lab11

#include <iostream>

#include <cmath>

#include <string>

using namespace std;

long long modExp(long long base, long long exp, long long mod) {

    long long result = 1;

    while (exp > 0) {

        if (exp % 2 == 1)

            result = (result * base) % mod;

        base = (base * base) % mod;

        exp /= 2;

    }

    return result;

}

string xorEncryptDecrypt(const string &text, long long sharedKey) {

    string result = text;

    for (size_t i = 0; i < text.size(); i++) {

        result[i] = text[i] ^ (sharedKey % 256);

    }

    return result;

}

int main() {

    long long p, g, a, b;

    cout << "Enter a large prime number (p): ";

    cin >> p;

    cout << "Enter a primitive root modulo p (g): ";

    cin >> g;

    cout << "Enter Alice's private key (a): ";

    cin >> a;

    cout << "Enter Bob's private key (b): ";

    cin >> b;

    long long A = modExp(g, a, p);

    long long B = modExp(g, b, p);

    cout << "Alice's Public Key (A): " << A << endl;

    cout << "Bob's Public Key (B): " << B << endl;

    long long shared_secret_Alice = modExp(B, a, p);

    long long shared_secret_Bob = modExp(A, b, p);

    if (shared_secret_Alice == shared_secret_Bob) {

        cout << "Shared Secret Computed Successfully: " << shared_secret_Alice << endl;

    } else {

        cout << "Error: Shared secrets do not match!" << endl;

        return 1;

    }

    cin.ignore();

    string plainText;

    cout << "Enter a plain text sentence or paragraph: ";

    getline(cin, plainText);

    string encryptedText = xorEncryptDecrypt(plainText, shared_secret_Alice);

    cout << "Encrypted Message: " << encryptedText << endl;

    string inputEncryptedText;

    cout << "\nEnter the encrypted message: ";

    getline(cin, inputEncryptedText);

    string decryptedText = xorEncryptDecrypt(inputEncryptedText, shared_secret_Alice);

    if (decryptedText == plainText) {

        cout << "Decryption successful! Decrypted Message: " << decryptedText << endl;

    } else {

        cout << "Decryption failed! The decrypted message does not match the original plaintext." << endl;

    }

    return 0;

}