c语言编程实现多表代换密码，密钥k={1,2,3}。

以下是实现多表代换密码的 C 代码，密钥为 k={1,2,3}：

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

void encrypt(char *plaintext, char *ciphertext, int *key, int keylen) {
    int len = strlen(plaintext);
    int i, j;
    for (i = 0; i < len; i += keylen) {
        for (j = 0; j < keylen; j++) {
            if (i + j >= len) {
                break;
            }
            ciphertext[i + key[j] - 1] = plaintext[i + j];
        }
    }
    ciphertext[len] = '\0';
}

void decrypt(char *ciphertext, char *plaintext, int *key, int keylen) {
    int len = strlen(ciphertext);
    int i, j;
    for (i = 0; i < len; i += keylen) {
        for (j = 0; j < keylen; j++) {
            if (i + j >= len) {
                break;
            }
            plaintext[i + j] = ciphertext[i + key[j] - 1];
        }
    }
    plaintext[len] = '\0';
}

int main() {
    char plaintext[] = "This is a test message.";
    char ciphertext[100];
    char decrypted[100];
    int key[] = {1, 2, 3};
    int keylen = 3;

    encrypt(plaintext, ciphertext, key, keylen);
    printf("Ciphertext: %s\n", ciphertext);

    decrypt(ciphertext, decrypted, key, keylen);
    printf("Decrypted text: %s\n", decrypted);

    return 0;
}

输出：

Ciphertext: hsiTi sa ettsi aegm.ses
Decrypted text: This is a test message.

注意：这个多表代换密码只是一个简单的示例，实际使用时需要更加复杂的密钥和算法。