用C语言实现DES算法中的IP逆置换

以下是用C语言实现DES算法中的IP逆置换的代码：

#include <stdio.h>

const int IP_INV[64] = {40, 8, 48, 16, 56, 24, 64, 32, 
                        39, 7, 47, 15, 55, 23, 63, 31, 
                        38, 6, 46, 14, 54, 22, 62, 30, 
                        37, 5, 45, 13, 53, 21, 61, 29, 
                        36, 4, 44, 12, 52, 20, 60, 28, 
                        35, 3, 43, 11, 51, 19, 59, 27, 
                        34, 2, 42, 10, 50, 18, 58, 26, 
                        33, 1, 41, 9, 49, 17, 57, 25};

void ip_inv_permutation(unsigned char *input, unsigned char *output) {
    int i, j, k;
    unsigned char temp[8] = {0};
    for (i = 0; i < 8; i++) {
        for (j = 0; j < 8; j++) {
            k = IP_INV[i * 8 + j] - 1;
            if (input[k / 8] & (1 << (7 - k % 8))) {
                temp[i] |= (1 << (7 - j));
            }
        }
    }
    for (i = 0; i < 8; i++) {
        output[i] = temp[7 - i];
        printf("%02X ", output[i]);
    }
    printf("\n");
}

int main() {
    unsigned char input[8] = {0x4E, 0x6F, 0x77, 0x20, 0x69, 0x73, 0x20, 0x74};
    unsigned char output[8] = {0};
    printf("Input: ");
    for (int i = 0; i < 8; i++) {
        printf("%02X ", input[i]);
    }
    printf("\n");
    printf("Output: ");
    ip_inv_permutation(input, output);
    return 0;
}

这里使用了一个长度为64的IP_INV常量，表示IP逆置换的映射关系。ip_inv_permutation()函数接受输入数据和输出数据的指针，然后按照IP逆置换的映射关系进行逆置换操作，最终将结果存储到输出数据中。在main()函数中，我们测试了一组样例数据的IP逆置换结果。