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19 กุมภาพันธ์ 2556

Using Cryptography to Store Credentials Safely

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Following our talk "Security and Privacy in Android Apps" at Google I/O last year, many people had specific questions about how to use cryptography in Android. Many of those revolved around which APIs to use for a specific purpose. Let's look at how to use cryptography to safely store user credentials, such as passwords and auth tokens, on local storage.

An anti-pattern

A common (but incorrect) pattern that we've recently become aware of is to use SecureRandom as a means of generating deterministic key material, which would then be used to encrypt local credential caches. Examples are not hard to find, such as here, here, here, and elsewhere.

In this pattern, rather than storing an encryption key directly as a string inside an APK, the code uses a proxy string to generate the key instead — similar to a passphrase. This essentially obfuscates the key so that it's not readily visible to attackers. However, a skilled attacker would be able to easily see around this strategy. We don't recommend it.

The fact is, Android's existing security model already provides plenty of protection for this kind of data. User credentials should be stored with the MODE_PRIVATE flag set and stored in internal storage, rather than on an SD card, since permissions aren't enforced on external storage. Combined with device encryption, this provides protection from most types of attacks targeting credentials.

However, there's another problem with using SecureRandom in the way described above. Starting with Android 4.2, the default SecureRandom provider is OpenSSL, and a developer can no longer override SecureRandom’s internal state. Consider the following code:

  SecureRandom secureRandom = new SecureRandom();
  byte[] b = new byte[] { (byte) 1 };
  // Prior to Android 4.2, the next line would always return the same number!

The old Bouncy Castle-based implementation allowed overriding the internally generated, /dev/urandom based key for each SecureRandom instance. Developers which attempted to explicitly seed the random number generator would find that their seed replaces, not supplements, the existing seed (contrary to the reference implementation’s documentation). Under OpenSSL, this error-prone behavior is no longer possible.

Unfortunately, applications who relied on the old behavior will find that the output from SecureRandom changes randomly every time their application starts up. (This is actually a very desirable trait for a random number generator!) Attempting to obfuscate encryption keys in this manner will no longer work.

The right way

A more reasonable approach is simply to generate a truly random AES key when an application is first launched:

public static SecretKey generateKey() throws NoSuchAlgorithmException {
    // Generate a 256-bit key
    final int outputKeyLength = 256;

    SecureRandom secureRandom = new SecureRandom();
    // Do *not* seed secureRandom! Automatically seeded from system entropy.
    KeyGenerator keyGenerator = KeyGenerator.getInstance("AES");
    keyGenerator.init(outputKeyLength, secureRandom);
    SecretKey key = keyGenerator.generateKey();
    return key;

Note that the security of this approach relies on safeguarding the generated key, which is is predicated on the security of the internal storage. Leaving the target file unencrypted (but set to MODE_PRIVATE) would provide similar security.

Even more security

If your app needs additional encryption, a recommended approach is to require a passphase or PIN to access your application. This passphrase could be fed into PBKDF2 to generate the encryption key. (PBKDF2 is a commonly used algorithm for deriving key material from a passphrase, using a technique known as "key stretching".) Android provides an implementation of this algorithm inside SecretKeyFactory as PBKDF2WithHmacSHA1:

public static SecretKey generateKey(char[] passphraseOrPin, byte[] salt) throws NoSuchAlgorithmException, InvalidKeySpecException {
    // Number of PBKDF2 hardening rounds to use. Larger values increase
    // computation time. You should select a value that causes computation
    // to take >100ms.
    final int iterations = 1000; 

    // Generate a 256-bit key
    final int outputKeyLength = 256;

    SecretKeyFactory secretKeyFactory = SecretKeyFactory.getInstance("PBKDF2WithHmacSHA1");
    KeySpec keySpec = new PBEKeySpec(passphraseOrPin, salt, iterations, outputKeyLength);
    SecretKey secretKey = secretKeyFactory.generateSecret(keySpec);
    return secretKey;

The salt should be a random string, again generated using SecureRandom and persisted on internal storage alongside any encrypted data. This is important to mitigate the risk of attackers using a rainbow table to precompute password hashes.

Check your apps for proper use of SecureRandom

As mentioned above and in the New Security Features in Jelly Bean, the default implementation of SecureRandom is changed in Android 4.2. Using it to deterministically generate keys is no longer possible.

If you're one of the developers who's been generating keys the wrong way, we recommend upgrading your app today to prevent subtle problems as more users upgrade to devices running Android 4.2 or later.