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 {
  privat         


        

Answer 1

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);
    }
}