aes encryption and decryption through tcp.Input length must be multiple of 16 when decrypting with padded cipher

Question

I see this "Input length must be multiple of 16 when decrypting with padded cipher" error when I run the program

RealEchoServer.java

import java.io.*;
import java.net.*;
import java.security.*;
import javax.crypto.*;
import javax.crypto.spec.*;
public class RealEchoServer {

public static void main(String[] args) {
    int i = 1;
    try {
        ServerSocket s = new ServerSocket(9003);

        for (;;) {
            Socket incoming = s.accept();
            System.out.println("Spawning " + i);
            new RealEchoHandler(incoming, i).start();
            i++;
        }
    } catch (Exception e) {
        System.out.println(e);
    }
 }
}

class RealEchoHandler extends Thread {

DataInputStream in;
DataOutputStream out;
private Socket incoming;
private int counter;

public RealEchoHandler(Socket i, int c) {
    incoming = i;
    counter = c;
}

public void run() {
    try {

        String key1 = "1234567812345678";
        byte[] key2 = key1.getBytes();
        SecretKeySpec secret = new SecretKeySpec(key2, "AES");
        String msg = "Singapore Malaysia Japan India Indonesia HongKong Taiwan China England";
        Cipher cipher = Cipher.getInstance("AES");
        cipher.init(Cipher.ENCRYPT_MODE, secret);
        byte[] encrypted = cipher.doFinal(msg.getBytes());

        in = new DataInputStream(incoming.getInputStream());
        out = new DataOutputStream(incoming.getOutputStream());

        boolean done = false;
        String str = "";
        out.writeUTF("Connected!\n");
        out.flush();
        while (!done) {
            out.writeUTF(">");
            out.flush();
            str = in.readUTF();
            System.out.println(in + ":" + str);
            if (str == null) {
                done = true;
            } else {
                System.out.println("Sending Ciphertext : " + new String(encrypted));
                out.writeUTF(new String(encrypted));
                out.flush();
            }
        }
        incoming.close();
    } catch (Exception e) {
        System.out.println(e);
    }
 }
}

RealSocketTest.java

   import java.io.*;
   import java.net.*;
   import java.security.*;
   import javax.crypto.*;
   import javax.crypto.spec.*;
   import java.util.*;

 class RealSocketTest {
 public static void main(String[] args) throws NoSuchAlgorithmException, NoSuchPaddingException, InvalidKeyException, IllegalBlockSizeException, BadPaddingException {

    String str = "";
    String str2 = "";
    DataOutputStream out;
    DataInputStream in;

    try {
        Socket t = new Socket("127.0.0.1", 9003);
        in = new DataInputStream(t.getInputStream());
        out = new DataOutputStream(t.getOutputStream());
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

        boolean more = true;
        System.out.println(in.readUTF());

        while (more) {
            str = in.readUTF();
            System.out.print(str);
            str2 = br.readLine();
            out.writeUTF(str2);
            out.flush();
            str = in.readUTF();

            System.out.println("Encrypted Info: " + str);

            try {

                String key1 = "1234567812345678";
                byte[] key2 = key1.getBytes();
                SecretKeySpec secret = new SecretKeySpec(key2, "AES");
                Cipher cipher = Cipher.getInstance("AES");
                cipher.init(Cipher.DECRYPT_MODE, secret);
                byte[] decrypted = cipher.doFinal(str.getBytes());
                System.out.println("Decrypted Info: " + new String(decrypted));
            } catch (BadPaddingException e) {
                System.out.println("Wrong Key!");
            } catch (InvalidKeyException f) {
                System.out.println("Invalid Key!");
            }
        }
    } catch (IOException e) {
        System.out.println("Error");
    }
  }
}

I've read a similar problem here Illegal Block Size Exception Input length must be multiple of 16 when decrypting with padded cipher , but I don't understand how I could change mine, because it looks very different from mine.

So what should be added/changed to get it decrypted?

`String` is not a container for binary data. – user207421 Apr 20 '15 at 21:14 — user207421, Apr 20 '15 at 21:14

score 1 · Accepted Answer · answered Apr 20 '15 at 19:59

The problem is that ciphertexts may contain every possible byte value. Many byte values on the other hand are not printable and therefore not a valid UTF-8 encoding. When you make a String out of it new String(encrypted), it will silently drop some bytes and you won't be able to successfully decrypt the ciphertext.

Two possible fixes:

Encode the ciphertext as Base64 or Hex to be used in a text-based protocol.
Use the DataOutputStream::write() method to make it a binary protocol without encoding it.

Other security stuff:

Always specify the complete String for the expected Cipher instance. Different providers might have different defaults and it may happen that client and server don't use the same method. Example: AES/ECB/PKCS5Padding.
Never use ECB mode. It is not semantically secure. At least use CBC with a random IV (prepend the IV in front of the ciphertex or write it to the stream in order).
Check your ciphertext for manipulation. This is easily done, by employing an authenticated mode like GCM (AES/GCM/NoPadding with GCMParameters). If you don't want that, then at least try to implement an encrypt-then-MAC scheme where you HMAC the ciphertext (with a different key) and check it before decryption.

that really made sense, and the security stuff was deep(good). i'm just a starter , i probably got the code from the internet so could you post the code precisely for either of the possible fixes ? i appreciate it very much — pavanleawesome, Apr 20 '15 at 20:13
Here are [some ways](http://stackoverflow.com/questions/13109588/base64-encoding-in-java) you can use Base64 encoders. Encode `encrypted` before writing it to the socket and decode `str = in.readUTF();` before passing it to `cipher.doFinal()`. I'm not really sure whether this will fix all your problems, because the protocol seems strange. — Artjom B., Apr 20 '15 at 20:38

aes encryption and decryption through tcp.Input length must be multiple of 16 when decrypting with padded cipher

1 Answers1