vigenere密码算法c

下面是Vigenere密码算法的C代码实现，包括加密和解密：

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

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

int main() {
    char plaintext[] = "Vigenere cipher is a polyalphabetic cipher";
    char key[] = "SECRET";
    char* ciphertext = encrypt(plaintext, key);
    printf("Ciphertext: %s\n", ciphertext);
    char* decryptedtext = decrypt(ciphertext, key);
    printf("Decrypted text: %s\n", decryptedtext);

    free(ciphertext);
    free(decryptedtext);

    return 0;
}

char* encrypt(char* plaintext, char* key) {
    int plaintext_len = strlen(plaintext);
    int key_len = strlen(key);
    char* ciphertext = (char*) malloc(sizeof(char) * (plaintext_len + 1));
    for (int i = 0; i < plaintext_len; i++) {
        ciphertext[i] = ((plaintext[i] + key[i % key_len]) % 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* decryptedtext = (char*) malloc(sizeof(char) * (ciphertext_len + 1));
    for (int i = 0; i < ciphertext_len; i++) {
        decryptedtext[i] = (((ciphertext[i] - key[i % key_len]) + 26) % 26) + 'A';
    }
    decryptedtext[ciphertext_len] = '\0';
    return decryptedtext;
}

这个代码使用了纯文本作为明文和密钥，返回的密文和解密后的明文也是纯文本。在实际使用中，需要先将明文和密钥转换成数字或二进制形式，再进行加密和解密。