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

media/crypto/hmac_aes_decryptor.cc

+// Copyright (c) 2012 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "media/crypto/hmac_aes_decryptor.h"
+
+#include "base/logging.h"
+#include "base/stl_util.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"
+
+namespace media {
+
+// Derives a key from an HAMC using SHA1. |secret| is the base secret to derive
+// the key from. |seed| is the knwon input to the HMAC. |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 std::string& secret,
+                             const std::string& seed,
+                             int key_size) {
+  CHECK_EQ(secret.length(), static_cast<size_t>(DecryptConfig::kKeySize));
+  CHECK(!seed.empty());
+  CHECK_GT(key_size, 0);
+
+  std::string key;
+  crypto::HMAC hmac(crypto::HMAC::SHA1);
+  if (!hmac.Init(reinterpret_cast<const uint8*>(secret.data()),
+                 secret.size())) {
+    DVLOG(1) << "Could not initialize HMAC with secret data.";
+    return key;
+  }
+
+  uint8 calculated_hmac[HmacAesDecryptor::kSha1DigestSize];
+  if (!hmac.Sign(seed, calculated_hmac, HmacAesDecryptor::kSha1DigestSize)) {
+    DVLOG(1) << "Could not calculate HMAC.";
+    return key;
+  }
+  key.assign(reinterpret_cast<const char*>(calculated_hmac), key_size);
+  return key;
+}
+
+// Integrity check of |input|'s data. Checks that the first
+// |kWebMIntegrityCheckSize| in bytes of |ipunt| matches the rest of the data
+// in |input|. The check is defined in the WebM specification. Current WebM
+// encrypted request for comments specification is here
+// http://wiki.webmproject.org/encryption/webm-encryption-rfc.
+// |hmac_key| is the key of the HMAC algorithm. Returns true if the integrity
+// check is good.
+static bool CheckData(const DecoderBuffer& input,
+                      const std::string& hmac_key) {
+  CHECK(input.GetDataSize());
+  CHECK(input.GetDecryptConfig());
+  CHECK(!hmac_key.empty());
+
+  crypto::HMAC hmac(crypto::HMAC::SHA1);
+  if (!hmac.Init(reinterpret_cast<const uint8*>(hmac_key.data()),
+                 hmac_key.size())) {
+    DVLOG(1) << "Could not initialize HMAC.";
+    return false;
+  }
+
+  // The HMAC covers the IV and the frame data.
+  const char* iv_frame = reinterpret_cast<const char*>(input.GetData() +
+      DecryptConfig::kWebMIntegrityCheckSize);
+  int iv_frame_size =
+      input.GetDataSize() - DecryptConfig::kWebMIntegrityCheckSize;
+  std::string data_to_check(iv_frame, iv_frame_size);
+
+  uint8 calculated_hmac[HmacAesDecryptor::kSha1DigestSize];
+  if (!hmac.Sign(data_to_check,
+                 calculated_hmac,
+                 HmacAesDecryptor::kSha1DigestSize)) {
+    DVLOG(1) << "Could not calculate HMAC.";
+    return false;
+  }
+
+  if (memcmp(input.GetDecryptConfig()->hmac(),
+             calculated_hmac,
+             input.GetDecryptConfig()->hmac_size()) != 0) {
+    DVLOG(1) << "Integrity check failure.";
+    return false;
+  }
+  return true;
+}
+
+const char HmacAesDecryptor::kHmacSeed[] = "hmac-key";
+const char HmacAesDecryptor::kEncryptionSeed[] = "encryption-key";
+
+HmacAesDecryptor::HmacAesDecryptor() {}
+
+HmacAesDecryptor::~HmacAesDecryptor() {
+  STLDeleteValues(&keys_map_);
+}
+
+void HmacAesDecryptor::AddKey(const uint8* key_id, int key_id_size,
+                              const uint8* key, int key_size) {
+  CHECK(key_id && key);
+  CHECK_GT(key_id_size, 0);
+  CHECK_GT(key_size, 0);
+
+  std::string key_id_string(reinterpret_cast<const char*>(key_id), key_id_size);
+  std::string key_string(reinterpret_cast<const char*>(key) , key_size);
+
+  HmacEncryptionKeys* keys = new HmacEncryptionKeys(key_string);
+  if (!keys) {
+    DVLOG(1) << "Could not create keys.";
+    return;
+  }
+  if (!keys->Init()) {
+    delete keys;
+    DVLOG(1) << "Could not create keys.";
+    return;
+  }
+
+  base::AutoLock auto_lock(lock_);
+  KeysMap::iterator found = keys_map_.find(key_id_string);
+  if (found != keys_map_.end()) {
+    delete found->second;
+    keys_map_.erase(found);
+  }
+  keys_map_[key_id_string] = keys;
+}
+
+scoped_refptr<DecoderBuffer> HmacAesDecryptor::Decrypt(
+    const scoped_refptr<DecoderBuffer>& encrypted) {
+  CHECK(encrypted->GetDecryptConfig());
+  const uint8* key_id = encrypted->GetDecryptConfig()->key_id();
+  const int key_id_size = encrypted->GetDecryptConfig()->key_id_size();
+
+  // TODO(xhwang): Avoid always constructing a string with StringPiece?
+  std::string key_id_string(reinterpret_cast<const char*>(key_id), key_id_size);
+
+  HmacEncryptionKeys* keys = NULL;
+  {
+    base::AutoLock auto_lock(lock_);
+    KeysMap::const_iterator found = keys_map_.find(key_id_string);
+    if (found == keys_map_.end()) {
+      DVLOG(1) << "Could not find a matching key for given key ID.";
+      return NULL;
+    }
+    keys = found->second;
+  }
+
+  if (!CheckData(*encrypted, keys->hmac_key())) {
+    DVLOG(1) << "Integrity check failed.";
+    return NULL;
+  }
+
+  scoped_refptr<DecoderBuffer> decrypted =
+      Decryptor::DecryptData(*encrypted,
+                             keys->encryption_key(),
+                             DecryptConfig::kWebMIntegrityCheckSize +
+                                 DecryptConfig::kIvSize);
+  if (decrypted) {
+    decrypted->SetTimestamp(encrypted->GetTimestamp());
+    decrypted->SetDuration(encrypted->GetDuration());
+  }
+
+  return decrypted;
+}
+
+HmacAesDecryptor::HmacEncryptionKeys::HmacEncryptionKeys(
+    const std::string& secret)
+    : secret_(secret) {
+}
+
+HmacAesDecryptor::HmacEncryptionKeys::~HmacEncryptionKeys() {}
+
+bool HmacAesDecryptor::HmacEncryptionKeys::Init() {
+  CHECK(!secret_.empty());
+
+  std::string raw_key = DeriveKey(secret_,
+                                  kEncryptionSeed,
+                                  DecryptConfig::kKeySize);
+  if (raw_key.empty()) {
+    DVLOG(1) << "Could not create encryption key.";
+    return false;
+  }
+  encryption_key_.reset(crypto::SymmetricKey::Import(crypto::SymmetricKey::AES,
+                                                     raw_key));
+  if (!encryption_key_.get()) {
+    DVLOG(1) << "Could not create encryption key.";
+    return false;
+  }
+
+  hmac_key_ = DeriveKey(secret_, kHmacSeed, kSha1DigestSize);
+  if (hmac_key_.empty()) {
+    DVLOG(1) << "Could not create HMAC key.";
+    return false;
+  }
+  return true;
+}
+
+}  // namespace media