We Discussed about using 4 bytes state and 2 bytes key for stream generation. As

Question

We Discussed about using 4 bytes state and 2 bytes key for stream generation. Assuming that K = [2,1] and the permutation S = [1,2,3,0] has been obtained after Step 2 (initial permutation of S). Write a Program to complete the followings:

1. Complete the Stream generation step (step 3) to obtain streams.

2. USing the streams to encrypt the message: "HI" (H = 01001000 and I = 01001001)

Your output should look like the following screenshot.

        String res="";

        // xor
        int c =   p ^ k;

        // Binary representation
        String b4p = String.format("%8s", Integer.toBinaryString((int)p)).replace(' ', '0');
        String b4k = String.format("%8s", Integer.toBinaryString(k)).replace(' ', '0');
        String b4c = String.format("%8s", Integer.toBinaryString(c)).replace(' ', '0');

        // print out
        System.out.printf("%s (%2c) ", b4p,p);
        System.out.printf("%s (%2d) ", b4k, k);
        System.out.printf("-------------- ");
        System.out.printf("%s (%c^%d) ", b4c, p,k);

        // print out the corresponding letter
        res += (char) c;

        return res;
    }

    public static int [] streamGeneration (int keylen) {
        int key [] = new int[keylen];
        int S[] = {1,2,3,0};

        int i = 0;
        int j = 0;
        int count = 0;
        int temp, t;

        // ......

        return key;
    }

    public static void main () {

       
        // key generation
        int [] key = streamGeneration (2);

        System.out.print("Key: ");
        for (int k=0; k             System.out.print(key[k] + " ");
        }
        System.out.println(" ");

        // encryption
        String cipher = encryption ('A', 3);
        System.out.printf("Cipher text: %s ", cipher);

    }
}

Options Key: 3 91881008 (H) 00000011 (3) 91001811 (HA3) 91001001 I) eee00eee ) 91881881 (IAB) Cipher text: KI an only enter input whi ure 1: A screen shot of program output

Explanation / Answer

Hi I have implemented how to find the next permutation and then take number of key len elements from S[] array.

after that we need to call the encryption method for H and I as well.

Please find the below code and output after running it in any java compiler.

/*package whatever //do not write package name here */

import java.io.*;

public class StreamCipher
{
public static String encryption (char p, int k) {

String res="";

// xor
int c = p ^ k;

// Binary representation
String b4p = String.format("%8s", Integer.toBinaryString((int)p)).replace(' ', '0');
String b4k = String.format("%8s", Integer.toBinaryString(k)).replace(' ', '0');
String b4c = String.format("%8s", Integer.toBinaryString(c)).replace(' ', '0');

// print out
System.out.printf("%s (%2c) ", b4p,p);
System.out.printf("%s (%2d) ", b4k, k);
System.out.printf("-------------- ");
System.out.printf("%s (%c^%d) ", b4c, p,k);

// print out the corresponding letter
res += (char) c;

return res;
}

public static int [] streamGeneration (int keylen) {
int key [] = new int[keylen];
int S[] = {1,2,3,0};

int i = 0;
int j = 0;
int count = 0;
int temp, t;

// ......
//this piece of code will find the next permutation
i = S.length - 1;
while (i > 0 && S[i - 1] >= S[i]) {
i--;
}

// if (i <= 0) {
// return false;
// }

j = S.length - 1;

while (S[j] <= S[i - 1]) {
j--;
}

temp = S[i - 1];
S[i - 1] = S[j];
S[j] = temp;

j = S.length - 1;

while (i < j) {
temp = S[i];
S[i] = S[j];
S[j] = temp;
i++;
j--;
}
  
for(i=0;i<4;i++){
//System.out.print(" "+S[i]);
  
  
if(count<keylen){
key[count] = S[keylen+i-1];
}
count++;
}

return key;
}

public static void main (String[] args) {

// key generation
int [] key = streamGeneration (2);

System.out.print("Key: ");
for (int k=0; k<2;k++){
System.out.print(key[k] + " ");
}
System.out.println(" ");

// encryption
String cipher = encryption ('H', 3);
//System.out.printf("Cipher text: %s ", cipher);
String encrypted_cipher = cipher;
cipher = encryption ('I', 0);
encrypted_cipher+=cipher;
System.out.printf("Cipher text: %s ", encrypted_cipher);

}
}

Output :

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