Fun Things in Life

… Answer a call for help, hopefully I would get the same when I call for help …

The Problem: Part 1 and Part 2 has been quite interesting. I was thinking of what to do next and Robi Ilham asked me to try to solve the problem of encrypting a PDF file (in PHP) and decrypting it in Java.

Now this seems hard, but once you look at how it is done, this is pretty simple stuff. You just need to know the appropriate tools to use and right encryption method (in this case, AES).

The Solution:

1. Encrypt a message in PHP and force it down the browser:

   001	<?php
   002	  $secret_key   = "01234567890abcde";
   003	  $iv           = "fedcba9876543210";
   004	  $infile       = "plain.pdf";
   005	  $outfile      = "encrypted.pdf";
   006
   007	  $plaintext = file_get_contents($infile);
   008	  $crypttext = mcrypt_encrypt(MCRYPT_RIJNDAEL_128,
   009                      $secret_key, $plaintext, MCRYPT_MODE_CBC, $iv);
   010
   011	  header('Content-Type: application/octet-stream');
   012	  header('Content-Length: ' . strlen($crypttext));
   013	  header('Content-Disposition: attachment; filename=' . ($outfile));
   014	  echo $crypttext;
   015	?>

2. The above PHP program will force an encrypted PDF file down from your browser. It takes the unencrypted PDF file (plain.pdf) and force a download of encrypted.pdf (the encrypted file).
3. Now after saving the encrypted.pdf, we use a Java program to decrypt it:

   001	import java.io.*;
   002	import javax.crypto.*;
   003	import javax.crypto.spec.*;
   004
   005	public class decrypt {
   006	  public static void main(String args[]) throws Exception {
   007	    Cipher cipher = Cipher.getInstance("AES/CBC/NoPadding");
   008	    SecretKeySpec keySpec = new SecretKeySpec("01234567890abcde".getBytes(), "AES");
   009	    IvParameterSpec ivSpec = new IvParameterSpec("fedcba9876543210".getBytes());
   010	    cipher.init(Cipher.DECRYPT_MODE, keySpec, ivSpec);
   011
   012	    FileInputStream fis   = new FileInputStream(new File("encrypted.pdf"));
   013	    CipherInputStream cis = new CipherInputStream(fis, cipher);
   014	    FileOutputStream fos  = new FileOutputStream(new File("decrypted.pdf"));
   015
   016	    byte[] b = new byte[8];
   017	    int i;
   018
   019	    while ((i = cis.read(b)) != -1) {
   020	      fos.write(b, 0, i);
   021	    }
   022	    fos.flush(); fos.close();
   023	    cis.close(); fis.close();
   024	?>

4. The Java program takes the encrypted.pdf and write out the decrypted.pdf which should be the same as the plain.pdf.