用c语言实现simple des

Simple DES（S-DES）是一种简化版的DES加密算法，它使用了一个8位的密钥和一个8位的明文块，输出一个8位的密文块。

这里提供一个用C语言实现Simple DES加密算法的示例代码，其中包括了加密和解密两个函数。

#include <stdio.h>

/* S-boxes */
int s1[4][4] = {
    {1, 0, 3, 2},
    {3, 2, 1, 0},
    {0, 2, 1, 3},
    {3, 1, 3, 2}
};

int s2[4][4] = {
    {0, 1, 2, 3},
    {2, 0, 1, 3},
    {3, 0, 1, 0},
    {2, 1, 0, 3}
};

/* Permutation tables */
int p10[10] = {3, 5, 2, 7, 4, 10, 1, 9, 8, 6};
int p10_len = 10;

int p8[8] = {6, 3, 7, 4, 8, 5, 10, 9};
int p8_len = 8;

int ip[8] = {2, 6, 3, 1, 4, 8, 5, 7};
int ip_len = 8;

int ep[8] = {4, 1, 2, 3, 2, 3, 4, 1};
int ep_len = 8;

int ip_inv[8] = {4, 1, 3, 5, 7, 2, 8, 6};
int ip_inv_len = 8;

/* Key generation */
void keygen(int key, int* k1, int* k2) {
    int p10_res[10], p10_res_len = 10;
    int p8_res[8], p8_res_len = 8;
    int l1[5], l1_len = 5, r1[5], r1_len = 5;
    int l2[5], l2_len = 5, r2[5], r2_len = 5;
    int tmp1[10], tmp1_len = 10, tmp2[10], tmp2_len = 10;

    /* Apply P10 permutation */
    for (int i = 0; i < p10_len; i++) {
        p10_res[i] = (key >> (10 - p10[i])) & 1;
    }

    /* Split into left and right halves */
    for (int i = 0; i < p10_len / 2; i++) {
        l1[i] = p10_res[i];
        r1[i] = p10_res[i + p10_len / 2];
    }

    /* Left circular shift */
    l2[0] = l1[1];
    l2[1] = l1[2];
    l2[2] = l1[3];
    l2[3] = l1[4];
    l2[4] = l1[0];

    r2[0] = r1[1];
    r2[1] = r1[2];
    r2[2] = r1[3];
    r2[3] = r1[4];
    r2[4] = r1[0];

    /* Combine and apply P8 permutation */
    for (int i = 0; i < p8_len / 2; i++) {
        p8_res[i] = l2[p8[i] - 1];
        p8_res[i + p8_len / 2] = r2[p8[i + p8_len / 2] - 1];
    }

    /* Save keys */
    *k1 = 0;
    *k2 = 0;
    for (int i = 0; i < p8_len; i++) {
        if (i < p8_len / 2) {
            *k1 |= p8_res[i] << (p8_len / 2 - i - 1);
        } else {
            *k2 |= p8_res[i] << (p8_len - i - 1);
        }
    }
}

/* F function */
int f(int r, int k) {
    int ep_res[8], ep_res_len = 8;
    int xor_res[8], xor_res_len = 8;
    int s1_row, s1_col, s1_res, s2_row, s2_col, s2_res;

    /* Apply EP permutation */
    for (int i = 0; i < ep_len; i++) {
        ep_res[i] = (r >> (4 - ep[i])) & 1;
    }

    /* XOR with k */
    for (int i = 0; i < ep_len; i++) {
        xor_res[i] = ep_res[i] ^ ((k >> (8 - ep[i])) & 1);
    }

    /* S-box lookup */
    s1_row = (xor_res[0] << 1) | xor_res[3];
    s1_col = (xor_res[1] << 1) | xor_res[2];
    s1_res = s1[s1_row][s1_col];

    s2_row = (xor_res[4] << 1) | xor_res[7];
    s2_col = (xor_res[5] << 1) | xor_res[6];
    s2_res = s2[s2_row][s2_col];

    /* Combine and apply P4 permutation */
    int p4_res = (s1_res << 2) | s2_res;
    int p4[4] = {2, 4, 3, 1};
    int p4_res_perm[4];

    for (int i = 0; i < 4; i++) {
        p4_res_perm[i] = (p4_res >> (4 - p4[i])) & 1;
    }

    /* Return result */
    return p4_res_perm[0] << 3 | p4_res_perm[1] << 2 | p4_res_perm[2] << 1 | p4_res_perm[3];
}

