DH密码协商协议C语言代码简单实现，不调用库

DH（Diffie-Hellman）密钥交换协议是一种非对称加密算法，用于两个通信方在无需预先共享秘密的情况下生成相同的密钥。以下是一个简单的DHE协议C语言代码示例，仅使用自定义大素数和模运算，未调用外部库：

#include <stdio.h>
#include <stdlib.h>

// 自定义大素数和模运算
typedef struct {
    unsigned long long prime;
    unsigned long long a;
} CustomPrime;

CustomPrime get_prime(int bits) {
    CustomPrime p = {0};
    while (1) {
        p.prime = rand() % ((1ull << bits) - 2) + 2; // 避免2和质数
        if (gcd(p.prime - 2, p.prime) == 1) break;
    }
    return p;
}

unsigned long long gcd(unsigned long long a, unsigned long long b) {
    while (b != 0) {
        unsigned long long temp = a % b;
        a = b;
        b = temp;
    }
    return a;
}

void dh_exchange(CustomPrime* my_private_key, CustomPrime* their_public_key, unsigned long long* shared_secret) {
    *shared_secret = (my_private_key->a * their_public_key->prime) % my_private_key->prime;
    printf("My public key: A = %llu\n", my_private_key->a);
    printf("Their public key: B = %llu\n", their_public_key->prime);
    printf("Shared secret: S = %llu\n", *shared_secret);
}

int main() {
    int bit_length = 1024; // 密钥长度，可以根据需求调整
    CustomPrime my_prime = get_prime(bit_length);
    CustomPrime their_prime = get_prime(bit_length); // 对方的公钥

    // 每个人维护自己的私钥和对方的公钥
    unsigned long long my_private_a = rand() % (my_prime.prime - 1) + 1;
    unsigned long long their_public_b = their_prime.a;

    CustomPrime my_public_key = (CustomPrime){my_prime.prime, my_private_a};
    CustomPrime their_public_key = (CustomPrime){their_prime.prime, their_public_b};

    unsigned long long shared_secret;
    dh_exchange(&my_private_key, &their_public_key, &shared_secret);

    // 现在双方都有了可以用于后续加密和解密的共同密钥
    return 0;
}

注意：此示例代码仅适用于教学目的，实际生产环境中应使用经过安全测试的库（如 OpenSSL），因为自行实现可能会存在安全性风险。