凯撒密码加密、解密、暴力破解（C语言实现）

 C语言设计，替换密码开发设计，2022年9月信息安全课程作业分享

#include
#include
#include
#include 
 
#define N 1000
 
void encrypt();
 
void decrypt();
 
void decryptAll();
 
int key() {
    char k[N];
    printf("输入密钥(1~26)：");
    gets(k);      
    while (1) {
        if (atoi(k) > 0 && atoi(k) <= 26)
            break;
        printf("格式错误，输入密钥(1~26)：");
        gets(k);
    }
    return atoi(k);
}
 
void change(char code[],char code2[] ,int n) {
	char min[26]={'a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z'};
	char max[26]={'A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z'};
	int k,m;
    for (int i = 0; i < strlen(code); i++) {
    	if('a'<=code[i] && code[i]<='z')
		{
			k=code[i]-'a';
			m=(26+k-n)%26;
			code2[i]=min[m];
		}	
		else if('A'<=code[i] && code[i]<='Z')
		{
			k=code[i]-'A';
			m=(26+k-n)%26;
			code2[i]=max[m];
		}else{
			code2[i]=code[i];
		}		
    }
}
 
 
void encrypt() {
	
	char min[26]={'a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z'};
	char max[26]={'A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z'};
    char code[N] = {};//接受字符串 
    int n,k,m;
    getchar();
    printf("请输入要加密的内容：");
    gets(code);
    n = key();//获得加密顺序的密钥	
    for (int i = 0; i < strlen(code); i++) {
    	    if('a'<=code[i] && code[i]<='z')
			{
				k=code[i]-'a';
				m=(26+k+n)%26;
				code[i]=min[m];
			}	
			else if('A'<=code[i] && code[i]<='Z')
			{
				k=code[i]-'A';
				m=(26+k+n)%26;
				code[i]=max[m];
			}
			else{
				code[i]=code[i];
			}   
    }
    printf("加密之后的内容为：");
    puts(code);
}
 
 
void decrypt() {
    char code[N] = {}, code2[N] = {};
    int k;
    getchar();
    printf("输入密文：");
    gets(code);
    k = key();   
    change(code,code2, k);
    printf("解密的内容为：");
    puts(code2);
}
 
void decryptAll() {
    char code[N] = {},code2[N]={};
    getchar();
    printf("输入密文：");
    gets (code);
    for (int k = 1; k <= 26; ++k) {   
        change(code,code2, k);   
        printf("当key = %d时，解密是：", k);
        puts(code2);
    }
}
 
int main() {
    int x;
    while (1) { 
		printf("\n\n\t================= 欢迎使用凯撒密码加密系统 =============\n\n");
  		printf("\t=====================请选择服务=====================\n");
  		printf("\t*                                                  *\n");
  		printf("\t*    1  加密                   2   解密            *\n");
		printf("\t*                                                  *\n");
  		printf("\t*    3  暴力解密               4   退出            *\n");
  		printf("\t====================================================\n");
  		printf("\t\t\t请作出选择(1-4):\n");   
        scanf("%d", &x);
        if (x == 1)
            encrypt();     
        else if (x == 2)
            decrypt();     
        else if (x == 3)
            decryptAll();     
        else  if(x==4){
        	printf("已退出，谢谢使用\n");
			printf("欢迎您下次使用，凯撒密码加密系统时刻在。"); 
			break;   
		}else{ 
			printf("警告输入错误！！！\n");
			} 
    }
}