1/*
2 * Copyright (C) 2009 The Android Open Source Project
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 *      http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16
17package com.android.server;
18
19import java.io.File;
20import java.io.FileOutputStream;
21import java.io.IOException;
22import java.io.OutputStream;
23import java.io.PrintWriter;
24
25import android.os.Binder;
26import android.os.Environment;
27import android.os.Handler;
28import android.os.Message;
29import android.os.SystemProperties;
30import android.util.Slog;
31
32/**
33 * A service designed to load and periodically save "randomness"
34 * for the Linux kernel.
35 *
36 * <p>When a Linux system starts up, the entropy pool associated with
37 * {@code /dev/random} may be in a fairly predictable state.  Applications which
38 * depend strongly on randomness may find {@code /dev/random} or
39 * {@code /dev/urandom} returning predictable data.  In order to counteract
40 * this effect, it's helpful to carry the entropy pool information across
41 * shutdowns and startups.
42 *
43 * <p>This class was modeled after the script in
44 * <a href="http://www.kernel.org/doc/man-pages/online/pages/man4/random.4.html">man
45 * 4 random</a>.
46 *
47 * <p>TODO: Investigate attempting to write entropy data at shutdown time
48 * instead of periodically.
49 */
50public class EntropyMixer extends Binder {
51    private static final String TAG = "EntropyMixer";
52    private static final int ENTROPY_WHAT = 1;
53    private static final int ENTROPY_WRITE_PERIOD = 3 * 60 * 60 * 1000;  // 3 hrs
54    private static final long START_TIME = System.currentTimeMillis();
55    private static final long START_NANOTIME = System.nanoTime();
56
57    private final String randomDevice;
58    private final String entropyFile;
59
60    /**
61     * Handler that periodically updates the entropy on disk.
62     */
63    private final Handler mHandler = new Handler() {
64        @Override
65        public void handleMessage(Message msg) {
66            if (msg.what != ENTROPY_WHAT) {
67                Slog.e(TAG, "Will not process invalid message");
68                return;
69            }
70            writeEntropy();
71            scheduleEntropyWriter();
72        }
73    };
74
75    public EntropyMixer() {
76        this(getSystemDir() + "/entropy.dat", "/dev/urandom");
77    }
78
79    /** Test only interface, not for public use */
80    public EntropyMixer(String entropyFile, String randomDevice) {
81        if (randomDevice == null) { throw new NullPointerException("randomDevice"); }
82        if (entropyFile == null) { throw new NullPointerException("entropyFile"); }
83
84        this.randomDevice = randomDevice;
85        this.entropyFile = entropyFile;
86        loadInitialEntropy();
87        addDeviceSpecificEntropy();
88        writeEntropy();
89        scheduleEntropyWriter();
90    }
91
92    private void scheduleEntropyWriter() {
93        mHandler.removeMessages(ENTROPY_WHAT);
94        mHandler.sendEmptyMessageDelayed(ENTROPY_WHAT, ENTROPY_WRITE_PERIOD);
95    }
96
97    private void loadInitialEntropy() {
98        try {
99            RandomBlock.fromFile(entropyFile).toFile(randomDevice, false);
100        } catch (IOException e) {
101            Slog.w(TAG, "unable to load initial entropy (first boot?)", e);
102        }
103    }
104
105    private void writeEntropy() {
106        try {
107            RandomBlock.fromFile(randomDevice).toFile(entropyFile, true);
108        } catch (IOException e) {
109            Slog.w(TAG, "unable to write entropy", e);
110        }
111    }
112
113    /**
114     * Add additional information to the kernel entropy pool.  The
115     * information isn't necessarily "random", but that's ok.  Even
116     * sending non-random information to {@code /dev/urandom} is useful
117     * because, while it doesn't increase the "quality" of the entropy pool,
118     * it mixes more bits into the pool, which gives us a higher degree
119     * of uncertainty in the generated randomness.  Like nature, writes to
120     * the random device can only cause the quality of the entropy in the
121     * kernel to stay the same or increase.
122     *
123     * <p>For maximum effect, we try to target information which varies
124     * on a per-device basis, and is not easily observable to an
125     * attacker.
126     */
127    private void addDeviceSpecificEntropy() {
128        PrintWriter out = null;
129        try {
130            out = new PrintWriter(new FileOutputStream(randomDevice));
131            out.println("Copyright (C) 2009 The Android Open Source Project");
132            out.println("All Your Randomness Are Belong To Us");
133            out.println(START_TIME);
134            out.println(START_NANOTIME);
135            out.println(SystemProperties.get("ro.serialno"));
136            out.println(SystemProperties.get("ro.bootmode"));
137            out.println(SystemProperties.get("ro.baseband"));
138            out.println(SystemProperties.get("ro.carrier"));
139            out.println(SystemProperties.get("ro.bootloader"));
140            out.println(SystemProperties.get("ro.hardware"));
141            out.println(SystemProperties.get("ro.revision"));
142            out.println(new Object().hashCode());
143            out.println(System.currentTimeMillis());
144            out.println(System.nanoTime());
145        } catch (IOException e) {
146            Slog.w(TAG, "Unable to add device specific data to the entropy pool", e);
147        } finally {
148            if (out != null) {
149                out.close();
150            }
151        }
152    }
153
154    private static String getSystemDir() {
155        File dataDir = Environment.getDataDirectory();
156        File systemDir = new File(dataDir, "system");
157        systemDir.mkdirs();
158        return systemDir.toString();
159    }
160}
161