Question

I have a problem in encrypting/decrypting log file generated by log4j’s RollingFileAppender. For the encryption I have tried to extend the RollingFileAppender, just call it EncryptedRollingFileAppender. I override the method

setFile(String fileName, boolean append, boolean bufferedIO, int bufferSize)

and basically I use CipherOutputStream and Base64OutputStream to encrypt and encode everything written to the output stream. Here’s part of the code:

...
setImmediateFlush(true);

FileOutputStream ostream = null;
CipherOutputStream cstream = null;
Base64OutputStream b64stream = null;
try {        
    byte[] keyBytes = "1234123412341234".getBytes();  //example
    final byte[] ivBytes = new byte[] { 0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 
         0x07, 0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f }; //example

    final SecretKey key = new SecretKeySpec(keyBytes, "AES");
    final IvParameterSpec IV = new IvParameterSpec(ivBytes);
    final Cipher cipher = Cipher.getInstance("AES/CFB8/NoPadding");
    cipher.init(Cipher.ENCRYPT_MODE, key, IV);

    ostream = new FileOutputStream(fileName, true);
    b64stream = new Base64OutputStream(ostream);
    cstream = new CipherOutputStream(b64stream, cipher);

    } catch(Exception ex) {
        ex.printStackTrace();
    }

Writer cw = createWriter(cstream);
...

And then i decrypt the file with this code:

private static void decryptFile(String filename) throws Exception {
    FileInputStream fis = null;
    BufferedReader br = new BufferedReader(new FileReader(filename));

    File file = new File(filename + "-decrypted");
    file.createNewFile();
    Writer out = new OutputStreamWriter(new FileOutputStream(filename + "-decrypted"), "UTF-8");

    String line = null;
    try {
         while (( line = br.readLine()) != null){
             line = decrypt(Base64.decodeBase64(line));
             out.write(line);
         }
    } catch (Exception e) {
        e.printStackTrace();
    } finally {
        if (br != null) {
            br.close();
        }
        if (fis != null) {
            fis.close();
        }
        if (out != null) {
            out.close();
        }
    }
}

public static String decrypt(byte[] line) throws Exception {
    byte[] keyBytes = "1234123412341234".getBytes();
    final byte[] ivBytes = new byte[] { 0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09,
                0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f };

    final SecretKey secretkey = new SecretKeySpec(keyBytes, "AES");
    final IvParameterSpec IV = new IvParameterSpec(ivBytes);
    final Cipher decipher = Cipher.getInstance("AES/CFB8/NoPadding");
    decipher.init(Cipher.DECRYPT_MODE, secretkey, IV);
    final byte[] plainText = decipher.doFinal(line);

    return new String(plainText, "UTF-8").trim();
}

It worked but only partially. Some texts in the result file were decrypted correctly but some others were not. If you’re curious, this is what I mean by partially:

07 Jul 11 13:13:13, DEBUG  MrBean.java:checkUserVal���̥V;��ƃ�˨�� - username: squall,password: 4GROmr95Qcf����v�M�7�y�5�@CGO09 ,active: true 

I also have tried changing the algorithm to “DESede” but it was still partially decrypted. Then I tried to use “CBC/PKCS5Padding” in both ends but I got an exception

javax.crypto.IllegalBlockSizeException: Input length must be multiple of 16 when decrypting with padded cipher

I assume the encryption is not properly padding the input but I wonder why… because when I use the same encryption and decryption algorithms without the CipherOutputStream the padding worked just fine.
Anyone can help me to make this works? Any help will be appreciated.

PS: Sorry for my English, it’s not my native language.

Answer 1

Looks pretty good. Are you resetting/initializing the cipher object for every message? You probably don’t want to do that, but if you do then you need to think carefully about the structure of the cipher file, because the decryptor needs to know where the message boundaries are.