1// Aim: Program to perform RC4 encryption with Diffie-Hellman key exchange (RC4 with Diffie-Hellman Key Exchange).
2import javax.crypto.*;
3import javax.crypto.spec.SecretKeySpec;
4import java.util.*;
5import java.math.BigInteger;
6
7public class rc4_with_diffie {
8    public static void main(String[] args) throws Exception {
9        // Diffie-Hellman Key Exchange
10        Scanner sc = new Scanner(System.in);
11        System.out.print("Enter a prime number (q): ");
12        BigInteger q = sc.nextBigInteger();
13        System.out.print("Enter a primitive root: ");
14        BigInteger a = sc.nextBigInteger();
15        System.out.print("Enter private key for user A: ");
16        BigInteger xA = sc.nextBigInteger();
17        System.out.print("Enter private key for user B: ");
18        BigInteger xB = sc.nextBigInteger();
19        BigInteger yA = a.modPow(xA, q);
20        BigInteger yB = a.modPow(xB, q);
21        BigInteger kA = yB.modPow(xA, q); // Secret key
22        BigInteger kB = yA.modPow(xB, q); // Secret key verification
23
24        if (!kA.equals(kB)) {
25            System.out.println("Error: Keys do not match!");
26            return;
27        }
28
29        System.out.println("Shared Secret Key: " + kA);
30        byte[] keyBytes = Arrays.copyOf(kA.toByteArray(), 16); // Using 16 bytes for RC4 key
31        SecretKey key = new SecretKeySpec(keyBytes, "RC4");
32
33        // RC4 Encryption and Decryption
34        System.out.print("Enter text: ");
35        sc.nextLine(); // Consume the newline
36        String msg = sc.nextLine();
37        sc.close();
38
39        Cipher cpr = Cipher.getInstance("RC4");
40        cpr.init(Cipher.ENCRYPT_MODE, key);
41        byte[] encrypted = cpr.doFinal(msg.getBytes("UTF-8"));
42        String enctext = Base64.getEncoder().encodeToString(encrypted);
43
44        cpr.init(Cipher.DECRYPT_MODE, key);
45        byte[] decrypted = cpr.doFinal(Base64.getDecoder().decode(enctext));
46        String dectext = new String(decrypted, "UTF-8");
47
48        System.out.println("Original: " + msg);
49        System.out.println("Encrypted: " + enctext);
50        System.out.println("Decrypted: " + dectext);
51    }
52}