Rename EntropyService to EntropyMixer
EntropyService implies that this program provides entropy to other
programs, and is misleading. The EntropyMixer class is designed purely
to stir the existing entropy pool with some possibily random-ish data,
and carryover entropy across device reboots.
Change-Id: I086cd339a3b652d32371521e61e1b1f555ce2280
diff --git a/services/java/com/android/server/EntropyMixer.java b/services/java/com/android/server/EntropyMixer.java
new file mode 100644
index 0000000..b63a70e
--- /dev/null
+++ b/services/java/com/android/server/EntropyMixer.java
@@ -0,0 +1,160 @@
+/*
+ * 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.FileOutputStream;
+import java.io.IOException;
+import java.io.OutputStream;
+import java.io.PrintWriter;
+
+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.
+ *
+ * <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>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>.
+ *
+ * <p>TODO: Investigate attempting to write entropy data at shutdown time
+ * instead of periodically.
+ */
+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();
+ }
+ };
+
+ public EntropyMixer() {
+ this(getSystemDir() + "/entropy.dat", "/dev/urandom");
+ }
+
+ /** Test only interface, not for public use */
+ public EntropyMixer(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();
+ }
+
+ 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 (IOException e) {
+ Slog.w(TAG, "unable to load initial entropy (first boot?)", e);
+ }
+ }
+
+ private void writeEntropy() {
+ try {
+ 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(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();
+ }
+}