C语言 凯撒加密

古罗马时期，凯撒大帝曾经使用密码来传递信息，它是一种替代密码，对于信件中的每个字母，会用其它字母代替，比如：a用s代替，b用u来代替，具体替换规则如下表所示：

 

#include <stdio.h>
void jiami(char []);
void jiemi(char []); 
int i=0;
int main(){
	int u;
	char str[100];
	printf("加密请键入“1 ”，解密请键入“2 ”:  "); 
	scanf("%d",&u);
	if(u==1){
	printf("请输入要加密的字段(100个字符以内)：\n");
	fflush(stdin);//清空文件缓冲区
	//scanf("%s",str); 
	gets(str);
	printf("加密结果为：\n");
	jiami(str);//调用加密 
	}else{
	    if(u==2){
	    printf("请输入要解密的字段(100个字符以内)：\n");
	    fflush(stdin);//清空文件缓冲区
	    //scanf("%s",str); 
	    gets(str);
	    printf("解密结果为：\n");
	    jiemi(str);//调用解密 
	    //scanf("%s",str); 
	}else{
	printf("输入错误"); 
	}
	}
}
void jiami(char str[100]){//加密算法 
	if(str[i]!='\0'){
		if(str[i]==65||str[i]==97) str[i]+=18; 
		else 
	    if(str[i]==66||str[i]==98) str[i]+=19;
		else 
		if(str[i]==67||str[i]==99) str[i]+=20;
		else 
		if(str[i]==68||str[i]==100) str[i]+=21;
		else 
		if(str[i]==69||str[i]==101) str[i]-=4;
		else 
		if(str[i]==70||str[i]==102) str[i]-=3;
		else 
		if(str[i]==71||str[i]==103) str[i]-=2;
		else 
		if(str[i]==72||str[i]==104) str[i]-=1;
		else 
		if(str[i]==73||str[i]==105) ;
		else 
		if(str[i]==74||str[i]==106) str[i]+=1;
		else 
		if(str[i]==75||str[i]==107) str[i]+=2;
		else 
		if(str[i]==76||str[i]==108) str[i]+=3;
		else 
		if(str[i]==77||str[i]==109) str[i]+=4;
		else 
		if(str[i]==78||str[i]==110) str[i]+=4;
		else 
		if(str[i]==79||str[i]==111) str[i]+=5;
		else 
		if(str[i]==80||str[i]==112) str[i]+=6;
		else 
		if(str[i]==81||str[i]==113) str[i]+=7;
		else 
		if(str[i]==82||str[i]==114) str[i]+=8;
		else 
		if(str[i]==83||str[i]==115) str[i]-=17;
		else 
		if(str[i]==84||str[i]==116) str[i]-=16;
		else 
		if(str[i]==85||str[i]==117) str[i]-=15;
		else 
		if(str[i]==86||str[i]==118) str[i]-=14;
		else 
		if(str[i]==87||str[i]==119) str[i]-=13;
		else
		if(str[i]==88||str[i]==120) str[i]-=12;
		else
		if(str[i]==89||str[i]==121) str[i]-=11;
		else
		if(str[i]==90|str[i]==122) str[i]-=10;
		
		printf("%c",str[i]);
	    i++;
		jiami(str);
		}
}
void jiemi(char str[100]){//解密算法 
	if(str[i]!='\0'){
		if(str[i]==65||str[i]==97) str[i]+=4;
		else 
	    if(str[i]==66||str[i]==98) str[i]+=17;
		else 
		if(str[i]==67||str[i]==99) str[i]+=3;
		else 
		if(str[i]==68||str[i]==100) str[i]+=16;
		else 
		if(str[i]==69||str[i]==101) str[i]+=2;
		else 
		if(str[i]==70||str[i]==102) str[i]+=15;
		else 
		if(str[i]==71||str[i]==103) str[i]+=1;
		else 
		if(str[i]==72||str[i]==104) str[i]+=14;
		else 
		if(str[i]==73||str[i]==105) ;
		else 
		if(str[i]==74||str[i]==106) str[i]+=13;
		else 
		if(str[i]==75||str[i]==107) str[i]-=1;
		else 
		if(str[i]==76||str[i]==108) str[i]+=12;
		else 
		if(str[i]==77||str[i]==109) str[i]+=4;
		else 
		if(str[i]==78||str[i]==110) str[i]+=11;
		else 
		if(str[i]==79||str[i]==111) str[i]-=3;
		else 
		if(str[i]==80||str[i]==112) str[i]+=10;
		else 
		if(str[i]==81||str[i]==113) str[i]-=4;
		else 
		if(str[i]==82||str[i]==114) str[i]-=4;
		else 
		if(str[i]==83||str[i]==115) str[i]-=18;
		else 
		if(str[i]==84||str[i]==116) str[i]-=5;
		else 
		if(str[i]==85||str[i]==117) str[i]-=17;
		else 
		if(str[i]==86||str[i]==118) str[i]-=6;
		else 
		if(str[i]==87||str[i]==119) str[i]-=20;
		else
		if(str[i]==88||str[i]==120) str[i]-=7;
		else
		if(str[i]==89||str[i]==121) str[i]-=21;
		else
		if(str[i]==90|str[i]==122) str[i]-=8;
		
		printf("%c",str[i]);
	    i++;
		jiemi(str);
		} 
}