使用C语言实现仿射密码和Vigenere加解密，要求密码算法采用模块化设计，一般密码算法要包括主函数、设置密钥函数、加密函数和解密函数等几个部分，示例

代码如下：

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

#define MAX_SIZE 1000
#define MODULO 26

void set_key(char *key) {
    printf("Please enter the key: ");
    scanf("%s", key);
}

void affine_encrypt(char *plaintext, char *ciphertext, int a, int b) {
    int i, len;
    len = strlen(plaintext);
    for (i = 0; i < len; i++) {
        if (plaintext[i] >= 'a' && plaintext[i] <= 'z') {
            ciphertext[i] = (a * (plaintext[i] - 'a') + b) % MODULO + 'a';
        } else if (plaintext[i] >= 'A' && plaintext[i] <= 'Z') {
            ciphertext[i] = (a * (plaintext[i] - 'A') + b) % MODULO + 'A';
        } else {
            ciphertext[i] = plaintext[i];
        }
    }
    ciphertext[i] = '\0';
}

void affine_decrypt(char *ciphertext, char *plaintext, int a, int b) {
    int i, len, t;
    len = strlen(ciphertext);
    for (i = 0; i < len; i++) {
        if (ciphertext[i] >= 'a' && ciphertext[i] <= 'z') {
            t = (ciphertext[i] - 'a' - b) % MODULO;
            while (t < 0) {
                t += MODULO;
            }
            plaintext[i] = (a * t) % MODULO + 'a';
        } else if (ciphertext[i] >= 'A' && ciphertext[i] <= 'Z') {
            t = (ciphertext[i] - 'A' - b) % MODULO;
            while (t < 0) {
                t += MODULO;
            }
            plaintext[i] = (a * t) % MODULO + 'A';
        } else {
            plaintext[i] = ciphertext[i];
        }
    }
    plaintext[i] = '\0';
}

void vigenere_encrypt(char *plaintext, char *ciphertext, char *key) {
    int i, j, len_plaintext, len_key;
    len_plaintext = strlen(plaintext);
    len_key = strlen(key);
    for (i = 0; i < len_plaintext; i++) {
        if (plaintext[i] >= 'a' && plaintext[i] <= 'z') {
            ciphertext[i] = 'a' + (plaintext[i] - 'a' + key[i % len_key] - 'a') % MODULO;
        } else if (plaintext[i] >= 'A' && plaintext[i] <= 'Z') {
            ciphertext[i] = 'A' + (plaintext[i] - 'A' + key[i % len_key] - 'a') % MODULO;
        } else {
            ciphertext[i] = plaintext[i];
        }
    }
    ciphertext[i] = '\0';
}

void vigenere_decrypt(char *ciphertext, char *plaintext, char *key) {
    int i, j, len_ciphertext, len_key;
    len_ciphertext = strlen(ciphertext);
    len_key = strlen(key);
    for (i = 0; i < len_ciphertext; i++) {
        if (ciphertext[i] >= 'a' && ciphertext[i] <= 'z') {
            plaintext[i] = 'a' + (ciphertext[i] - 'a' - key[i % len_key] + 'a') % MODULO;
            while (plaintext[i] < 'a') {
                plaintext[i] += MODULO;
            }
        } else if (ciphertext[i] >= 'A' && ciphertext[i] <= 'Z') {
            plaintext[i] = 'A' + (ciphertext[i] - 'A' - key[i % len_key] + 'a') % MODULO;
            while (plaintext[i] < 'A') {
                plaintext[i] += MODULO;
            }
        } else {
            plaintext[i] = ciphertext[i];
        }
    }
    plaintext[i] = '\0';
}

int main() {
    char plaintext[MAX_SIZE], ciphertext[MAX_SIZE], decrypted[MAX_SIZE], key[MAX_SIZE];
    int a, b;
    printf("[Affine Cipher]\n");
    printf("Please enter the plaintext: ");
    scanf("%s", plaintext);
    printf("Please enter the coefficient a: ");
    scanf("%d", &a);
    printf("Please enter the constant b: ");
    scanf("%d", &b);
    affine_encrypt(plaintext, ciphertext, a, b);
    printf("The ciphertext is: %s\n", ciphertext);
    affine_decrypt(ciphertext, decrypted, a, b);
    printf("The decrypted plaintext is: %s\n", decrypted);
    printf("\n[Vigenere Cipher]\n");
    printf("Please enter the plaintext: ");
    scanf("%s", plaintext);
    set_key(key);
    vigenere_encrypt(plaintext, ciphertext, key);
    printf("The ciphertext is: %s\n", ciphertext);
    vigenere_decrypt(ciphertext, decrypted, key);
    printf("The decrypted plaintext is: %s\n", decrypted);
    return 0;
}