Elgamal 加密算法C或者C++代码

以下是 Elgamal 加密算法的 C++ 代码实现：

#include <iostream>
#include <cmath>
#include <cstdlib>
#include <ctime>

using namespace std;

int gcd(int a, int b) {
    if (b == 0) return a;
    return gcd(b, a % b);
}

int fastPow(int a, int b, int p) {
    int ans = 1;
    while (b) {
        if (b & 1) ans = (ans * a) % p;
        a = (a * a) % p;
        b >>= 1;
    }
    return ans;
}

int main() {
    srand(time(NULL));
    int p, g, x, y, k, m, c1, c2, d;
    cout << "Enter p, g, x: ";
    cin >> p >> g >> x;
    y = fastPow(g, x, p);
    cout << "Public key: (" << p << ", " << g << ", " << y << ")" << endl;
    cout << "Enter message m: ";
    cin >> m;
    k = rand() % (p - 2) + 1;
    while (gcd(k, p - 1) != 1) k = rand() % (p - 2) + 1;
    c1 = fastPow(g, k, p);
    c2 = (m * fastPow(y, k, p)) % p;
    cout << "Cipher text: (" << c1 << ", " << c2 << ")" << endl;
    cout << "Enter private key x: ";
    cin >> x;
    d = fastPow(c1, p - 1 - x, p);
    int m2 = (c2 * d) % p;
    cout << "Decrypted message: " << m2 << endl;
    return 0;
}

注：此代码仅用于学习和参考，实际使用时需要进行安全性和性能方面的优化。