content: /*
   * Copyright (C) 2009 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.
   */
  
  package com.android.server;
  
  import java.io.File;
  import java.io.FileNotFoundException;
  import java.io.FileOutputStream;
  import java.io.IOException;
  import java.io.PrintWriter;
  
  import android.content.BroadcastReceiver;
  import android.content.Context;
  import android.content.Intent;
  import android.content.IntentFilter;
  import android.os.Binder;
  import android.os.Environment;
  import android.os.Handler;
  import android.os.Message;
  import android.os.SystemProperties;
  import android.util.Slog;
  
  /**
   * A service designed to load and periodically save &quot;randomness&quot;
   * for the Linux kernel RNG and to mix in data from Hardware RNG (if present)
   * into the Linux RNG.
   *
   * <p>When a Linux system starts up, the entropy pool associated with
   * {@code /dev/random} may be in a fairly predictable state.  Applications which
   * depend strongly on randomness may find {@code /dev/random} or
   * {@code /dev/urandom} returning predictable data.  In order to counteract
   * this effect, it's helpful to carry the entropy pool information across
   * shutdowns and startups.
   *
   * <p>On systems with Hardware RNG (/dev/hw_random), a block of output from HW
   * RNG is mixed into the Linux RNG on EntropyMixer's startup and whenever
   * EntropyMixer periodically runs to save a block of output from Linux RNG on
   * disk. This mixing is done in a way that does not increase the Linux RNG's
   * entropy estimate is not increased. This is to avoid having to trust/verify
   * the quality and authenticity of the &quot;randomness&quot; of the HW RNG.
   *
   * <p>This class was modeled after the script in
   * <a href="http://www.kernel.org/doc/man-pages/online/pages/man4/random.4.html">man
   * 4 random</a>.
   */
  public class EntropyMixer extends Binder {
      private static final String TAG = "EntropyMixer";
      private static final int ENTROPY_WHAT = 1;
      private static final int ENTROPY_WRITE_PERIOD = 3 * 60 * 60 * 1000;  // 3 hrs
      private static final long START_TIME = System.currentTimeMillis();
      private static final long START_NANOTIME = System.nanoTime();
  
      private final String randomDevice;
      private final String hwRandomDevice;
      private final String entropyFile;
  
      /**
       * Handler that periodically updates the entropy on disk.
       */
      private final Handler mHandler = new Handler(IoThread.getHandler().getLooper()) {
          // IMPLEMENTATION NOTE: This handler runs on the I/O thread to avoid I/O on the main thread.
          // The reason we're using our own Handler instead of IoThread.getHandler() is to create our
          // own ID space for the "what" parameter of messages seen by the handler.
          @Override
          public void handleMessage(Message msg) {
              if (msg.what != ENTROPY_WHAT) {
                  Slog.e(TAG, "Will not process invalid message");
                  return;
              }
              addHwRandomEntropy();
              writeEntropy();
              scheduleEntropyWriter();
          }
      };
  
      private final BroadcastReceiver mBroadcastReceiver = new BroadcastReceiver() {
          @Override
          public void onReceive(Context context, Intent intent) {
              writeEntropy();
          }
      };
  
      public EntropyMixer(Context context) {
          this(context, getSystemDir() + "/entropy.dat", "/dev/urandom", "/dev/hw_random");
      }
  
      /** Test only interface, not for public use */
      public EntropyMixer(
              Context context,
              String entropyFile,
              String randomDevice,
              String hwRandomDevice) {
          if (randomDevice == null) { throw new NullPointerException("randomDevice"); }
          if (hwRandomDevice == null) { throw new NullPointerException("hwRandomDevice"); }
          if (entropyFile == null) { throw new NullPointerException("entropyFile"); }
  
