HELP: How come getOutputSize() returns 8 bytes more than what I need?

[Vernon Chin]

Hi all,

 

Using BC lightweight API to do 3DES with PaddedBufferedBlockCipher and ZeroBytePadding.

 

This is how I create the cipher:

 

BlockCipher engine = new DESedeEngine();
PaddedBufferedBlockCipher cipher = new PaddedBufferedBlockCipher(engine, new ZeroBytePadding());

 

When I have plaintext input that is a multiple of the block size (8 bytes), I get 8 more bytes when I call getOutputSize(input).  For example, an input length of 8 gives me 16, 24 --> 32 etc.  If the input length is not a multiple of 8, it does what is expected (e.g. 7 byte length --> 8, 14 --> 16).

 

I am using 2 64-bit keys, and encrypting and decrypting works fine, except at the end I get 8 extra empty bytes after decrypting.

 

Why does getOutputSize() do this?  How come it doesn't think an 8/16/24 byte output buffer is big enough for 8/16/24 byte input?

 

Thanks in advance,

Vernon

 

 

==

private final static String ENCODE_METHOD = FrameworkConstants.ENCODING_64;

 

public static String encrypt(String encodedKey, String plainText)
{
    if (StringUtil.isEmpty(plainText))
    {

        return plainText;

    }
 
    String retVal = null;

 

    try
    {
        BlockCipher engine = new DESedeEngine();
        PaddedBufferedBlockCipher cipher = new PaddedBufferedBlockCipher(engine, new ZeroBytePadding());
 

        
        byte[] key = Base64.decodeBase64(encodedKey.getBytes(ENCODE_METHOD));
        byte[] input = plainText.getBytes();
 
        cipher.init(true, new KeyParameter(key));
 
        byte[] cipherText = new byte[cipher.getOutputSize(input.length)];
 
        int outputLen = cipher.processBytes(input, 0, input.length, cipherText, 0);
        cipher.doFinal(cipherText, outputLen);

 

        retVal = new String(Base64.encodeBase64(cipherText), ENCODE_METHOD);
    }
    catch (CryptoException ce)
    {
        log.error("Problem encrypting: " + ce);
    }
    catch (UnsupportedEncodingException uee)
    {
        log.error("bad encoding: " + uee);
    }

 

    return retVal;

}

 

 

___________________________

 

Vernon Chin

Developer, Professional Services

Infowave Software Inc.

[hidden email]