Add support for encrypted WebM files as defined in the RFC.

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 f677f0d4206f186683b7c30d36fa4eb1f7b117c7..e69f4062a8afb3f00ee9d609a19785da40269efe 100644
--- a/media/crypto/aes_decryptor.cc
+++ b/media/crypto/aes_decryptor.cc
@@ -7,8 +7,8 @@
 #include "base/logging.h"
 #include "base/stl_util.h"
 #include "base/string_number_conversions.h"
-#include "base/string_piece.h"
 #include "crypto/encryptor.h"
+#include "crypto/hmac.h"
 #include "crypto/symmetric_key.h"
 #include "media/base/decoder_buffer.h"
 #include "media/base/decrypt_config.h"
@@ -16,31 +16,111 @@
 
 namespace media {
 
-// TODO(xhwang): Get real IV from frames.
-static const char kInitialCounter[] = "0000000000000000";
+// The size is from the WebM encrypted specification. Current encrypted WebM
+// request for comments specification is here
+// http://wiki.webmproject.org/encryption/webm-encryption-rfc
+static const int kWebmSha1DigestSize = 20;
+static const char kWebmHmacSeed[] = "hmac-key";
+static const char kWebmEncryptionSeed[] = "encryption-key";
 
 uint32 AesDecryptor::next_session_id_ = 1;
 
-// Decrypt |input| using |key|.
-// Return a DecoderBuffer with the decrypted data if decryption succeeded.
-// Return NULL if decryption failed.
+// Derives a key using SHA1 HMAC. |secret| is the base secret to derive
+// the key from. |seed| is the known message to the HMAC algorithm. |key_size|
+// is how many bytes are returned in the key. Returns a string containing the
+// key on success. Returns an empty string on failure.
+static std::string DeriveKey(const base::StringPiece& secret,
+                             const base::StringPiece& seed,
+                             int key_size) {
+  CHECK(!secret.empty());
+  CHECK(!seed.empty());
+  CHECK_GT(key_size, 0);
+
+  crypto::HMAC hmac(crypto::HMAC::SHA1);
+  if (!hmac.Init(secret)) {
+    DVLOG(1) << "Could not initialize HMAC with secret data.";
+    return std::string();
+  }
+
+  scoped_array<uint8> calculated_hmac(new uint8[hmac.DigestLength()]);
+  if (!hmac.Sign(seed, calculated_hmac.get(), hmac.DigestLength())) {
+    DVLOG(1) << "Could not calculate HMAC.";
+    return std::string();
+  }
+
+  return std::string(reinterpret_cast<const char*>(calculated_hmac.get()),
+                     key_size);
+}
+
+// Checks data in |input| matches the HMAC in |input|. The check is using the
+// SHA1 algorithm. |hmac_key| is the key of the HMAC algorithm. Returns true if
+// the integrity check passes.
+static bool CheckData(const DecoderBuffer& input,
+                      const base::StringPiece& hmac_key) {
+  CHECK(input.GetDataSize());
+  CHECK(input.GetDecryptConfig());
+  CHECK_GT(input.GetDecryptConfig()->checksum_size(), 0);
+  CHECK(!hmac_key.empty());
+
+  crypto::HMAC hmac(crypto::HMAC::SHA1);
+  if (!hmac.Init(hmac_key))
+    return false;
+
+  // The HMAC covers the IV and the frame data.
+  base::StringPiece data_to_check(
+      reinterpret_cast<const char*>(input.GetData()), input.GetDataSize());
+
+  scoped_array<uint8> calculated_hmac(new uint8[hmac.DigestLength()]);
+  if (!hmac.Sign(data_to_check, calculated_hmac.get(), hmac.DigestLength()))
+    return false;
+
+  DCHECK(input.GetDecryptConfig()->checksum_size() <=
+         static_cast<int>(hmac.DigestLength()));
+  if (memcmp(input.GetDecryptConfig()->checksum(),
+             calculated_hmac.get(),
+             input.GetDecryptConfig()->checksum_size()) != 0)

[Ryan Sleevi, 2012/07/23 21:50:59]

Note: crypto::HMAC specifically exposes a Verify method to avoid timing issues
with memcmp that may leak keying material.

