From b8cba95ffd4d9be0edace7a9eb42286e668ef3e3 Mon Sep 17 00:00:00 2001
From: Nick Kralevich <nnk@google.com>
Date: Fri, 19 Jun 2009 09:45:35 -0700
Subject: At boot time, add additional per-device information to the kernel
 randomness pool.  This helps increase the quality / uniqueness of the random
 numbers, and is especially important during the device's first boot, when
 insufficient randomness is available.

---
 .../java/com/android/server/EntropyService.java    | 49 +++++++++++++++++++++-
 1 file changed, 48 insertions(+), 1 deletion(-)

diff --git a/services/java/com/android/server/EntropyService.java b/services/java/com/android/server/EntropyService.java
index e51a0afb4ef2..28f09f53ff14 100644
--- a/services/java/com/android/server/EntropyService.java
+++ b/services/java/com/android/server/EntropyService.java
@@ -17,12 +17,16 @@
 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.Log;
 
 /**
@@ -49,6 +53,8 @@ public class EntropyService extends Binder {
     private static final int ENTROPY_WHAT = 1;
     private static final int ENTROPY_WRITE_PERIOD = 3 * 60 * 60 * 1000;  // 3 hrs
     private static final String RANDOM_DEV = "/dev/urandom";
+    private static final long START_TIME = System.currentTimeMillis();
+    private static final long START_NANOTIME = System.nanoTime();
 
     /**
      * Handler that periodically updates the entropy on disk.
@@ -67,6 +73,7 @@ public class EntropyService extends Binder {
 
     public EntropyService() {
         loadInitialEntropy();
+        addDeviceSpecificEntropy();
         writeEntropy();
         scheduleEntropyWriter();
     }
@@ -88,7 +95,47 @@ public class EntropyService extends Binder {
         try {
             RandomBlock.fromFile(RANDOM_DEV).toFile(ENTROPY_FILENAME);
         } catch (IOException e) {
-            Log.e(TAG, "unable to write entropy", e);
+            Log.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(RANDOM_DEV));
+            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(System.currentTimeMillis());
+            out.println(System.nanoTime());
+        } catch (IOException e) {
+            Log.w(TAG, "Unable to add device specific data to the entropy pool", e);
+        } finally {
+            if (out != null) {
+                out.close();
+            }
         }
     }
 
-- 
cgit v1.2.3-59-g8ed1b