RC4 encryption java

Question

Hi there I am trying to implement the RC4 algorithm in Java. I found this code as an example that help me to understand the idea:

public class RC4 {
  private int[] S = new int[256];
  private int[] T = new int[256];
  private int keylen;

  public RC4(byte[] key) throws Exception {
    if (key.length < 1 || key.length > 256) {
      throw new Exception("key must be between 1 and 256 bytes");
    } else {
      keylen = key.length;
      for (int i = 0; i < 256; i++) {
        S[i] = i;
        T[i] = key[i % keylen];
      }
      int j = 0;
      for (int i = 0; i < 256; i++) {
        j = (j + S[i] + T[i]) % 256;
        S[i] ^= S[j];
        S[j] ^= S[i];
        S[i] ^= S[j];
      }
    }
  }

  public int[] encrypt(int[] plaintext) {
    int[] ciphertext = new int[plaintext.length];
    int i = 0, j = 0, k, t;
    for (int counter = 0; counter < plaintext.length; counter++) {
      i = (i + 1) % 256;
      j = (j + S[i]) % 256;
      S[i] ^= S[j];
      S[j] ^= S[i];
      S[i] ^= S[j];
      t = (S[i] + S[j]) % 256;
      k = S[t];
      ciphertext[counter] = plaintext[counter] ^ k;
    }
    return ciphertext;
  }

  public int[] decrypt(int[] ciphertext) {
    return encrypt(ciphertext);
  }
}

I have few question:

1. Why is the plain-text an int array in the above code?

2. When I test this code I get strange result, can somebody explain to me? Here my code to test:

   public class RC4_Main {
   
       public static void main(String args[]) throws Exception {
           String keyword = "hello";
           byte[] keytest = keyword.getBytes(); //convert keyword to byte
   
           int[] text = {1, 2, 3, 4, 5}; // text as 12345
   
           RC4 rc4 = new RC4(keytest);
   
           System.out.print("\noriginal text: ");
           for (int i = 0; i < text.length; i++) {          
               System.out.print(text[i]);          
           }    
   
           int[] cipher = rc4.encrypt(text); //encryption      
           System.out.print("\ncipher: ");
           for (int i = 0; i < cipher.length; i++) {          
               System.out.print(cipher[i]);          
           }    
   
           int[] backtext = rc4.decrypt(cipher); //decryption
           System.out.print("\nback to text: ");
           for (int i = 0; i < backtext.length; i++) {          
               System.out.print(backtext[i]);            
           } 
           System.out.println();
       }
   }
   

Here is the result: (original and back to text are not SAME) why???

original text: 12345
cipher: 1483188254174
back to text: 391501310217

Answer 1

There are a few things to notice:

• Java is not very easy to use when you require unsigned bytes (e.g. for indexing);
• if you create a state in S and T, you should really notice that these values change, when you decrypt with the same instance you take the state used for encryption;
• the above code is not very efficient memory wise, and you can easily rewrite it to take byte arrays;
• to use a String, after refactoring the arguments to byte[], you first need to use character-encoding first, e.g. using String.getBytes(Charset charset);

To make life easier, and to have some fun late night hacking, I improved your code and tested it against a single vector in rfc6229 using a zero'd out byte array.

UPDATE: As micahk points out below, the evil C XOR swap that was used prevented this code from encrypting the final byte of input in Java. Using regular old swaps fixes it.

Warning: the code below should be considered a coding exercise. Please use a well vetted library instead of the code snippet below to perform RC4 (or Ron's Code 4, ARC4 etc.) in your application. That means using Cipher.getInstance("RC4"); or the ARC4 classes in Bouncy Castle.

