c语言实现Vigenere密码算法

以下是C语言实现Vigenere密码算法的示例代码：

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

#define MAX_LEN 100

char* encrypt(char* plaintext, char* key);
char* decrypt(char* ciphertext, char* key);

int main() {
    char plaintext[MAX_LEN];
    char key[MAX_LEN];
    char ciphertext[MAX_LEN];

    printf("Enter plaintext: ");
    fgets(plaintext, MAX_LEN, stdin);
    plaintext[strcspn(plaintext, "\n")] = 0;  // remove newline character

    printf("Enter key: ");
    fgets(key, MAX_LEN, stdin);
    key[strcspn(key, "\n")] = 0;  // remove newline character

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

    strcpy(plaintext, decrypt(ciphertext, key));
    printf("Decrypted plaintext: %s\n", plaintext);

    return 0;
}

char* encrypt(char* plaintext, char* key) {
    int plaintext_len = strlen(plaintext);
    int key_len = strlen(key);
    char* ciphertext = malloc(plaintext_len + 1);

    for (int i = 0; i < plaintext_len; i++) {
        ciphertext[i] = ((plaintext[i] - 'A') + (key[i % key_len] - 'A')) % 26 + 'A';
    }
    ciphertext[plaintext_len] = '\0';

    return ciphertext;
}

char* decrypt(char* ciphertext, char* key) {
    int ciphertext_len = strlen(ciphertext);
    int key_len = strlen(key);
    char* plaintext = malloc(ciphertext_len + 1);

    for (int i = 0; i < ciphertext_len; i++) {
        plaintext[i] = ((ciphertext[i] - 'A') - (key[i % key_len] - 'A') + 26) % 26 + 'A';
    }
    plaintext[ciphertext_len] = '\0';

    return plaintext;
}

该程序从用户输入获取明文和密钥，并使用`encrypt()`函数加密明文并返回密文，使用`decrypt()`函数解密密文并返回明文。在加密和解密过程中，程序将明文和密文中的字母从'A'到'Z'映射到0到25，执行Vigenere算法，然后将结果映射回字母形式。请注意，该程序假定输入的明文和密钥都是大写字母。