Android Chromoting: Initialize SecureRandom generator.

remoting/android/java/src/org/chromium/chromoting/ThirdPartyTokenFetcher.java

Index: remoting/android/java/src/org/chromium/chromoting/ThirdPartyTokenFetcher.java
diff --git a/remoting/android/java/src/org/chromium/chromoting/ThirdPartyTokenFetcher.java b/remoting/android/java/src/org/chromium/chromoting/ThirdPartyTokenFetcher.java
index ac195cf617acbc1cbb598000b5dd1fa87c2dd55c..71a23f50bd8f70e7ec29ea0dbe42ffa86c2d221b 100644
--- a/remoting/android/java/src/org/chromium/chromoting/ThirdPartyTokenFetcher.java
+++ b/remoting/android/java/src/org/chromium/chromoting/ThirdPartyTokenFetcher.java
@@ -4,6 +4,7 @@
 
 package org.chromium.chromoting;
 
+import android.annotation.SuppressLint;
 import android.app.Activity;
 import android.content.ActivityNotFoundException;
 import android.content.ComponentName;
@@ -14,6 +15,8 @@ import android.text.TextUtils;
 import android.util.Base64;
 import android.util.Log;
 
+import java.io.FileInputStream;
+import java.io.IOException;
 import java.security.SecureRandom;
 import java.util.ArrayList;
 
@@ -38,8 +41,21 @@ public class ThirdPartyTokenFetcher {
      */
     private static final String RESPONSE_TYPE = "code token";
 
+    /** Used to initialize the random number generator. */
+    private static final int NUM_RANDOM_BYTES = 16;
+
     /** This is used to securely generate an opaque 128 bit for the |mState| variable. */
-    private static SecureRandom sSecureRandom = new SecureRandom();
+    @SuppressLint("TrulyRandom")
+    private static SecureRandom sSecureRandom;
+
+    static {
+        // Versions of SecureRandom from Android <= 4.3 do not seed themselves as
+        // securely as possible. This workaround should suffice until the fixed version
+        // is deployed to all users. It reads random data from /dev/urandom, which is as good as
+        // the platform can get.
+        sSecureRandom = new SecureRandom();
+        sSecureRandom.setSeed(getRandomBytes(NUM_RANDOM_BYTES));

[kelvinp, 2014/08/13 02:33:26]

As discussed, maybe it is better to move the initialization to a static
singleton instead of a static block so that we only crash when the user tries
third party auth instead of during launch.

[Lambros, 2014/08/13 23:48:51]

Discussed offline. The refactoring to do this is non-trivial so I've added a
TODO and will file a bug for this.

+    }
 
     /** This is used to launch the third party login page in the browser. */
     private Activity mContext;
@@ -61,6 +77,28 @@ public class ThirdPartyTokenFetcher {
 
     private final String mRedirectUri;
 
+    private static byte[] getRandomBytes(int numBytes) {
+        FileInputStream fis = null;
+        try {
+            fis = new FileInputStream("/dev/urandom");
+            byte[] bytes = new byte[numBytes];
+            if (bytes.length != fis.read(bytes)) {
+                throw new SecurityException("Failed to get enough random data.");
+            }
+            return bytes;
+        } catch (IOException e) {
+            throw new SecurityException("Error reading random data.");
+        } finally {
+            try {
+                if (fis != null) {
+                    fis.close();
+                }
+            } catch (IOException e) {
+                // Ignore exception closing the device.
+            }
+        }
+    }
+
     public ThirdPartyTokenFetcher(Activity context,
                                   ArrayList<String> tokenUrlPatterns,
                                   Callback callback) {