RC4加解密源码（C语言描述）

RC4是一种流密码算法，常用于对网络数据进行实时加密。下面是C语言的一个简化版RC4加解密源码概述：

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

// 初始化S盒数组
void init_key(char *key, int key_len) {
    unsigned char key_schedule[key_len + 256];
    int j = 0, x = 0;

    for (int i = 0; i < 256; ++i) {
        key_schedule[i] = i;
    }

    for (int i = 0; i < key_len; ++i) {
        x = (x + key_schedule[i] + key[i]) % 256;
        swap(&key_schedule[i], &key_schedule[x]);
    }

    // 将剩余部分复制到前256位
    for (j = key_len; j < 256; ++j) {
        x = (x + 1) % 256;
        swap(&key_schedule[j], &key_schedule[x]);
    }
}

// 执行一次轮函数
void shift_row(unsigned char *state) {
    unsigned char temp = state[0];
    state[0] = state[4];
    state[4] = state[8];
    state[8] = state[12];
    state[12] = temp;
}

// 执行一次混洗
void mix_column(unsigned char *column) {
    static const unsigned char sbox[] = {
        0x0, 0x3, 0x6, 0x9, 0xc, 0xf, 0xa, 0xd,
        0xe, 0xb, 0x8, 0x5, 0x2, 0x1, 0x7, 0x4
    };

    unsigned char t = column[0];
    for (int i = 0; i < 4; ++i) {
        column[i] = column[i] ^ sbox[(t >> 5) & 0xf];
        t <<= 1;
        if (t >= 0x100)
            t ^= 0x11b;
    }
    column[4] = t;
}

// 加密和解密通用函数
void rc4_encrypt_or_decrypt(char *input, char *output, char *key, int len, int is_encrypt) {
    init_key(key, strlen(key));
    unsigned char state[256];

    for (int i = 0; i < len; ++i) {
        x = (x + 1) % 256;
        y = (y + state[x]) % 256;
        swap(&state[x], &state[y]);

        if (is_encrypt)
            output[i] = input[i] ^ state[(state[x] + state[y]) % 256];
        else
            output[i] = input[i] ^ state[state[(state[x] + state[y]) % 256]];
    }
}

int main() {
    char key[] = "example"; // 示例密钥
    char plaintext[] = "Hello, World!";
    char ciphertext[sizeof(plaintext)];

    // 加密
    rc4_encrypt_or_decrypt(plaintext, ciphertext, key, sizeof(plaintext), 1);
    printf("Encrypted text: %s\n", ciphertext);

    // 解密
    rc4_encrypt_or_decrypt(ciphertext, ciphertext, key, sizeof(plaintext), 0);
    printf("Decrypted text: %s\n", ciphertext);

    return 0;
}

这个示例展示了如何创建一个简单的RC4加密和解密函数，实际应用中可能会有更复杂的错误处理和性能优化。请注意，RC4已经被认为不够安全，不适合现代加密需求。