diff --git a/common/prefs.cc b/common/prefs.cc
new file mode 100644
index 0000000..9d3a30f
--- /dev/null
+++ b/common/prefs.cc
@@ -0,0 +1,143 @@
+//
+// Copyright (C) 2012 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+#include "update_engine/common/prefs.h"
+
+#include <algorithm>
+
+#include <base/files/file_util.h>
+#include <base/logging.h>
+#include <base/strings/string_number_conversions.h>
+#include <base/strings/string_util.h>
+
+#include "update_engine/common/utils.h"
+
+using std::string;
+
+namespace chromeos_update_engine {
+
+bool Prefs::Init(const base::FilePath& prefs_dir) {
+  prefs_dir_ = prefs_dir;
+  return true;
+}
+
+bool Prefs::GetString(const string& key, string* value) const {
+  base::FilePath filename;
+  TEST_AND_RETURN_FALSE(GetFileNameForKey(key, &filename));
+  if (!base::ReadFileToString(filename, value)) {
+    LOG(INFO) << key << " not present in " << prefs_dir_.value();
+    return false;
+  }
+  return true;
+}
+
+bool Prefs::SetString(const string& key, const string& value) {
+  base::FilePath filename;
+  TEST_AND_RETURN_FALSE(GetFileNameForKey(key, &filename));
+  if (!base::DirectoryExists(filename.DirName())) {
+    // Only attempt to create the directory if it doesn't exist to avoid calls
+    // to parent directories where we might not have permission to write to.
+    TEST_AND_RETURN_FALSE(base::CreateDirectory(filename.DirName()));
+  }
+  TEST_AND_RETURN_FALSE(base::WriteFile(filename, value.data(), value.size()) ==
+                        static_cast<int>(value.size()));
+  const auto observers_for_key = observers_.find(key);
+  if (observers_for_key != observers_.end()) {
+    std::vector<ObserverInterface*> copy_observers(observers_for_key->second);
+    for (ObserverInterface* observer : copy_observers)
+      observer->OnPrefSet(key);
+  }
+  return true;
+}
+
+bool Prefs::GetInt64(const string& key, int64_t* value) const {
+  string str_value;
+  if (!GetString(key, &str_value))
+    return false;
+  base::TrimWhitespaceASCII(str_value, base::TRIM_ALL, &str_value);
+  TEST_AND_RETURN_FALSE(base::StringToInt64(str_value, value));
+  return true;
+}
+
+bool Prefs::SetInt64(const string& key, const int64_t value) {
+  return SetString(key, base::Int64ToString(value));
+}
+
+bool Prefs::GetBoolean(const string& key, bool* value) const {
+  string str_value;
+  if (!GetString(key, &str_value))
+    return false;
+  base::TrimWhitespaceASCII(str_value, base::TRIM_ALL, &str_value);
+  if (str_value == "false") {
+    *value = false;
+    return true;
+  }
+  if (str_value == "true") {
+    *value = true;
+    return true;
+  }
+  return false;
+}
+
+bool Prefs::SetBoolean(const string& key, const bool value) {
+  return SetString(key, value ? "true" : "false");
+}
+
+bool Prefs::Exists(const string& key) const {
+  base::FilePath filename;
+  TEST_AND_RETURN_FALSE(GetFileNameForKey(key, &filename));
+  return base::PathExists(filename);
+}
+
+bool Prefs::Delete(const string& key) {
+  base::FilePath filename;
+  TEST_AND_RETURN_FALSE(GetFileNameForKey(key, &filename));
+  TEST_AND_RETURN_FALSE(base::DeleteFile(filename, false));
+  const auto observers_for_key = observers_.find(key);
+  if (observers_for_key != observers_.end()) {
+    std::vector<ObserverInterface*> copy_observers(observers_for_key->second);
+    for (ObserverInterface* observer : copy_observers)
+      observer->OnPrefDeleted(key);
+  }
+  return true;
+}
+
+void Prefs::AddObserver(const string& key, ObserverInterface* observer) {
+  observers_[key].push_back(observer);
+}
+
+void Prefs::RemoveObserver(const string& key, ObserverInterface* observer) {
+  std::vector<ObserverInterface*>& observers_for_key = observers_[key];
+  auto observer_it =
+      std::find(observers_for_key.begin(), observers_for_key.end(), observer);
+  if (observer_it != observers_for_key.end())
+    observers_for_key.erase(observer_it);
+}
+
+bool Prefs::GetFileNameForKey(const string& key,
+                              base::FilePath* filename) const {
+  // Allows only non-empty keys containing [A-Za-z0-9_-].
+  TEST_AND_RETURN_FALSE(!key.empty());
+  for (size_t i = 0; i < key.size(); ++i) {
+    char c = key.at(i);
+    TEST_AND_RETURN_FALSE(IsAsciiAlpha(c) || IsAsciiDigit(c) ||
+                          c == '_' || c == '-');
+  }
+  *filename = prefs_dir_.Append(key);
+  return true;
+}
+
+}  // namespace chromeos_update_engine