I am trying to crack an RC4 encryption in java. I am given 8 bytes of an 11 byte key. I need to find the remaining 3 bytes,: Pretending I didn't have the last 3 bytes of the key, how would I change to cipher text to plain text. My test code is:

class ARC4CrackerTest {

@org.junit.jupiter.api.Test

void noHintCrack() throws NoSuchPaddingException,

NoSuchAlgorithmException, InvalidKeyException, BadPaddingException,

IllegalBlockSizeException {

// setup the secret

byte secretBytes[] = Arrays.copyOf(ARC4Cracker.keyPrefix, 11);

secretBytes[8] = (byte) 0xd0;

secretBytes[9] = (byte) 0xff;

secretBytes[10] = 1;

SecretKey secretKey = new SecretKeySpec(secretBytes, "ARCFOUR");

// setup the cipher

// encrypt and base64 encode

String plainText = "this is the secret plain text";

String base64CipherText =

Base64.getEncoder().encodeToString(rc4Encrypt(secretKey,

plainText.getBytes()));

String base64KnownString =

Base64.getEncoder().encodeToString("secret".getBytes());

// see if the cracker can give us the answer

ARC4Cracker cracker = new ARC4Cracker();

String base64CrackedText = cracker.crack(base64CipherText,

base64KnownString);

Assertions.assertEquals(plainText, new

String(Base64.getDecoder().decode(base64CrackedText)));

}

private byte[] rc4Encrypt(SecretKey secretKey, byte[] plainText)

throws NoSuchAlgorithmException, NoSuchPaddingException,

InvalidKeyException, IllegalBlockSizeException, BadPaddingException {

Cipher arc4 = Cipher.getInstance("ARCFOUR");

arc4.init(Cipher.ENCRYPT_MODE, secretKey);

return arc4.doFinal(plainText);

}

My code for cracking is:

public class ARC4Cracker {

/**

* the know prefix. (we just need to discover the remaining 3 bytes)

*/

static public byte keyPrefix[] = { 0x13, 0x37, (byte)0xd0, 0x0d, 0x15,

0x50, (byte)0xc0, 0x01};

/**

* this method provides a hint of known plaintext, and the

corresponding cipherText

* @param base64CipherText

* @param base64PlainText base64 encoded known plain text from

* @param position the position in the stream where the text was

known

*/

public void crackedText(String base64CipherText, String

base64PlainText, int position) { }

/**

* the method will crack cipher text by searching for the correct

plain text containing

* the known string

* @param base64CipherText base64 encoded cipher text to crack

* @param base64KnownText a base64 encoded string that is know to

exist in the plain text

* @return the base64 encoded plain text or null if couldn't crack

*/

public String crack(String base64CipherText, String base64KnownText) {

return null; }

}

Thanks