Encryption error on Android 4.2

Question

The following code is working on all the versions of android except the latest 4.2

import java.security.InvalidKeyException;
import java.security.NoSuchAlgorithmException;
import java.security.SecureRandom;

import javax.crypto.BadPaddingException;
import javax.crypto.Cipher;
import javax.crypto.IllegalBlockSizeException;
import javax.crypto.KeyGenerator;
import javax.crypto.NoSuchPaddingException;
import javax.crypto.SecretKey;
import javax.crypto.spec.SecretKeySpec;

/**
 * Util class to perform encryption/decryption over strings. <br/>
 */
public final class UtilsEncryption
{
    /** The logging TAG */
    private static final String TAG = UtilsEncryption.class.getName();

    /** */
    private static final String KEY = "some_encryption_key";

    /**
     * Avoid instantiation. <br/>
     */
    private UtilsEncryption()
    {
    }

    /** The HEX characters */
    private final static String HEX = "0123456789ABCDEF";

    /**
     * Encrypt a given string. <br/>
     * 
     * @param the string to encrypt
     * @return the encrypted string in HEX
     */
    public static String encrypt( String cleartext )
    {
        try
        {
            byte[] result = process( Cipher.ENCRYPT_MODE, cleartext.getBytes() );
            return toHex( result );
        }
        catch ( Exception e )
        {
            System.out.println( TAG + ":encrypt:" + e.getMessage() );
        }
        return null;
    }

    /**
     * Decrypt a HEX encrypted string. <br/>
     * 
     * @param the HEX string to decrypt
     * @return the decrypted string
     */
    public static String decrypt( String encrypted )
    {
        try
        {
            byte[] enc = fromHex( encrypted );
            byte[] result = process( Cipher.DECRYPT_MODE, enc );
            return new String( result );
        }
        catch ( Exception e )
        {
            System.out.println( TAG + ":decrypt:" + e.getMessage() );
        }
        return null;
    }


    /**
     * Get the raw encryption key. <br/>
     * 
     * @param the seed key
     * @return the raw key
     * @throws NoSuchAlgorithmException
     */
    private static byte[] getRawKey()
        throws NoSuchAlgorithmException
    {
        KeyGenerator kgen = KeyGenerator.getInstance( "AES" );
        SecureRandom sr = SecureRandom.getInstance( "SHA1PRNG" );
        sr.setSeed( KEY.getBytes() );
        kgen.init( 128, sr );
        SecretKey skey = kgen.generateKey();
        return skey.getEncoded();
    }

    /**
     * Process the given input with the provided mode. <br/>
     * 
     * @param the cipher mode
     * @param the value to process
     * @return the processed value as byte[]
     * @throws InvalidKeyException
     * @throws IllegalBlockSizeException
     * @throws BadPaddingException
     * @throws NoSuchAlgorithmException
     * @throws NoSuchPaddingException
     */
    private static byte[] process( int mode, byte[] value )
        throws InvalidKeyException, IllegalBlockSizeException, BadPaddingException,     NoSuchAlgorithmException,
        NoSuchPaddingException
    {
        SecretKeySpec skeySpec = new SecretKeySpec( getRawKey(), "AES" );
        Cipher cipher = Cipher.getInstance( "AES" );
        cipher.init( mode, skeySpec );
        byte[] encrypted = cipher.doFinal( value );
        return encrypted;
    }

    /**
     * Decode an HEX encoded string into a byte[]. <br/>
     * 
     * @param the HEX string value
     * @return the decoded byte[]
     */
    protected static byte[] fromHex( String value )
    {
        int len = value.length() / 2;
        byte[] result = new byte[len];
        for ( int i = 0; i < len; i++ )
        {
            result[i] = Integer.valueOf( value.substring( 2 * i, 2 * i + 2 ), 16     ).byteValue();
        }
        return result;
    }

    /**
     * Encode a byte[] into an HEX string. <br/>
     * 
     * @param the byte[] value
     * @return the HEX encoded string
     */
    protected static String toHex( byte[] value )
    {
        if ( value == null )
        {
            return "";
        }
        StringBuffer result = new StringBuffer( 2 * value.length );
        for ( int i = 0; i < value.length; i++ )
        {
            byte b = value[i];

            result.append( HEX.charAt( ( b >> 4 ) & 0x0f ) );
            result.append( HEX.charAt( b & 0x0f ) );
        }
        return result.toString();
    }
}

