javax.crypto.BadPaddingException

Question

I am working on AES algorithm, and I have this exception which I couldn't solve.

javax.crypto.BadPaddingException: Given final block not properly padded at com.sun.crypto.provider.SunJCE_f.b(DashoA13*..) at com.sun.crypto.provider.SunJCE_f.b(DashoA13*..) at com.sun.crypto.provider.AESCipher.engineDoFinal(DashoA13*..) at javax.crypto.Cipher.doFinal(DashoA13*..) 

the exception happens in the decryption part. I initialize the key in a different place from where the decryption algorithm is

KeyGenerator kgen = KeyGenerator.getInstance("AES");//key generation for AES kgen.init(128); // 192 and 256 bits may not be available 

then I pass it with the cipher text which I read from file to the following method

 public String decrypt(String message, SecretKey skey) {      byte[] raw = skey.getEncoded();     SecretKeySpec skeySpec = new SecretKeySpec(raw, "AES");     // Instantiate the cipher     Cipher cipher;      byte[] original = null;     try {         cipher = Cipher.getInstance("AES");          cipher.init(Cipher.DECRYPT_MODE, skeySpec);         System.out.println("Original string: "                 + message);         original = cipher.doFinal(message.trim().getBytes());  //here where I got the exception         String originalString = new String(original);        }  //catches 

EDIT here's the encryption method.

public String encrypt(String message, SecretKey skey) {     byte[] raw = skey.getEncoded();     SecretKeySpec skeySpec = new SecretKeySpec(raw, "AES");      // Instantiate the cipher      Cipher cipher;     byte[] encrypted = null;     try {         cipher = Cipher.getInstance("AES");          cipher.init(Cipher.ENCRYPT_MODE, skeySpec);          encrypted = cipher.doFinal(message.getBytes());         System.out.println("raw is " + encrypted);      } catches     return asHex(encrypted); } 

and here's the asHex method

  public static String asHex(byte buf[]) {     StringBuffer strbuf = new StringBuffer(buf.length * 2);     int i;      for (i = 0; i < buf.length; i++) {         if (((int) buf[i] & 0xff) < 0x10) {             strbuf.append("0");         }          strbuf.append(Long.toString((int) buf[i] & 0xff, 16));     }      return strbuf.toString(); } 

Here's where I read the cipher text form the file

static public String readFile(String filePath) {     StringBuilder file = new StringBuilder();     String line = null;     try {         FileReader reader = new FileReader(filePath);         BufferedReader br = new BufferedReader(reader);         if (br != null) {             line = br.readLine();             while (line != null) {                 file.append(line);                 //      System.out.println("line is " + line);                 line = br.readLine();              }         }         br.close();         reader.close();     } catch (IOException ex) {         Logger.getLogger(FileManagement.class.getName()).log(Level.SEVERE, null, ex);     }     System.out.println("line is " + file.toString());     return String.valueOf(file);  } 

can someone help?

Answer 1

Ok, so the problem is that you are converting the encrypted bytes to a hex string (using the asHex method) but are not converting the hex string back to a byte array correctly for decryption. You can't use getBytes.

You can use the following method to convert a hex string to a byte array:

public static byte[] fromHexString(String s) {     int len = s.length();     byte[] data = new byte[len / 2];     for (int i = 0; i < len; i += 2) {         data[i / 2] = (byte) ((Character.digit(s.charAt(i), 16) << 4)                              + Character.digit(s.charAt(i+1), 16));     }     return data; } 

and then change your decrypt method to use:

original = cipher.doFinal(fromHexString(message));