/*
* 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 "randomness"
* 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 "randomness" 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() {
@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);
}
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() {
try {
RandomBlock.fromFile(hwRandomDevice).toFile(randomDevice, false);
Slog.i(TAG, "Added HW RNG output to entropy pool");
} catch (FileNotFoundException ignored) {
// HW RNG not present/exposed -- ignore
} 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();
}
}
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