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

//加密
int encrypt(char* plaintext, char* ciphertext, int k)
{
int i, z = 0;
int l = strlen(plaintext); //获取明文的长度
for (i = 0; i < l; i++)
    {
//判断大小写
if (plaintext[i] >= 'A' && plaintext[i] <= 'Z') {
ciphertext[z] = ( (plaintext[i] - 'A') + k) % 26 + 'A';
        }
else if (plaintext[i] >= 'a' && plaintext[i] <= 'z') {
ciphertext[z] = ((plaintext[i] - 'a') + k) % 26 + 'a';
        }
else {  //判断是否是空格
ciphertext[z] = plaintext[i];
        }
z++;
    }
return 0;
}

//解密
int decrypt(char* plaintext, char* ciphertext, int k)
{
int i, z = 0;
int l = strlen(plaintext); //获取明文的长度
for (i = 0; i < l; i++)
    {
//判断大小写
if (plaintext[i] >= 'A' && plaintext[i] <= 'Z') {
ciphertext[z] = (((plaintext[i] - 'A') - k)) % 26 + 'A';
if (((plaintext[i] - 'A') - k) < 0) {
ciphertext[z] = ciphertext[z] + 26;
            }
        }
else if (plaintext[i] >= 'a' && plaintext[i] <= 'z') {
ciphertext[z] = ( ((plaintext[i] - 'a') - k)) % 26 + 'a';
if (((plaintext[i] - 'a') - k) < 0) {  //处理负数
ciphertext[z] = ciphertext[z] + 26;
            }
        }
else {  //判断是否是空格
ciphertext[z] = plaintext[i];
        }
z++;
    }
return 0;
}

int main()
{
char plaintext[50] = "";
char ciphertext[50] = "";
int k;
int type;
printf("请填写明文或者密文:\n");
scanf("%s", plaintext);
printf("请选择加密方式，输入1加密，输入2解密\n");
scanf("%d", &type);
if (type == 1) {
//加密
printf("请输入密钥k:\n");
scanf("%d", &k);
encrypt(plaintext, ciphertext, k);
printf("明文%s的密文为:%s\n", plaintext, ciphertext);
    }
else if (type == 2) {
//解密
printf("请输入密钥k:\n");
scanf("%d", &k);
decrypt(plaintext, ciphertext, k);
printf("密文%s的明文为:%s\n", plaintext, ciphertext);
    }
return 0;
}