java - 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));