// Copyright 2013 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
package org.chromium.chrome.browser;
import android.content.Context;
import android.os.AsyncTask;
import android.util.Log;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.security.GeneralSecurityException;
import java.security.SecureRandom;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.FutureTask;
import javax.crypto.KeyGenerator;
import javax.crypto.Mac;
import javax.crypto.SecretKey;
import javax.crypto.spec.SecretKeySpec;
/**
* Authenticate the source of Intents to launch web apps (see e.g. {@link #FullScreenActivity}).
*
* Chrome does not keep a store of valid URLs for installed web apps (because it cannot know when
* any have been uninstalled). Therefore, upon installation, it tells the Launcher a message
* authentication code (MAC) along with the URL for the web app, and then Chrome can verify the MAC
* when starting e.g. {@link #FullScreenActivity}. Chrome can thus distinguish between legitimate,
* installed web apps and arbitrary other URLs.
*/
public class WebappAuthenticator {
private static final String TAG = "WebappAuthenticator";
private static final String MAC_ALGORITHM_NAME = "HmacSHA256";
private static final String MAC_KEY_BASENAME = "webapp-authenticator";
private static final int MAC_KEY_BYTE_COUNT = 32;
private static final Object sLock = new Object();
private static FutureTask<SecretKey> sMacKeyGenerator;
private static SecretKey sKey = null;
/**
* @see #getMacForUrl
*
* @param url The URL to validate.
* @param mac The bytes of a previously-calculated MAC.
*
* @return true if the MAC is a valid MAC for the URL, false otherwise.
*/
public static boolean isUrlValid(Context context, String url, byte[] mac) {
byte[] goodMac = getMacForUrl(context, url);
if (goodMac == null) {
return false;
}
return constantTimeAreArraysEqual(goodMac, mac);
}
/**
* @see #isUrlValid
*
* @param url A URL for which to calculate a MAC.
*
* @return The bytes of a MAC for the URL, or null if a secure MAC was not available.
*/
public static byte[] getMacForUrl(Context context, String url) {
Mac mac = getMac(context);
if (mac == null) {
return null;
}
return mac.doFinal(url.getBytes());
}
// TODO(palmer): Put this method, and as much of this class as possible, in a utility class.
private static boolean constantTimeAreArraysEqual(byte[] a, byte[] b) {
if (a.length != b.length) {
return false;
}
int result = 0;
for (int i = 0; i < a.length; i++) {
result |= a[i] ^ b[i];
}
return result == 0;
}
private static SecretKey readKeyFromFile(
Context context, String basename, String algorithmName) {
FileInputStream input = null;
File file = context.getFileStreamPath(basename);
try {
if (file.length() != MAC_KEY_BYTE_COUNT) {
Log.w(TAG, "Could not read key from '" + file + "': invalid file contents");
return null;
}
byte[] keyBytes = new byte[MAC_KEY_BYTE_COUNT];
input = new FileInputStream(file);
if (MAC_KEY_BYTE_COUNT != input.read(keyBytes)) {
return null;
}
try {
return new SecretKeySpec(keyBytes, algorithmName);
} catch (IllegalArgumentException e) {
return null;
}
} catch (Exception e) {
Log.w(TAG, "Could not read key from '" + file + "': " + e);
return null;
} finally {
try {
if (input != null) {
input.close();
}
} catch (Exception e) {
Log.e(TAG, "Could not close key input stream '" + file + "': " + e);
}
}
}
private static boolean writeKeyToFile(Context context, String basename, SecretKey key) {
File file = context.getFileStreamPath(basename);
byte[] keyBytes = key.getEncoded();
if (MAC_KEY_BYTE_COUNT != keyBytes.length) {
Log.e(TAG, "writeKeyToFile got key encoded bytes length " + keyBytes.length +
"; expected " + MAC_KEY_BYTE_COUNT);
return false;
}
try {
FileOutputStream output = new FileOutputStream(file);
output.write(keyBytes);
output.close();
return true;
} catch (Exception e) {
Log.e(TAG, "Could not write key to '" + file + "': " + e);
return false;
}
}
private static SecretKey getKey(Context context) {
synchronized (sLock) {
if (sKey == null) {
SecretKey key = readKeyFromFile(context, MAC_KEY_BASENAME, MAC_ALGORITHM_NAME);
if (key != null) {
sKey = key;
return sKey;
}
triggerMacKeyGeneration();
try {
sKey = sMacKeyGenerator.get();
sMacKeyGenerator = null;
if (!writeKeyToFile(context, MAC_KEY_BASENAME, sKey)) {
sKey = null;
return null;
}
return sKey;
} catch (InterruptedException e) {
throw new RuntimeException(e);
} catch (ExecutionException e) {
throw new RuntimeException(e);
}
}
return sKey;
}
}
/**
* Generates the authentication encryption key in a background thread (if necessary).
*/
private static void triggerMacKeyGeneration() {
synchronized (sLock) {
if (sKey != null || sMacKeyGenerator != null) {
return;
}
sMacKeyGenerator = new FutureTask<SecretKey>(new Callable<SecretKey>() {
@Override
public SecretKey call() throws Exception {
KeyGenerator generator = KeyGenerator.getInstance(MAC_ALGORITHM_NAME);
SecureRandom random = SecureRandom.getInstance("SHA1PRNG");
// 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. getRandomBytes, which reads from /dev/urandom,
// which is as good as the platform can get.
//
// TODO(palmer): Consider getting rid of this once the updated platform has
// shipped to everyone. Alternately, leave this in as a defense against other
// bugs in SecureRandom.
byte[] seed = getRandomBytes(MAC_KEY_BYTE_COUNT);
if (seed == null) {
return null;
}
random.setSeed(seed);
generator.init(MAC_KEY_BYTE_COUNT * 8, random);
return generator.generateKey();
}
});
AsyncTask.THREAD_POOL_EXECUTOR.execute(sMacKeyGenerator);
}
}
private static byte[] getRandomBytes(int count) {
FileInputStream fis = null;
try {
fis = new FileInputStream("/dev/urandom");
byte[] bytes = new byte[count];
if (bytes.length != fis.read(bytes)) {
return null;
}
return bytes;
} catch (Throwable t) {
// This causes the ultimate caller, i.e. getMac, to fail.
return null;
} finally {
try {
if (fis != null) {
fis.close();
}
} catch (IOException e) {
// Nothing we can do.
}
}
}
/**
* @return A Mac, or null if it is not possible to instantiate one.
*/
private static Mac getMac(Context context) {
try {
SecretKey key = getKey(context);
if (key == null) {
// getKey should have invoked triggerMacKeyGeneration, which should have set the
// random seed and generated a key from it. If not, there is a problem with the
// random number generator, and we must not claim that authentication can work.
return null;
}
Mac mac = Mac.getInstance(MAC_ALGORITHM_NAME);
mac.init(key);
return mac;
} catch (GeneralSecurityException e) {
Log.w(TAG, "Error in creating MAC instance", e);
return null;
}
}
}