public class RC4 {
    private final byte[] S = new byte[256];
    private final byte[] T = new byte[256];
    private final int keylen;

    public RC4(final byte[] key) {
        if (key.length < 1 || key.length > 256) {
            throw new IllegalArgumentException(
                    "key must be between 1 and 256 bytes");
        } else {
            keylen = key.length;
            for (int i = 0; i < 256; i++) {
                S[i] = (byte) i;
                T[i] = key[i % keylen];
            }
            int j = 0;
            byte tmp;
            for (int i = 0; i < 256; i++) {
                j = (j + S[i] + T[i]) & 0xFF;
                tmp = S[j];
                S[j] = S[i];
                S[i] = tmp;
            }
        }
    }

    public byte[] encrypt(final byte[] plaintext) {
        final byte[] ciphertext = new byte[plaintext.length];
        int i = 0, j = 0, k, t;
        byte tmp;
        for (int counter = 0; counter < plaintext.length; counter++) {
            i = (i + 1) & 0xFF;
            j = (j + S[i]) & 0xFF;
            tmp = S[j];
            S[j] = S[i];
            S[i] = tmp;
            t = (S[i] + S[j]) & 0xFF;
            k = S[t];
            ciphertext[counter] = (byte) (plaintext[counter] ^ k);
        }
        return ciphertext;
    }

    public byte[] decrypt(final byte[] ciphertext) {
        return encrypt(ciphertext);
    }
}

Happy coding.

Answer 2

The Java code has a bug due to the use of the xor-swap technique:

        S[i] ^= S[j];
        S[j] ^= S[i];
        S[i] ^= S[j];

Instead of this, you'll want to use a temp variable as in the below. I haven't delved into why the result isn't as expected with the xor swap, but I had decryption errors with this that were resolved by simply doing a straight-forward swap. I suspect it to be a subtle-side effect of the implicit cast from byte to int that occurs in order to do the xor operation.

public class RC4 {
    private final byte[] S = new byte[256];
    private final byte[] T = new byte[256];
    private final int keylen;

    public RC4(final byte[] key) {
        if (key.length < 1 || key.length > 256) {
            throw new IllegalArgumentException(
                    "key must be between 1 and 256 bytes");
        } else {
            keylen = key.length;
            for (int i = 0; i < 256; i++) {
                S[i] = (byte) i;
                T[i] = key[i % keylen];
            }
            int j = 0;
            for (int i = 0; i < 256; i++) {
                j = (j + S[i] + T[i]) & 0xFF;
                byte temp = S[i];
                S[i] = S[j];
                S[j] = temp;
            }
        }
    }

    public byte[] encrypt(final byte[] plaintext) {
        final byte[] ciphertext = new byte[plaintext.length];
        int i = 0, j = 0, k, t;
        for (int counter = 0; counter < plaintext.length; counter++) {
            i = (i + 1) & 0xFF;
            j = (j + S[i]) & 0xFF;
            byte temp = S[i];
            S[i] = S[j];
            S[j] = temp;
            t = (S[i] + S[j]) & 0xFF;
            k = S[t];
            ciphertext[counter] = (byte) (plaintext[counter] ^ k);
        }
        return ciphertext;
    }

    public byte[] decrypt(final byte[] ciphertext) {
        return encrypt(ciphertext);
    }
}

Answer 3

Your integer arrays S andT have not been constructed. Hence you get a NullPointerException as soon as you attempt to use them.

Looking at the rest of the code, I guess they should have been 256-item arrays:

private int[] S = new int[256];
private int[] T = new int[256];

Answer 4

(I know this is a old thread, but maybe my answer can help who is reading it)

The problem is not in the RC4 code but in how you are using it. What you have to understand is every time that encript method is invoked, the S array is modified to generate a pseudo random key.

In this code your are using the decript method after encript over the same instance of RC4 class. But RC4 class have the key creation in the constructor, so when you execute decript method, the key is not recently created as it has been modified by the previous encript. Instead of this code:

int[] cipher = rc4.encrypt(text); //encryption      
System.out.print("\ncipher: ");
for (int i = 0; i < cipher.length; i++) {          
    System.out.print(cipher[i]);          
}    

int[] backtext = rc4.decrypt(cipher); //decryption
System.out.print("\nback to text: ");
for (int i = 0; i < backtext.length; i++) {          
    System.out.print(backtext[i]);            
} 

Use a rc4 new instance before decript:

int[] cipher = rc4.encrypt(text); //encryption      
System.out.print("\ncipher: ");
for (int i = 0; i < cipher.length; i++) {          
    System.out.print(cipher[i]);          
}    

rc4 = new RC4(keytest);
int[] backtext = rc4.decrypt(cipher); //decryption
System.out.print("\nback to text: ");
for (int i = 0; i < backtext.length; i++) {          
    System.out.print(backtext[i]);            
} 

So the decript method will be have a clean S array, and it will be able to obtain S sequence in the same order than the previous encript method.

