C语言实现Playfair密码加密与解密

#include<iostream>
#include<cstring>
#include<stdio.h>

//#define k 100
void encrypt();
void decrypt();

void  main()
{

printf("        |-------------------------------------------------------------------|\n");
printf("        |                                                                   |\n");
printf("        |               欢迎使用Playfair多表代换加解密程序                  |\n");
    printf("        |                                                                   |\n");
printf("        |-------------------------------------------------------------------|\n");
    while(true)
{
int n=0;
printf("请输入您要进行的操作代码:\n");
printf("1-->加密                  2-->解密                   3-->退出程序\n");
printf("---------------------------------------------------------------------");
printf("------------------->");
scanf("%d",&n);
switch(n)
{
case 1:
encrypt();

break;

case 2:
decrypt();

break;
case 3:
exit(1);

break;
default :

printf("您的输入有误，请重输。\n");

break;

}

}
}

void encrypt()
{
const int k=100;
char letters[26]="ABCDEFGHIKLMNOPQRSTUVWXYZ";//用于填充矩阵
int flag[25]={0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0};//字母是否已在矩阵中，与letters数组对应
char ch[5][5];//5X5矩阵
char key[k];//密钥
char p[k];//明文
char r;

 int flg=1;