cryptohome: Move Encrypt/DecryptWithSystemSalt() out of CryptohomeLibrary

chrome/browser/chromeos/settings/token_encryptor.cc

+// Copyright 2013 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 "chrome/browser/chromeos/settings/token_encryptor.h"
+
+#include <vector>
+
+#include "base/logging.h"
+#include "base/strings/string_number_conversions.h"
+#include "base/strings/string_util.h"
+#include "base/sys_info.h"
+#include "chromeos/cryptohome/cryptohome_library.h"
+#include "crypto/encryptor.h"
+#include "crypto/nss_util.h"
+#include "crypto/sha2.h"
+#include "crypto/symmetric_key.h"
+
+namespace chromeos {
+
+namespace {
+const size_t kNonceSize = 16;
+}  // namespace
+
+CryptohomeTokenEncryptor::CryptohomeTokenEncryptor() {
+}
+
+CryptohomeTokenEncryptor::~CryptohomeTokenEncryptor() {
+}
+
+std::string CryptohomeTokenEncryptor::EncryptWithSystemSalt(
+    const std::string& token) {
+  // Don't care about token encryption while debugging.
+  if (!base::SysInfo::IsRunningOnChromeOS())
+    return token;
+
+  if (!LoadSystemSaltKey()) {
+    LOG(WARNING) << "System salt key is not available for encrypt.";
+    return std::string();
+  }
+  return EncryptTokenWithKey(system_salt_key_.get(),
+                             system_salt_,
+                             token);
+}
+
+std::string CryptohomeTokenEncryptor::DecryptWithSystemSalt(
+    const std::string& encrypted_token_hex) {
+  // Don't care about token encryption while debugging.
+  if (!base::SysInfo::IsRunningOnChromeOS())
+    return encrypted_token_hex;
+
+  if (!LoadSystemSaltKey()) {
+    LOG(WARNING) << "System salt key is not available for decrypt.";
+    return std::string();
+  }
+  return DecryptTokenWithKey(system_salt_key_.get(),
+                             system_salt_,
+                             encrypted_token_hex);
+}
+
+// TODO: should this use the system salt for both the password and the salt
+// value, or should this use a separate salt value?
+bool CryptohomeTokenEncryptor::LoadSystemSaltKey() {
+  if (system_salt_.empty())
+    system_salt_ = CryptohomeLibrary::Get()->GetSystemSalt();

[satorux1, 2013/10/04 06:11:14]

As mentioned in the patch description, the two lines are the only difference
from the original code in cryptohome_library.cc

[hashimoto, 2013/10/04 06:22:28]

I don't think we should add a new user of CryptohomeLibrary::GetSystemSalt here
because the method may make D-Bus method call synchronously.

[satorux1, 2013/10/04 06:44:01]

That's a good point. In the original code, there was a hidden dependency between
LoadSystemSalt() and LoadSystemSaltKey(), where LoadSystemSaltKey() expects that
LoadSystemSalt() is called beforehand. I guess it's guaranteed that
GetSystemSalt() is called beforehand at login time.

That said, I think we need a function that simply returns a cached system salt
here. Added CryptohomeLibrary::GetCachedSystemSalt() and used it here.

+  if (system_salt_.empty())
+    return false;
+  if (!system_salt_key_.get())
+    system_salt_key_.reset(PassphraseToKey(system_salt_, system_salt_));
+  return system_salt_key_.get();
+}
+
+crypto::SymmetricKey* CryptohomeTokenEncryptor::PassphraseToKey(
+    const std::string& passphrase,
+    const std::string& salt) {
+  return crypto::SymmetricKey::DeriveKeyFromPassword(
+      crypto::SymmetricKey::AES, passphrase, salt, 1000, 256);
+}
+
+
+// Encrypts (AES) the token given |key| and |salt|.
+std::string CryptohomeTokenEncryptor::EncryptTokenWithKey(
+    crypto::SymmetricKey* key,
+    const std::string& salt,
+    const std::string& token) {
+  crypto::Encryptor encryptor;
+  if (!encryptor.Init(key, crypto::Encryptor::CTR, std::string())) {
+    LOG(WARNING) << "Failed to initialize Encryptor.";
+    return std::string();
+  }
+  std::string nonce = salt.substr(0, kNonceSize);
+  std::string encoded_token;
+  CHECK(encryptor.SetCounter(nonce));
+  if (!encryptor.Encrypt(token, &encoded_token)) {
+    LOG(WARNING) << "Failed to encrypt token.";
+    return std::string();
+  }
+
+  return StringToLowerASCII(base::HexEncode(
+      reinterpret_cast<const void*>(encoded_token.data()),
+      encoded_token.size()));
+}
+
+// Decrypts (AES) hex encoded encrypted token given |key| and |salt|.
+std::string CryptohomeTokenEncryptor::DecryptTokenWithKey(
+    crypto::SymmetricKey* key,
+    const std::string& salt,
+    const std::string& encrypted_token_hex) {
+  std::vector<uint8> encrypted_token_bytes;
+  if (!base::HexStringToBytes(encrypted_token_hex, &encrypted_token_bytes)) {
+    LOG(WARNING) << "Corrupt encrypted token found.";
+    return std::string();
+  }
+
+  std::string encrypted_token(
+      reinterpret_cast<char*>(encrypted_token_bytes.data()),
+      encrypted_token_bytes.size());
+  crypto::Encryptor encryptor;
+  if (!encryptor.Init(key, crypto::Encryptor::CTR, std::string())) {
+    LOG(WARNING) << "Failed to initialize Encryptor.";
+    return std::string();
+  }
+
+  std::string nonce = salt.substr(0, kNonceSize);
+  std::string token;
+  CHECK(encryptor.SetCounter(nonce));
+  if (!encryptor.Decrypt(encrypted_token, &token)) {
+    LOG(WARNING) << "Failed to decrypt token.";
+    return std::string();
+  }
+  return token;
+}
+
+}  // namespace chromeos