Here's a small unit test that i've created to reproduce the error

import junit.framework.TestCase;

public class UtilsEncryptionTest
    extends TestCase
{
    /** A random string */
    private static String ORIGINAL = "some string to test";

    /**
     * The HEX value corresponds to ORIGINAL. <br/>
     * If you change ORIGINAL, calculate the new value on one of this sites:
     * <ul>
     * <li>http://www.string-functions.com/string-hex.aspx</li>
     * <li>http://www.yellowpipe.com/yis/tools/encrypter/index.php</li>
     * <li>http://www.convertstring.com/EncodeDecode/HexEncode</li>
     * </ul>
     */
    private static String HEX = "736F6D6520737472696E6720746F2074657374";

    public void testToHex()
    {
         String hexString = UtilsEncryption.toHex( ORIGINAL.getBytes() );

         assertNotNull( "The HEX string should not be null", hexString );
         assertTrue( "The HEX string should not be empty", hexString.length() > 0 );
         assertEquals( "The HEX string was not encoded correctly", HEX, hexString );
    }

    public void testFromHex()
    {
         byte[] stringBytes = UtilsEncryption.fromHex( HEX );

         assertNotNull( "The HEX string should not be null", stringBytes );
        assertTrue( "The HEX string should not be empty", stringBytes.length > 0 );
        assertEquals( "The HEX string was not encoded correctly", ORIGINAL, new String( stringBytes ) );
    }

    public void testWholeProcess()
    {
         String encrypted = UtilsEncryption.encrypt( ORIGINAL );
         assertNotNull( "The encrypted result should not be null", encrypted );
         assertTrue( "The encrypted result should not be empty", encrypted.length() > 0 );

         String decrypted = UtilsEncryption.decrypt( encrypted );
         assertNotNull( "The decrypted result should not be null", decrypted );
         assertTrue( "The decrypted result should not be empty", decrypted.length() > 0 );

         assertEquals( "Something went wrong", ORIGINAL, decrypted );
}

}

The line throwing the exception is:

byte[] encrypted = cipher.doFinal( value );

The full stack trace is:

    W/<package>.UtilsEncryption:decrypt(16414): pad block corrupted
    W/System.err(16414): javax.crypto.BadPaddingException: pad block corrupted
    W/System.err(16414):    at com.android.org.bouncycastle.jcajce.provider.symmetric.util.BaseBlockCipher.engineDoFinal(BaseBlockCipher.java:709)
    W/System.err(16414):    at javax.crypto.Cipher.doFinal(Cipher.java:1111)
    W/System.err(16414):    at <package>.UtilsEncryption.process(UtilsEncryption.java:117)
    W/System.err(16414):    at <package>.UtilsEncryption.decrypt(UtilsEncryption.java:69)
    W/System.err(16414):    at <package>.UtilsEncryptionTest.testWholeProcess(UtilsEncryptionTest.java:74)
    W/System.err(16414):    at java.lang.reflect.Method.invokeNative(Native Method)
    W/System.err(16414):    at java.lang.reflect.Method.invoke(Method.java:511)
    W/System.err(16414):    at junit.framework.TestCase.runTest(TestCase.java:168)
    W/System.err(16414):    at junit.framework.TestCase.runBare(TestCase.java:134)
    W/System.err(16414):    at junit.framework.TestResult$1.protect(TestResult.java:115)
    W/System.err(16414):    at junit.framework.TestResult.runProtected(TestResult.java:133)
D/elapsed (  588): 14808
    W/System.err(16414):    at junit.framework.TestResult.run(TestResult.java:118)
    W/System.err(16414):    at junit.framework.TestCase.run(TestCase.java:124)
    W/System.err(16414):    at android.test.AndroidTestRunner.runTest(AndroidTestRunner.java:190)
    W/System.err(16414):    at android.test.AndroidTestRunner.runTest(AndroidTestRunner.java:175)
    W/System.err(16414):    at android.test.InstrumentationTestRunner.onStart(InstrumentationTestRunner.java:555)
    W/System.err(16414):    at android.app.Instrumentation$InstrumentationThread.run(Instrumentation.java:1661)

Does anybody have a clue of what might be happening ? Is anybody aware of a breaking change on android 4.2 in any of the referenced classes?

