用C语言写一个古典密码学算法的加密解密实验，其中包含移位密码，代替密码，置换密码

好的，以下是一个简单的C语言实现古典密码学算法的加密和解密的示例代码，其中包括移位密码，代替密码和置换密码。

移位密码：

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

void encrypt(char *message, int shift);
void decrypt(char *message, int shift);

int main()
{
    char message[100];
    int shift;

    printf("Enter message to encrypt: ");
    fgets(message, 100, stdin);

    printf("Enter shift: ");
    scanf("%d", &shift);

    encrypt(message, shift);
    printf("Encrypted message: %s\n", message);

    decrypt(message, shift);
    printf("Decrypted message: %s\n", message);

    return 0;
}

void encrypt(char *message, int shift)
{
    int i;
    for(i = 0; message[i] != '\0'; i++)
    {
        if(message[i] >= 'a' && message[i] <= 'z')
        {
            message[i] = ((message[i] - 'a') + shift) % 26 + 'a';
        }
        else if(message[i] >= 'A' && message[i] <= 'Z')
        {
            message[i] = ((message[i] - 'A') + shift) % 26 + 'A';
        }
    }
}

void decrypt(char *message, int shift)
{
    int i;
    for(i = 0; message[i] != '\0'; i++)
    {
        if(message[i] >= 'a' && message[i] <= 'z')
        {
            message[i] = ((message[i] - 'a') - shift + 26) % 26 + 'a';
        }
        else if(message[i] >= 'A' && message[i] <= 'Z')
        {
            message[i] = ((message[i] - 'A') - shift + 26) % 26 + 'A';
        }
    }
}

代替密码：

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

void encrypt(char *message, char *key);
void decrypt(char *message, char *key);

int main()
{
    char message[100];
    char key[26];

    printf("Enter message to encrypt: ");
    fgets(message, 100, stdin);

    printf("Enter key: ");
    fgets(key, 26, stdin);

    encrypt(message, key);
    printf("Encrypted message: %s\n", message);

    decrypt(message, key);
    printf("Decrypted message: %s\n", message);

    return 0;
}

void encrypt(char *message, char *key)
{
    int i, j;
    for(i = 0; message[i] != '\0'; i++)
    {
        if(message[i] >= 'a' && message[i] <= 'z')
        {
            j = message[i] - 'a';
            message[i] = key[j];
        }
        else if(message[i] >= 'A' && message[i] <= 'Z')
        {
            j = message[i] - 'A';
            message[i] = key[j];
        }
    }
}

void decrypt(char *message, char *key)
{
    int i, j;
    char temp;

    for(i = 0; message[i] != '\0'; i++)
    {
        if(message[i] >= 'a' && message[i] <= 'z')
        {
            for(j = 0; j < 26; j++)
            {
                if(key[j] == message[i])
                {
                    temp = j + 'a';
                    break;
                }
            }
            message[i] = temp;
        }
        else if(message[i] >= 'A' && message[i] <= 'Z')
        {
            for(j = 0; j < 26; j++)
            {
                if(key[j] == message[i])
                {
                    temp = j + 'A';
                    break;
                }
            }
            message[i] = temp;
        }
    }
}

置换密码：

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

void encrypt(char *message, char *key);
void decrypt(char *message, char *key);

int main()
{
    char message[100];
    char key[26];

    printf("Enter message to encrypt: ");
    fgets(message, 100, stdin);

    printf("Enter key: ");
    fgets(key, 26, stdin);

    encrypt(message, key);
    printf("Encrypted message: %s\n", message);

    decrypt(message, key);
    printf("Decrypted message: %s\n", message);

    return 0;
}

void encrypt(char *message, char *key)
{
    int i, j, k;
    char temp[100];

    for(i = 0, k = 0; message[i] != '\0'; i++)
    {
        if(message[i] >= 'a' && message[i] <= 'z')
        {
            j = message[i] - 'a';
            temp[k++] = key[j];
        }
        else if(message[i] >= 'A' && message[i] <= 'Z')
        {
            j = message[i] - 'A';
            temp[k++] = key[j];
        }
    }
    temp[k] = '\0';

    strcpy(message, temp);
}

void decrypt(char *message, char *key)
{
    int i, j, k;
    char temp[100];

    for(i = 0, k = 0; message[i] != '\0'; i++)
    {
        if(message[i] >= 'a' && message[i] <= 'z')
        {
            for(j = 0; j < 26; j++)
            {
                if(key[j] == message[i])
                {
                    temp[k++] = j + 'a';
                    break;
                }
            }
        }
        else if(message[i] >= 'A' && message[i] <= 'Z')
        {
            for(j = 0; j < 26; j++)
            {
                if(key[j] == message[i])
                {
                    temp[k++] = j + 'A';
                    break;
                }
            }
        }
    }
    temp[k] = '\0';

    strcpy(message, temp);
}