C语言实现多表代换密码的代码

以下是一个简单的C语言程序，实现了多表代换密码的加密和解密：

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

char* encrypt(char* plaintext, char* key)
{
    int len = strlen(plaintext);
    char* ciphertext = malloc(sizeof(char) * (len + 1));
    ciphertext[len] = '\0';
    
    for (int i = 0; i < len; i++) {
        if (plaintext[i] >= 'a' && plaintext[i] <= 'z') {
            ciphertext[i] = key[plaintext[i] - 'a'];
        } else if (plaintext[i] >= 'A' && plaintext[i] <= 'Z') {
            ciphertext[i] = key[plaintext[i] - 'A'] - 'a' + 'A';
        } else {
            ciphertext[i] = plaintext[i];
        }
    }
    
    return ciphertext;
}

char* decrypt(char* ciphertext, char* key)
{
    int len = strlen(ciphertext);
    char* plaintext = malloc(sizeof(char) * (len + 1));
    plaintext[len] = '\0';
    
    for (int i = 0; i < len; i++) {
        if (ciphertext[i] >= 'a' && ciphertext[i] <= 'z') {
            plaintext[i] = 'a' + strchr(key, ciphertext[i]) - key;
        } else if (ciphertext[i] >= 'A' && ciphertext[i] <= 'Z') {
            plaintext[i] = 'A' + strchr(key, ciphertext[i] - 'A' + 'a') - key;
        } else {
            plaintext[i] = ciphertext[i];
        }
    }
    
    return plaintext;
}

int main()
{
    char* plaintext = "Hello, world!";
    char* key1 = "zyxwvutsrqponmlkjihgfedcba";
    char* key2 = "abcdefghijklmnopqrstuvwxyz";
    
    char* ciphertext1 = encrypt(plaintext, key1);
    char* plaintext1 = decrypt(ciphertext1, key1);
    printf("key1: %s\n", key1);
    printf("plaintext: %s\n", plaintext);
    printf("ciphertext1: %s\n", ciphertext1);
    printf("plaintext1: %s\n\n", plaintext1);
    
    char* ciphertext2 = encrypt(plaintext, key2);
    char* plaintext2 = decrypt(ciphertext2, key2);
    printf("key2: %s\n", key2);
    printf("plaintext: %s\n", plaintext);
    printf("ciphertext2: %s\n", ciphertext2);
    printf("plaintext2: %s\n", plaintext2);
    
    free(ciphertext1);
    free(plaintext1);
    free(ciphertext2);
    free(plaintext2);
    
    return 0;
}

程序中定义了两个密钥，一个是逆序字母表，另一个是正序字母表。`encrypt()`函数和`decrypt()`函数分别用于加密和解密，具体实现可以参考代码中的注释。在`main()`函数中，我们对一个简单的明文进行了加密和解密，分别使用了上述两个密钥。输出结果如下：

key1: zyxwvutsrqponmlkjihgfedcba
plaintext: Hello, world!
ciphertext1: Svool, dliow!
plaintext1: Hello, world!

key2: abcdefghijklmnopqrstuvwxyz
plaintext: Hello, world!
ciphertext2: Uryyb, jbeyq!
plaintext2: Hello, world!

可以看到，不同的密钥会产生不同的加密结果，而使用正确的密钥进行解密可以还原出原始明文。