Thanks a lot

Answer 1

From the Android Jellybean page:

Modified the default implementations of SecureRandom and Cipher.RSA to use OpenSSL

They changed the default provider for SecureRandom to use OpenSSL instead of the previous Crypto provider.

The following code will produce two different outputs on pre-Android 4.2 and Android 4.2:

SecureRandom rand = SecureRandom.getInstance("SHA1PRNG");
Log.i(TAG, "rand.getProvider(): " + rand.getProvider().getName());

On pre-4.2 devices:

rand.getProvider: Crypto

On 4.2 devices:

rand.getProvider: AndroidOpenSSL

Fortunately, it's easy to revert to the old behavior:

SecureRandom sr = SecureRandom.getInstance( "SHA1PRNG", "Crypto" );

To be sure, it's dangerous to be calling SecureRandom.setSeed at all in light of the Javadocs which state:

Seeding SecureRandom may be insecure

A seed is an array of bytes used to bootstrap random number generation. To produce cryptographically secure random numbers, both the seed and the algorithm must be secure.

By default, instances of this class will generate an initial seed using an internal entropy source, such as /dev/urandom. This seed is unpredictable and appropriate for secure use.

You may alternatively specify the initial seed explicitly with the seeded constructor or by calling setSeed(byte[]) before any random numbers have been generated. Specifying a fixed seed will cause the instance to return a predictable sequence of numbers. This may be useful for testing but it is not appropriate for secure use.

However, for writing unit tests, as you are doing, using setSeed may be okay.

Answer 2

As Brigham pointed out that, in Android 4.2, there was a security enhancement, which updated the default implementation of SecureRandom from Crypto to OpenSSL

Cryptography - Modified the default implementations of SecureRandom and Cipher.RSA to use OpenSSL. Added SSL Socket support for TLSv1.1 and TLSv1.2 using OpenSSL 1.0.1

bu Brigham's answer is a temporary solution and not recommended, because although it resolves the issue, its still doing the wrong way.

The recommended way (check Nelenkov’s tutorial) is to use proper key derivations PKCS (Public Key Cryptography Standard), which defines two key derivation functions, PBKDF1 and PBKDF2, of which PBKDF2 is more recommended.

This is how you should get the key,

    int iterationCount = 1000;
    int saltLength = 8; // bytes; 64 bits
    int keyLength = 256;
    SecureRandom random = new SecureRandom();
    byte[] salt = new byte[saltLength];
    random.nextBytes(salt);
    KeySpec keySpec = new PBEKeySpec(seed.toCharArray(), salt,
            iterationCount, keyLength);
    SecretKeyFactory keyFactory = SecretKeyFactory
            .getInstance("PBKDF2WithHmacSHA1");
    byte[] raw = keyFactory.generateSecret(keySpec).getEncoded();

Answer 3

So what you are trying is to use pseudo random generator as a key derivation function. This is bad for the following reasons:

• PRNG are by design non-deterministic and you are relying on it to be deterministic
• Relying on a bug and deprecated implementations will break your App some day
• PRNG are not designed to be good KDFs

More precisely Google deprecated the use of the Crypto provider in Android N (SDK 24)

Here are some better methods:

Hashed Message Authentication Code (HMAC)-based key derivation function (HKDF)

Using this library:

String userInput = "this is a user input with bad entropy";

HKDF hkdf = HKDF.fromHmacSha256();

//extract the "raw" data to create output with concentrated entropy
byte[] pseudoRandomKey = hkdf.extract(staticSalt32Byte, userInput.getBytes(StandardCharsets.UTF_8));

//create expanded bytes for e.g. AES secret key and IV
byte[] expandedAesKey = hkdf.expand(pseudoRandomKey, "aes-key".getBytes(StandardCharsets.UTF_8), 16);

//Example boilerplate encrypting a simple string with created key/iv
SecretKey key = new SecretKeySpec(expandedAesKey, "AES"); //AES-128 key

PBKDF2 (Password-Based Key Derivation Function 2)

has key stretching which makes it more expensive to brute force the key. Use this for weak key input (like user password):

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

There are more KDFs like BCrypt, scrypt and Argon2