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()) ||
+        !encryptor.SetCounter(iv)) {
+      DVLOG(1) << "Could not initialize encryptor.";
+      return NULL;
+    }
+  }
+
+  if (!input.GetDecryptConfig()->subsamples().empty()) {

[ddorwin, 2012/07/17 01:14:21]

Prefer positive logic (unless there's a good reason not to). In this case, it is
essentially if-else and neither is an error or early return. This also makes the
if block smaller.

[strobe_, 2012/07/19 02:43:35]

Done.

+    const std::vector<SubsampleEntry>& subsamples =
+        input.GetDecryptConfig()->subsamples();
+
+    int total_clear_size = 0;
+    int total_encrypted_size = 0;
+    for (size_t i = 0; i < subsamples.size(); i++) {
+      total_clear_size += subsamples[i].clear_bytes;
+      total_encrypted_size += subsamples[i].cypher_bytes;
+    }
+    if (total_clear_size + total_encrypted_size != input.GetDataSize()) {
+      DVLOG(1) << "Subsample sizes do not equal input size";
+      return NULL;
+    }
+
+    // The encrypted portions of all subsamples must form a contiguous block,
+    // such that an encrypted subsample that ends away from a block boundary is
+    // immediately followed by the start of the next encrypted subsample. We
+    // copy all encrypted subsamples to a contiguous buffer, decrypt them, then
+    // copy the decrypted bytes over the encrypted bytes in the output.
+    // TODO(strobe): attempt to reduce number of memory copies
+    scoped_array<uint8> encrypted_bytes(
+        new uint8[total_encrypted_size]);
+    CopySubsamples(subsamples, true, input.GetData(), encrypted_bytes.get());
+
+    std::string decrypted_text;
+    base::StringPiece encrypted_text(
+        reinterpret_cast<const char*>(encrypted_bytes.get()),
+        total_encrypted_size);
+    if (!encryptor.Decrypt(encrypted_text, &decrypted_text)) {
+      DVLOG(1) << "Could not decrypt data.";
+      return NULL;
+    }
+
+    scoped_refptr<DecoderBuffer> output = DecoderBuffer::CopyFrom(
+        input.GetData(), input.GetDataSize());
+    CopySubsamples(subsamples, false,
+                   reinterpret_cast<const uint8*>(decrypted_text.data()),
+                   output->GetWritableData());
+    return output;
   }
 
   std::string decrypted_text;