c实现feistel结构加密

Feistel结构是一种对称加密算法的设计思想，主要思想是将明文分成左右两部分，然后交替进行加密和解密，直到达到预定的轮数。最常见的Feistel结构加密算法是DES算法。

下面是一个简单的C语言实现Feistel结构加密的示例代码：

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

#define BLOCK_SIZE 8 // 块大小（字节数）
#define ROUNDS 16 // 加密轮数
#define KEY_SIZE 8 // 密钥大小（字节数）

// S盒
const unsigned char S_BOX[4][16] = {
    {0x0E, 0x04, 0x0D, 0x01, 0x02, 0x0F, 0x0B, 0x08, 0x03, 0x0A, 0x06, 0x0C, 0x05, 0x09, 0x00, 0x07},
    {0x00, 0x01, 0x07, 0x0D, 0x0F, 0x06, 0x0C, 0x0B, 0x05, 0x08, 0x04, 0x02, 0x09, 0x0E, 0x03, 0x0A},
    {0x0B, 0x03, 0x05, 0x08, 0x02, 0x0F, 0x0A, 0x0D, 0x0E, 0x01, 0x07, 0x04, 0x0C, 0x09, 0x00, 0x06},
    {0x07, 0x0E, 0x0C, 0x0B, 0x0D, 0x01, 0x03, 0x09, 0x00, 0x02, 0x04, 0x0A, 0x0F, 0x05, 0x08, 0x06}
};

// P盒
const unsigned char P_BOX[BLOCK_SIZE] = {2, 6, 3, 1, 4, 8, 5, 7};

// 左右分块
void split(unsigned char* block, unsigned char* left, unsigned char* right) {
    memcpy(left, block, BLOCK_SIZE / 2);
    memcpy(right, block + BLOCK_SIZE / 2, BLOCK_SIZE / 2);
}

// 合并左右块
void merge(unsigned char* block, unsigned char* left, unsigned char* right) {
    memcpy(block, left, BLOCK_SIZE / 2);
    memcpy(block + BLOCK_SIZE / 2, right, BLOCK_SIZE / 2);
}

// F函数
void F(unsigned char* right, unsigned char* subkey) {
    unsigned char temp[BLOCK_SIZE / 2];
    memcpy(temp, right, BLOCK_SIZE / 2);
    memset(right, 0, BLOCK_SIZE / 2);
    // 扩展置换
    unsigned char expand[BLOCK_SIZE] = {4, 1, 2, 3, 2, 3, 4, 1};
    unsigned char expanded[BLOCK_SIZE];
    for (int i = 0; i < BLOCK_SIZE; i++) {
        expanded[i] = temp[expand[i] - 1];
    }
    // 异或子密钥
    for (int i = 0; i < BLOCK_SIZE / 2; i++) {
        expanded[i] ^= subkey[i];
    }
    // S盒替换
    unsigned char row = (expanded[0] << 1) + expanded[3];
    unsigned char col = (expanded[1] << 3) + (expanded[2] << 2) + (expanded[4] << 1) + expanded[5];
    unsigned char sbox_out = S_BOX[row][col];
    for (int i = 0; i < 4; i++) {
        right[i] = (sbox_out >> (3 - i)) & 1;
    }
    // 置换
    unsigned char permuted[BLOCK_SIZE / 2];
    for (int i = 0; i < BLOCK_SIZE / 2; i++) {
        permuted[i] = right[P_BOX[i] - 1];
    }
    memcpy(right, permuted, BLOCK_SIZE / 2);
}

// Feistel加密
void feistel_encrypt(unsigned char* block, unsigned char* key) {
    unsigned char left[BLOCK_SIZE / 2], right[BLOCK_SIZE / 2];
    split(block, left, right);
    for (int i = 0; i < ROUNDS; i++) {
        F(right, key + i * (BLOCK_SIZE / 2));
        // 异或左右块
        for (int j = 0; j < BLOCK_SIZE / 2; j++) {
            left[j] ^= right[j];
        }
        // 交换左右块
        if (i != ROUNDS - 1) {
            unsigned char temp[BLOCK_SIZE / 2];
            memcpy(temp, left, BLOCK_SIZE / 2);
            memcpy(left, right, BLOCK_SIZE / 2);
            memcpy(right, temp, BLOCK_SIZE / 2);
        }
    }
    merge(block, right, left);
}

int main() {
    unsigned char block[BLOCK_SIZE] = {0x01, 0x23, 0x45, 0x67, 0x89, 0xAB, 0xCD, 0xEF};
    unsigned char key[KEY_SIZE] = {0x12, 0x34, 0x56, 0x78, 0x9A, 0xBC, 0xDE, 0xF0};
    feistel_encrypt(block, key);
    printf("Encrypted block: ");
    for (int i = 0; i < BLOCK_SIZE; i++) {
        printf("%02X", block[i]);
    }
    printf("\n");
    return 0;
}

上述示例代码实现了一个简单的Feistel结构加密算法，其中使用了S盒、P盒和异或操作等基本操作。需要注意的是，该实现仅供学习参考使用，实际应用中需要考虑更多的安全性问题。