AES cipher text is different

Question

I am using AES encrypting with bouncy castle implementation in both Android and in a Java servlet environment. The encryption part is ok in both scenario. However I get different result for those 2 platforms once I encrypt the same text with same key.

My intention is to do an encryption in Android and do the decryption in web environment.

This is the only change I have done for Android AES implementation.

    KeyGenerator kgen = KeyGenerator.getInstance("AES");
    SecureRandom sr = null;
    if (android.os.Build.VERSION.SDK_INT >= JELLY_BEAN_4_2) {
        sr = SecureRandom.getInstance("SHA1PRNG", "Crypto");
    } else {
        sr = SecureRandom.getInstance("SHA1PRNG");
    }
    sr.setSeed(key);
    kgen.init(128, sr);
    SecretKey skey = kgen.generateKey();
    byte[] raw = skey.getEncoded();

Above I just add the Crypto into get instance.

I used the spongy castle implementation as well to see whether I can accomplish this. Still it gave me the same result as Android gives. Not sure whether I have loaded it properly. I tested this on API level 14 and 17.

This leads to javax.crypto.BadPaddingException: pad block corrupted.

We can't see your mistake unless you post the encryption code from the Android part and the decryption code from the servlet part. The key generation code you provided is irrelevant. — Duncan Jones, Feb 15 '13 at 08:05
Actually, what you posted might be relevant if you can explain what `key` is? — Duncan Jones, Feb 15 '13 at 08:06
Servlet part work as the same. However based on @kroot answer above I went on searching and seems like he has also faced the same issue earlier. Found the needed info from his thread. — Dilshan, Feb 15 '13 at 08:08
`SecureRandom` is used to generate random numbers. If you provide a seed in production code, you're doing something wrong. Using a seed is fine to get reproducible behavior in a test, but misses the point of generating random numbers if its used in your actual crypto code. — CodesInChaos, Feb 15 '13 at 11:08
Either use `key` directly as AES key, or use a specialized key-derivation-function. Don't abuse `SecureRandom` as KDF. HKDF is pretty simple to implement, and is a great choice to derive multiple keys from a master key. If the key is actually a password, you need to use a password based KDF instead, such as PBKDF2. — CodesInChaos, Feb 15 '13 at 11:09

score 4 · Accepted Answer · edited May 23 '17 at 10:30

For anyone who is referencing this thread this is the change I did to my code. Now it works well in Android and server environment.

Answer is taken from,

Android 4.2 broke my encrypt/decrypt code and the provided solutions don't work

Thanks @kroot

    /* Store these things on disk used to derive key later: */
    int iterationCount = 1000;
    int saltLength = 32; // bytes; should be the same size as the output
                            // (256 / 8 = 32)
    int keyLength = 256; // 256-bits for AES-256, 128-bits for AES-128, etc
    byte[] salt = new byte[saltLength]; // Should be of saltLength

    /* When first creating the key, obtain a salt with this: */
    SecureRandom random = new SecureRandom();
    random.nextBytes(salt);

    /* Use this to derive the key from the password: */
    KeySpec keySpec = new PBEKeySpec(new String(key,
            Constants.CHAR_ENCODING).toCharArray(), key, iterationCount,
            keyLength);
    SecretKeyFactory keyFactory = SecretKeyFactory
            .getInstance("PBEWithSHA256And256BitAES-CBC-BC");
    byte[] keyBytes = keyFactory.generateSecret(keySpec).getEncoded();
    SecretKey secretKey = new SecretKeySpec(keyBytes, "AES");

    return secretKey.getEncoded();

AES cipher text is different

1 Answers1