102 lines
3.3 KiB
Java
102 lines
3.3 KiB
Java
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/*
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* Copyright (C) 2009 The Android Open Source Project
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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package com.android.server;
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import java.io.File;
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import java.io.IOException;
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import android.os.Binder;
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import android.os.Environment;
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import android.os.Handler;
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import android.os.Message;
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import android.util.Log;
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/**
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* A service designed to load and periodically save "randomness"
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* for the Linux kernel.
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*
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* <p>When a Linux system starts up, the entropy pool associated with
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* {@code /dev/random} may be in a fairly predictable state. Applications which
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* depend strongly on randomness may find {@code /dev/random} or
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* {@code /dev/urandom} returning predictable data. In order to counteract
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* this effect, it's helpful to carry the entropy pool information across
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* shutdowns and startups.
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*
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* <p>This class was modeled after the script in
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* <a href="http://www.kernel.org/doc/man-pages/online/pages/man4/random.4.html">man
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* 4 random</a>.
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*
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* <p>TODO: Investigate attempting to write entropy data at shutdown time
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* instead of periodically.
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*/
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public class EntropyService extends Binder {
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private static final String ENTROPY_FILENAME = getSystemDir() + "/entropy.dat";
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private static final String TAG = "EntropyService";
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private static final int ENTROPY_WHAT = 1;
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private static final int ENTROPY_WRITE_PERIOD = 3 * 60 * 60 * 1000; // 3 hrs
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private static final String RANDOM_DEV = "/dev/urandom";
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/**
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* Handler that periodically updates the entropy on disk.
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*/
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private final Handler mHandler = new Handler() {
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@Override
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public void handleMessage(Message msg) {
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if (msg.what != ENTROPY_WHAT) {
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Log.e(TAG, "Will not process invalid message");
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return;
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}
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writeEntropy();
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scheduleEntropyWriter();
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}
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};
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public EntropyService() {
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loadInitialEntropy();
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writeEntropy();
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scheduleEntropyWriter();
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}
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private void scheduleEntropyWriter() {
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mHandler.removeMessages(ENTROPY_WHAT);
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mHandler.sendEmptyMessageDelayed(ENTROPY_WHAT, ENTROPY_WRITE_PERIOD);
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}
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private void loadInitialEntropy() {
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try {
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RandomBlock.fromFile(ENTROPY_FILENAME).toFile(RANDOM_DEV);
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} catch (IOException e) {
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Log.w(TAG, "unable to load initial entropy (first boot?)", e);
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}
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}
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private void writeEntropy() {
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try {
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RandomBlock.fromFile(RANDOM_DEV).toFile(ENTROPY_FILENAME);
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} catch (IOException e) {
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Log.e(TAG, "unable to write entropy", e);
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}
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}
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private static String getSystemDir() {
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File dataDir = Environment.getDataDirectory();
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File systemDir = new File(dataDir, "system");
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systemDir.mkdirs();
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return systemDir.toString();
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}
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}
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