Answer 5

1) int array: probably because Java doesn't support unsigned bytes.

2) Null exception: I counted line 12 being this one: S[i] = i; It looks like the S array is not being constructed before it's used.

Answer 6

RC4 is a broken algorithm and recommendation is to not use the same anymore if the data is to be kept highly secure.

If you still need a working implementation, you don't need to recreate the algorithm in your code. Java API javax.crypto can do it for you. Just generate a key and call the init method with mode set to encryption/decryption.

static String decryptRC4() throws NoSuchAlgorithmException, NoSuchPaddingException, InvalidKeyException, IllegalBlockSizeException, BadPaddingException, InvalidAlgorithmParameterException{

    byte[] testDataBytes = "testString".getBytes();

    KeyGenerator rc4KeyGenerator = KeyGenerator.getInstance("RC4");
    SecretKey key = rc4KeyGenerator.generateKey();

    // Create Cipher instance and initialize it to encrytion mode
    Cipher cipher = Cipher.getInstance("RC4");  // Transformation of the algorithm
    cipher.init(Cipher.ENCRYPT_MODE, key);
    byte[] cipherBytes = cipher.doFinal(testDataBytes);

    // Reinitialize the Cipher to decryption mode
    cipher.init(Cipher.DECRYPT_MODE,key, cipher.getParameters());
    byte[] testDataBytesDecrypted = cipher.doFinal(cipherBytes);

    System.out.println("Decrypted Data : "+new String(testDataBytesDecrypted));
    return new String(testDataBytesDecrypted);
}

Output:

If you need to send the encrypted data as part of a url then use Base64Encoding and then send.

e.g.

    static String decryptRC4() throws NoSuchAlgorithmException, NoSuchPaddingException, InvalidKeyException, IllegalBlockSizeException, BadPaddingException, InvalidAlgorithmParameterException{

    byte[] plainBytes = "testString".getBytes();

    KeyGenerator rc4KeyGenerator = KeyGenerator.getInstance("RC4");
    SecretKey key = rc4KeyGenerator.generateKey();

    // Create Cipher instance and initialize it to encrytion mode
    Cipher cipher = Cipher.getInstance("RC4");  // Transformation of the algorithm
    cipher.init(Cipher.ENCRYPT_MODE, key);
    byte[] cipherBytes = cipher.doFinal(plainBytes);

    String encoded = encodeBase64(cipherBytes);

    String decoded = decodeBase64(encoded);

    // Reinitialize the Cipher to decryption mode
    cipher.init(Cipher.DECRYPT_MODE,key, cipher.getParameters());
    byte[] plainBytesDecrypted = cipher.doFinal(Hex.decode(decoded));

    System.out.println("Decrypted Data : "+new String(plainBytesDecrypted));
    return new String(plainBytesDecrypted);
}

static String decodeBase64(String encodedData){
    byte[] b = Base64.getDecoder().decode(encodedData);
    String decodedData = DatatypeConverter.printHexBinary(b);
    return decodedData;
}

static String encodeBase64(byte[] data){
    byte[] b = Base64.getEncoder().encode(data);
    String encodedData = new String(b);
    /*String encodedData = DatatypeConverter.printHexBinary(b);*/
    return encodedData;
}

Tip: Use Hex.decode as shown above to get bytes from the base64 decoded string or else you will get encoding issues. As much as possible do the conversions using Hex and convert to bytes array using bouncycastle methods.

Imports needed:

import java.io.IOException;
import java.security.InvalidAlgorithmParameterException;
import java.security.InvalidKeyException;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.util.Base64;

import javax.crypto.BadPaddingException;
import javax.crypto.Cipher;
import javax.crypto.IllegalBlockSizeException;
import javax.crypto.KeyGenerator;
import javax.crypto.NoSuchPaddingException;
import javax.crypto.SecretKey;
import javax.xml.bind.DatatypeConverter;

import org.apache.commons.codec.DecoderException;
import org.bouncycastle.util.encoders.Hex;

Also if you are generating a key from your own string you can use MD5Hashing for the same.

Please refer this for help on how to create a key using custom String: https://stackoverflow.com/a/52463858/5912424