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.