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Any way around awful SecretKeyFactory performance with LVL and AESObfuscator?

Asked 2010-09-28T16:31:24.927
9

I'm looking to use the new licensing (LVL) stuff with Android Marketplace, but I'm running into a performance problem with the stock AESObfuscator. Specifically, the constructor takes several seconds to run on a device (pure agony on emulator). Since this code needs to run to even check for cached license responses, it puts a serious damper on checking the license at startup.

Running the LVL sample app, here's my barbarian-style profiling of AESObfuscator's constructor:

public AESObfuscator(byte[] salt, String applicationId, String deviceId) {
        Log.w("AESObfuscator", "constructor starting");
        try {
            Log.w("AESObfuscator", "1");
            SecretKeyFactory factory = SecretKeyFactory.getInstance(KEYGEN_ALGORITHM);
            Log.w("AESObfuscator", "2");
            KeySpec keySpec =
                new PBEKeySpec((applicationId + deviceId).toCharArray(), salt, 1024, 256);
            Log.w("AESObfuscator", "3");
            SecretKey tmp = factory.generateSecret(keySpec);
            Log.w("AESObfuscator", "4");
            SecretKey secret = new SecretKeySpec(tmp.getEncoded(), "AES");
            Log.w("AESObfuscator", "5");
            mEncryptor = Cipher.getInstance(CIPHER_ALGORITHM);
            Log.w("AESObfuscator", "6");
            mEncryptor.init(Cipher.ENCRYPT_MODE, secret, new IvParameterSpec(IV));
            Log.w("AESObfuscator", "7");
            mDecryptor = Cipher.getInstance(CIPHER_ALGORITHM);
            Log.w("AESObfuscator", "8");
            mDecryptor.init(Cipher.DECRYPT_MODE, secret, new IvParameterSpec(IV));
        } catch (GeneralSecurityException e) {
            // This can't happen on a compatible Android device.
            throw new RuntimeException("Invalid environment", e);
        }
        Log.w("AESObfuscator", "constructor done");
    }

The output on a Nexus One:

09-28 09:29:02.799: INFO/System.out(12377): debugger has settled (1396)
09-28 09
android performance android-lvl
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1 Answer

6

After extensive searching and tinkering, my best workaround seems to be to create the AES key on my own, rather than using the PKCS#5 code in PBEKeySpec. I am somewhat amazed that other people have not posted this problem.

The workaround method is to combine a bunch of identifying data (device id, IMEI, package name, etc) into a string. I then take the SHA-1 hash of that string to get 20 bytes of the 24-byte AES key. Admittedly, there's not as much entropy as PKCS#5 and 4 bytes of the key are known. But, really, who is going to mount a crypto attack? It's still pretty sound and there are much weaker attack points in the LVL, despite my other attempts at hardening it.

Since even creating the AES cipher seems to be an expensive (2 secs on emulator) operation, I also defer creation of the encryptor and decryptor members until they are needed by calls to obfuscate and deobfuscate. When the app is using a cached license response, it does not need an encryptor; this cuts quite a bit of cycle out of the most common startup mode.

My new constructor is below. If anyone wants the whole source file, just drop me a line.

   /**
    * @param initialNoise device/app identifier. Use as many sources as possible to
    *    create this unique identifier.
    */
   public PixieObfuscator(String initialNoise) {
        try {
            // Hash up the initial noise into something smaller:
            MessageDigest md = MessageDigest.getInstance(HASH_ALGORITHM);
            md.update(initialNoise.getBytes());
            byte[] hash = md.digest();

            // Allocate a buffer for our actual AES key:
            byte[] aesKEY = new byte[AES_KEY_LENGTH];   

            // Fill it with our lucky byte to take up whatever slack is not filled by hash:
            Arrays.fill(aesKEY, LUCKY_BYTE);

            // Copy in as much of the hash as we got (should be twenty bytes, take as much as we can):
            for (int i = 0; i < ha
answered 2010-09-29T17:03:03.240

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