components/user_prefs/tracked/pref_hash_calculator.cc

   1 // Copyright 2013 The Chromium Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file.
   4
   5 #include "components/user_prefs/tracked/pref_hash_calculator.h"
   6
   7 #include <vector>
   8
   9 #include "base/bind.h"
  10 #include "base/json/json_string_value_serializer.h"
  11 #include "base/logging.h"
  12 #include "base/strings/string_number_conversions.h"
  13 #include "base/strings/string_util.h"
  14 #include "base/values.h"
  15 #include "crypto/hmac.h"
  16
  17 namespace {
  18
  19 // Calculates an HMAC of |message| using |key|, encoded as a hexadecimal string.
  20 std::string GetDigestString(const std::string& key,
  21                             const std::string& message) {
  22   crypto::HMAC hmac(crypto::HMAC::SHA256);
  23   std::vector<uint8> digest(hmac.DigestLength());
  24   if (!hmac.Init(key) || !hmac.Sign(message, &digest[0], digest.size())) {
  25     NOTREACHED();
  26     return std::string();
  27   }
  28   return base::HexEncode(&digest[0], digest.size());
  29 }
  30
  31 // Verifies that |digest_string| is a valid HMAC of |message| using |key|.
  32 // |digest_string| must be encoded as a hexadecimal string.
  33 bool VerifyDigestString(const std::string& key,
  34                         const std::string& message,
  35                         const std::string& digest_string) {
  36   crypto::HMAC hmac(crypto::HMAC::SHA256);
  37   std::vector<uint8> digest;
  38   return base::HexStringToBytes(digest_string, &digest) && hmac.Init(key) &&
  39          hmac.Verify(message,
  40                      base::StringPiece(reinterpret_cast<char*>(&digest[0]),
  41                                        digest.size()));
  42 }
  43
  44 // Renders |value| as a string. |value| may be NULL, in which case the result
  45 // is an empty string. This method can be expensive and its result should be
  46 // re-used rather than recomputed where possible.
  47 std::string ValueAsString(const base::Value* value) {
  48   // Dictionary values may contain empty lists and sub-dictionaries. Make a
  49   // deep copy with those removed to make the hash more stable.
  50   const base::DictionaryValue* dict_value;
  51   scoped_ptr<base::DictionaryValue> canonical_dict_value;
  52   if (value && value->GetAsDictionary(&dict_value)) {
  53     canonical_dict_value = dict_value->DeepCopyWithoutEmptyChildren();
  54     value = canonical_dict_value.get();
  55   }
  56
  57   std::string value_as_string;
  58   if (value) {
  59     JSONStringValueSerializer serializer(&value_as_string);
  60     serializer.Serialize(*value);
  61   }
  62
  63   return value_as_string;
  64 }
  65
  66 // Concatenates |device_id|, |path|, and |value_as_string| to give the hash
  67 // input.
  68 std::string GetMessage(const std::string& device_id,
  69                        const std::string& path,
  70                        const std::string& value_as_string) {
  71   std::string message;
  72   message.reserve(device_id.size() + path.size() + value_as_string.size());
  73   message.append(device_id);
  74   message.append(path);
  75   message.append(value_as_string);
  76   return message;
  77 }
  78
  79 // Generates a device ID based on the input device ID. The derived device ID has
  80 // no useful properties beyond those of the input device ID except that it is
  81 // consistent with previous implementations.
  82 // TODO(gab): Remove this once UMA reports for
  83 // Settings.TrackedPreferenceMigratedLegacyDeviceId become insignificant.
  84 std::string GenerateDeviceIdLikePrefMetricsServiceDid(
  85     const std::string& original_device_id) {
  86   if (original_device_id.empty())
  87     return std::string();
  88   return base::ToLowerASCII(
  89       GetDigestString(original_device_id, "PrefMetricsService"));
  90 }
  91
  92 }  // namespace
  93
  94 PrefHashCalculator::PrefHashCalculator(const std::string& seed,
  95                                        const std::string& device_id)
  96     : seed_(seed),
  97       device_id_(device_id),
  98       legacy_device_id_(GenerateDeviceIdLikePrefMetricsServiceDid(device_id)) {
  99 }
 100
 101 PrefHashCalculator::~PrefHashCalculator() {
 102 }
 103
 104 std::string PrefHashCalculator::Calculate(const std::string& path,
 105                                           const base::Value* value) const {
 106   return GetDigestString(seed_,
 107                          GetMessage(device_id_, path, ValueAsString(value)));
 108 }
 109
 110 PrefHashCalculator::ValidationResult PrefHashCalculator::Validate(
 111     const std::string& path,
 112     const base::Value* value,
 113     const std::string& digest_string) const {
 114   const std::string value_as_string(ValueAsString(value));
 115   if (VerifyDigestString(seed_, GetMessage(device_id_, path, value_as_string),
 116                          digest_string)) {
 117     return VALID;
 118   }
 119   if (VerifyDigestString(seed_,
 120                          GetMessage(legacy_device_id_, path, value_as_string),
 121                          digest_string)) {
 122     return VALID_SECURE_LEGACY;
 123   }
 124   return INVALID;
 125 }