/* Simple DES encryption */
int encrypt(int plaintext, int key) {
    int k1, k2;
    int ip_res[8], ip_res_len = 8;
    int l1[5], l1_len = 5, r1[5], r1_len = 5;
    int l2[5], l2_len = 5, r2[5], r2_len = 5;
    int f_res, xor_res[4], xor_res_len = 4;
    int cipher;

    /* Apply IP permutation */
    for (int i = 0; i < ip_len; i++) {
        ip_res[i] = (plaintext >> (8 - ip[i])) & 1;
    }

    /* Split into left and right halves */
    for (int i = 0; i < ip_len / 2; i++) {
        l1[i] = ip_res[i];
        r1[i] = ip_res[i + ip_len / 2];
    }

    /* Apply F function with k1 */
    keygen(key, &k1, &k2);
    f_res = f(r1[3] << 3 | r1[2] << 2 | r1[1] << 1 | r1[0], k1);

    /* XOR with left half */
    for (int i = 0; i < 4; i++) {
        xor_res[i] = l1[i] ^ ((f_res >> (3 - i)) & 1);
    }

    /* Swap left and right halves */
    for (int i = 0; i < r1_len; i++) {
        l2[i] = r1[i];
        r2[i] = xor_res[i];
    }

    /* Apply F function with k2 */
    f_res = f(r2[3] << 3 | r2[2] << 2 | r2[1] << 1 | r2[0], k2);

    /* XOR with left half */
    for (int i = 0; i < 4; i++) {
        xor_res[i] = l2[i] ^ ((f_res >> (3 - i)) & 1);
    }

    /* Combine and apply IP inverse permutation */
    cipher = xor_res[0] << 3 | xor_res[1] << 2 | xor_res[2] << 1 | xor_res[3];
    for (int i = 0; i < ip_inv_len; i++) {
        cipher |= ((cipher >> (8 - ip_inv[i])) & 1) << (ip_inv_len - i - 1);
    }

    /* Return result */
    return cipher;
}

/* Simple DES decryption */
int decrypt(int cipher, int key) {
    int k1, k2;
    int ip_res[8], ip_res_len = 8;
    int l1[5], l1_len = 5, r1[5], r1_len = 5;
    int l2[5], l2_len = 5, r2[5], r2_len = 5;
    int f_res, xor_res[4], xor_res_len = 4;
    int plaintext;

    /* Apply IP permutation */
    for (int i = 0; i < ip_len; i++) {
        ip_res[i] = (cipher >> (8 - ip[i])) & 1;
    }

    /* Split into left and right halves */
    for (int i = 0; i < ip_len / 2; i++) {
        l1[i] = ip_res[i];
        r1[i] = ip_res[i + ip_len / 2];
    }

    /* Apply F function with k2 */
    keygen(key, &k1, &k2);
    f_res = f(r1[3] << 3 | r1[2] << 2 | r1[1] << 1 | r1[0], k2);

    /* XOR with left half */
    for (int i = 0; i < 4; i++) {
        xor_res[i] = l1[i] ^ ((f_res >> (3 - i)) & 1);
    }

    /* Swap left and right halves */
    for (int i = 0; i < r1_len; i++) {
        l2[i] = r1[i];
        r2[i] = xor_res[i];
    }

    /* Apply F function with k1 */
    f_res = f(r2[3] << 3 | r2[2] << 2 | r2[1] << 1 | r2[0], k1);

    /* XOR with left half */
    for (int i = 0; i < 4; i++) {
        xor_res[i] = l2[i] ^ ((f_res >> (3 - i)) & 1);
    }

    /* Combine and apply IP inverse permutation */
    plaintext = xor_res[0] << 3 | xor_res[1] << 2 | xor_res[2] << 1 | xor_res[3];
    for (int i = 0; i < ip_inv_len; i++) {
        plaintext |= ((plaintext >> (8 - ip_inv[i])) & 1) << (ip_inv_len - i - 1);
    }

    /* Return result */
    return plaintext;
}

int main() {
    int plaintext = 0b10101010;
    int key = 0b0111111101;

    int cipher = encrypt(plaintext, key);
    printf("Cipher: %02x\n", cipher);

    int decrypted = decrypt(cipher, key);
    printf("Decrypted: %02x\n", decrypted);

    return 0;
}

需要注意的是，这只是一个简化版的DES加密算法，安全性并不高，仅能用于教学和学习研究。实际应用中应该使用更为安全的加密算法，例如AES等。