          this.randomDevice = randomDevice;
          this.hwRandomDevice = hwRandomDevice;
          this.entropyFile = entropyFile;
          loadInitialEntropy();
          addDeviceSpecificEntropy();
          addHwRandomEntropy();
          writeEntropy();
          scheduleEntropyWriter();
          IntentFilter broadcastFilter = new IntentFilter(Intent.ACTION_SHUTDOWN);
          broadcastFilter.addAction(Intent.ACTION_POWER_CONNECTED);
          broadcastFilter.addAction(Intent.ACTION_REBOOT);
          context.registerReceiver(
                  mBroadcastReceiver,
                  broadcastFilter,
                  null, // do not require broadcaster to hold any permissions
                  mHandler // process received broadcasts on the I/O thread instead of the main thread
                  );
      }
  
      private void scheduleEntropyWriter() {
          mHandler.removeMessages(ENTROPY_WHAT);
          mHandler.sendEmptyMessageDelayed(ENTROPY_WHAT, ENTROPY_WRITE_PERIOD);
      }
  
      private void loadInitialEntropy() {
          try {
              RandomBlock.fromFile(entropyFile).toFile(randomDevice, false);
          } catch (FileNotFoundException e) {
              Slog.w(TAG, "No existing entropy file -- first boot?");
          } catch (IOException e) {
              Slog.w(TAG, "Failure loading existing entropy file", e);
          }
      }
  
      private void writeEntropy() {
          try {
              Slog.i(TAG, "Writing entropy...");
              RandomBlock.fromFile(randomDevice).toFile(entropyFile, true);
          } catch (IOException e) {
              Slog.w(TAG, "Unable to write entropy", e);
          }
      }
  
      /**
       * Add additional information to the kernel entropy pool.  The
       * information isn't necessarily "random", but that's ok.  Even
       * sending non-random information to {@code /dev/urandom} is useful
       * because, while it doesn't increase the "quality" of the entropy pool,
       * it mixes more bits into the pool, which gives us a higher degree
       * of uncertainty in the generated randomness.  Like nature, writes to
       * the random device can only cause the quality of the entropy in the
       * kernel to stay the same or increase.
       *
       * <p>For maximum effect, we try to target information which varies
       * on a per-device basis, and is not easily observable to an
       * attacker.
       */
      private void addDeviceSpecificEntropy() {
          PrintWriter out = null;
          try {
              out = new PrintWriter(new FileOutputStream(randomDevice));
              out.println("Copyright (C) 2009 The Android Open Source Project");
              out.println("All Your Randomness Are Belong To Us");
              out.println(START_TIME);
              out.println(START_NANOTIME);
              out.println(SystemProperties.get("ro.serialno"));
              out.println(SystemProperties.get("ro.bootmode"));
              out.println(SystemProperties.get("ro.baseband"));
              out.println(SystemProperties.get("ro.carrier"));
              out.println(SystemProperties.get("ro.bootloader"));
              out.println(SystemProperties.get("ro.hardware"));
              out.println(SystemProperties.get("ro.revision"));
              out.println(SystemProperties.get("ro.build.fingerprint"));
              out.println(new Object().hashCode());
              out.println(System.currentTimeMillis());
              out.println(System.nanoTime());
          } catch (IOException e) {
              Slog.w(TAG, "Unable to add device specific data to the entropy pool", e);
          } finally {
              if (out != null) {
                  out.close();
              }
          }
      }
  
      /**
       * Mixes in the output from HW RNG (if present) into the Linux RNG.
       */
      private void addHwRandomEntropy() {
          if (!new File(hwRandomDevice).exists()) {
              // HW RNG not present/exposed -- ignore
              return;
          }
  
          try {
              RandomBlock.fromFile(hwRandomDevice).toFile(randomDevice, false);
              Slog.i(TAG, "Added HW RNG output to entropy pool");
          } catch (IOException e) {
              Slog.w(TAG, "Failed to add HW RNG output to entropy pool", e);
          }
      }
  
      private static String getSystemDir() {
          File dataDir = Environment.getDataDirectory();
          File systemDir = new File(dataDir, "system");
          systemDir.mkdirs();
          return systemDir.toString();
      }
  }
  