J2ME AES Decryption Error(org.bouncycastle.crypto.InvalidCipherTextException: pad block corrupted)

Question

I am doing encryption and decryption using AES Algorithm with bouncy castle

My encryption and decryption works ok but it gives me error when my plain text size is bigger

even sometimes it is giving non decrypted data

public static boolean setEncryptionKey(String keyText)
{
    byte[] keyBytes = keyText.getBytes();

    key = new KeyParameter(keyBytes);
    engine = new AESFastEngine();
    cipher = new PaddedBufferedBlockCipher(engine);

    return true;
}

Encryption:

public static String encryptString(String plainText)
{

        byte[] plainArray = plainText.getBytes();

        cipher.init(true, key);
        byte[] cipherBytes = new byte[cipher.getOutputSize(plainArray.length)];
        int cipherLength = cipher.processBytes(plainArray, 0, plainArray.length, cipherBytes, 0);
        cipher.doFinal(cipherBytes, cipherLength);
        String cipherString = new String(cipherBytes);
        return cipherString;
    }

Decryption:

public static String decryptString(String encryptedText)
{

        byte[] cipherBytes = encryptedText.getBytes();
        cipher.init(false, key);
        byte[] decryptedBytes = new byte[cipher.getOutputSize(cipherBytes.length)];
        int decryptedLength = cipher.processBytes(cipherBytes, 0, cipherBytes.length, decryptedBytes, 0);
        cipher.doFinal(decryptedBytes, decryptedLength);
        String decryptedString = new String(decryptedBytes);

        int index = decryptedString.indexOf("\u0000");
        if (index >= 0)
        {
            decryptedString = decryptedString.substring(0, index);
        }
        return decryptedString;
    }

This decryption is giving me following error

org.bouncycastle.crypto.InvalidCipherTextException: pad block corrupted
        at org.bouncycastle.crypto.paddings.PKCS7Padding.padCount(+30)
        at org.bouncycastle.crypto.paddings.PaddedBufferedBlockCipher.doFinal(+190)
        at com.NewCrypto.decryptString(NewCrypto.java:103)
        at com.New_Midlet.startApp(New_Midlet.java:23)
        at javax.microedition.midlet.MIDletProxy.startApp(MIDletProxy.java:44)
        at com.sun.midp.midlet.Scheduler.schedule(Scheduler.java:375)
        at com.sun.midp.main.Main.runLocalClass(Main.java:477)
        at com.sun.midp.main.Main.main(+80)

what could be the problem ?

Answer 1

The line

String cipherString = new String(cipherBytes);

is a bug. cipherBytes is a byte array with arbitrary values and cannot be converted to a string using any of the Java string decoders. You should just send/save the cipher as a byte array. If you must make it a string then you'll have to use an encoder. Base64 encoders are often used, as are Base16 (hex). You can use the Apache Commons Codec or my favorite, the Harder Base64 codec.