Question

Possible Duplicate:
Java AES Encrypt Entire String

Im having problems with the conversions back and forth between strings and byte arrays.

Basically I’ve made a small program to encrypt and decrypt messages using AES.

After encrypting the message this happens:

        byte[] result = cipher.doFinal(message.getBytes());
    String stringResult = new String(result);

Which converts the encrypted message to a string.
Now my decryptor changes the string back to a byte using:

byte[] result = stringResult.getBytes();

but when it decrypts the message (depending on the message) it may not be able to. There appears to be a padding problem and the error that I get is:

Exception in thread “main” javax.crypto.BadPaddingException: Given final block not properly padded

Any ideas why this occurs?

One example when this occurs for sure is when the encryption key is “1231231231231231” and the message encrypted is “read”.

Answer 1

You’re using the platform default encoding – once to convert message to bytes, and once to then convert the arbitrary binary output of encryption back into a string. Both steps are problematic.

Firstly, it’s best to use a fixed encoding which is known to cover the whole of Unicode when converting the string to bytes. UTF-8 is usually a good bet.

Then there’s the matter of representing arbitrary binary data as text. This isn’t text data represented in an encoding – it’s binary data. Interpreting it as if it were text data will almost certainly lose information. You need something more robust, capable of round-tripping arbitrary binary data. Base64 is usually the way to go.

There’s a public domain base64 encoder which works pretty well as far as I know. So your encryption code becomes:

byte[] result = cipher.doFinal(message.getBytes("UTF-8"));
String stringResult = Base64.encodeBytes(result);

The decrypting code would then be:

byte[] encrypted = Base64.decode(encryptedBase64);
byte[] decrypted = /* do the decryption here */
String message = new String(decrypted, "UTF-8");