+    return false;
+  return true;
+}
+
+// Decrypts |input| using |key|. |encrypted_data_offset| is the number of bytes
+// into |input| that the encrypted data starts.
+// Returns a DecoderBuffer with the decrypted data if decryption succeeded or
+// NULL if decryption failed.
 static scoped_refptr<DecoderBuffer> DecryptData(const DecoderBuffer& input,
-                                                crypto::SymmetricKey* key) {
+                                                crypto::SymmetricKey* key,
+                                                int encrypted_data_offset) {
   CHECK(input.GetDataSize());
+  CHECK(input.GetDecryptConfig());
   CHECK(key);
 
-  // Initialize encryption data.
-  // The IV must be exactly as long as the cipher block size.
+  // Initialize decryptor.
   crypto::Encryptor encryptor;
-  if (!encryptor.Init(key, crypto::Encryptor::CBC, kInitialCounter)) {
-    DVLOG(1) << "Could not initialize encryptor.";
+  if (!encryptor.Init(key, crypto::Encryptor::CTR, "")) {
+    DVLOG(1) << "Could not initialize decryptor.";
+    return NULL;
+  }
+
+  DCHECK_EQ(input.GetDecryptConfig()->iv_size(),
+            DecryptConfig::kDecryptionKeySize);
+  // Set the counter block.
+  base::StringPiece counter_block(
+      reinterpret_cast<const char*>(input.GetDecryptConfig()->iv()),
+      input.GetDecryptConfig()->iv_size());
+  if (counter_block.empty()) {
+    DVLOG(1) << "Could not generate counter block.";
+    return NULL;
+  }
+  if (!encryptor.SetCounter(counter_block)) {
+    DVLOG(1) << "Could not set counter block.";
     return NULL;
   }
 
   std::string decrypted_text;
-  base::StringPiece encrypted_text(
-      reinterpret_cast<const char*>(input.GetData()),
-      input.GetDataSize());
+  const char* frame =
+      reinterpret_cast<const char*>(input.GetData() + encrypted_data_offset);
+  int frame_size = input.GetDataSize() - encrypted_data_offset;
+  base::StringPiece encrypted_text(frame, frame_size);
   if (!encryptor.Decrypt(encrypted_text, &decrypted_text)) {
     DVLOG(1) << "Could not decrypt data.";
     return NULL;
@@ -85,9 +165,7 @@ void AesDecryptor::AddKey(const std::string& key_system,
 
   // TODO(xhwang): Add |session_id| check after we figure out how:
   // https://www.w3.org/Bugs/Public/show_bug.cgi?id=16550
-
-  const int kSupportedKeyLength = 16;  // 128-bit key.
-  if (key_length != kSupportedKeyLength) {
+  if (key_length != DecryptConfig::kDecryptionKeySize) {
     DVLOG(1) << "Invalid key length: " << key_length;
     client_->KeyError(key_system, session_id, Decryptor::kUnknownError, 0);
     return;
@@ -106,10 +184,17 @@ void AesDecryptor::AddKey(const std::string& key_system,
   std::string key_id_string(reinterpret_cast<const char*>(init_data),
                             init_data_length);
   std::string key_string(reinterpret_cast<const char*>(key) , key_length);
-  crypto::SymmetricKey* symmetric_key = crypto::SymmetricKey::Import(
-      crypto::SymmetricKey::AES, key_string);
-  if (!symmetric_key) {
-    DVLOG(1) << "Could not import key.";
+  scoped_ptr<DecryptionKey> decryption_key(new DecryptionKey(key_string));
+  if (!decryption_key.get()) {
+    DVLOG(1) << "Could not create key.";
+    client_->KeyError(key_system, session_id, Decryptor::kUnknownError, 0);
+    return;
+  }
+
+  // TODO(fgalligan): When ISO is added we will need to figure out how to
+  // detect if the encrypted data will contain an HMAC.
+  if (!decryption_key->Init(true)) {
+    DVLOG(1) << "Could not initialize decryption key.";
     client_->KeyError(key_system, session_id, Decryptor::kUnknownError, 0);
     return;
   }
@@ -121,7 +206,7 @@ void AesDecryptor::AddKey(const std::string& key_system,
       delete found->second;
       key_map_.erase(found);
     }
-    key_map_[key_id_string] = symmetric_key;
+    key_map_[key_id_string] = decryption_key.release();
   }
 
