/* * 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.OutputStream; 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 "randomness" * for the Linux kernel. * *
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. * *
This class was modeled after the script in * man * 4 random. */ 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 entropyFile; /** * Handler that periodically updates the entropy on disk. */ private final Handler mHandler = new Handler() { @Override public void handleMessage(Message msg) { if (msg.what != ENTROPY_WHAT) { Slog.e(TAG, "Will not process invalid message"); return; } 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"); } /** Test only interface, not for public use */ public EntropyMixer(Context context, String entropyFile, String randomDevice) { if (randomDevice == null) { throw new NullPointerException("randomDevice"); } if (entropyFile == null) { throw new NullPointerException("entropyFile"); } this.randomDevice = randomDevice; this.entropyFile = entropyFile; loadInitialEntropy(); addDeviceSpecificEntropy(); writeEntropy(); scheduleEntropyWriter(); IntentFilter broadcastFilter = new IntentFilter(Intent.ACTION_SHUTDOWN); broadcastFilter.addAction(Intent.ACTION_POWER_CONNECTED); broadcastFilter.addAction(Intent.ACTION_REBOOT); context.registerReceiver(mBroadcastReceiver, broadcastFilter); } 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. * *
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(); } } } private static String getSystemDir() { File dataDir = Environment.getDataDirectory(); File systemDir = new File(dataDir, "system"); systemDir.mkdirs(); return systemDir.toString(); } }