   client_->KeyAdded(key_system, session_id);
@@ -140,7 +225,7 @@ void AesDecryptor::Decrypt(const scoped_refptr<DecoderBuffer>& encrypted,
   // TODO(xhwang): Avoid always constructing a string with StringPiece?
   std::string key_id_string(reinterpret_cast<const char*>(key_id), key_id_size);
 
-  crypto::SymmetricKey* key = NULL;
+  DecryptionKey* key = NULL;
   {
     base::AutoLock auto_lock(key_map_lock_);
     KeyMap::const_iterator found = key_map_.find(key_id_string);
@@ -149,13 +234,28 @@ void AesDecryptor::Decrypt(const scoped_refptr<DecoderBuffer>& encrypted,
   }
 
   if (!key) {
+    // TODO(fgalligan): Fire a need_key event here and add a test.
     DVLOG(1) << "Could not find a matching key for given key ID.";
     decrypt_cb.Run(kError, NULL);
     return;
   }
 
-  scoped_refptr<DecoderBuffer> decrypted = DecryptData(*encrypted, key);
+  int checksum_size = encrypted->GetDecryptConfig()->checksum_size();
+  // According to the WebM encrypted specification, it is an open question
+  // what should happen when a frame fails the integrity check.
+  // http://wiki.webmproject.org/encryption/webm-encryption-rfc
+  if (checksum_size > 0 &&
+      !key->hmac_key().empty() &&
+      !CheckData(*encrypted, key->hmac_key())) {
+    DVLOG(1) << "Integrity check failed.";
+    decrypt_cb.Run(kError, NULL);
+    return;
+  }
 
+  scoped_refptr<DecoderBuffer> decrypted =
+      DecryptData(*encrypted,
+                  key->decryption_key(),
+                  encrypted->GetDecryptConfig()->encrypted_frame_offset());
   if (!decrypted) {
     DVLOG(1) << "Decryption failed.";
     decrypt_cb.Run(kError, NULL);
@@ -167,4 +267,42 @@ void AesDecryptor::Decrypt(const scoped_refptr<DecoderBuffer>& encrypted,
   decrypt_cb.Run(kSuccess, decrypted);
 }
 
+AesDecryptor::DecryptionKey::DecryptionKey(
+    const std::string& secret)
+    : secret_(secret) {
+}
+
+AesDecryptor::DecryptionKey::~DecryptionKey() {}
+
+bool AesDecryptor::DecryptionKey::Init(bool derive_webm_keys) {
+  CHECK(!secret_.empty());
+
+  if (derive_webm_keys) {
+    std::string raw_key = DeriveKey(secret_,
+                                    kWebmEncryptionSeed,
+                                    secret_.length());
+    if (raw_key.empty()) {
+      return false;
+    }
+    decryption_key_.reset(
+        crypto::SymmetricKey::Import(crypto::SymmetricKey::AES, raw_key));
+    if (!decryption_key_.get()) {
+      return false;
+    }
+
+    hmac_key_ = DeriveKey(secret_, kWebmHmacSeed, kWebmSha1DigestSize);
+    if (hmac_key_.empty()) {
+      return false;
+    }
+    return true;
+  }
+
+  decryption_key_.reset(
+      crypto::SymmetricKey::Import(crypto::SymmetricKey::AES, secret_));
+  if (!decryption_key_.get()) {
+    return false;
+  }
+  return true;
+}
+
 }  // namespace media