凯撒密码，广义凯撒密码加解密问题

#include "stdafx.h"

#include<stdio.h>
#include<stdlib.h>
//#include<math.h>

void main()
{
// int m;
// scanf("%d",m);
FILE *fp;
char ch,filename[10];
scanf("%s",filename);

if((fp=fopen(filename,"w"))==NULL)
{
printf("cannot open file\n");
exit(0);
}
ch=getchar();
ch=getchar();

while(ch!=EOF)
{
if(ch>96&&ch<123)
{
ch-=97;
ch+=m;
ch%=26;
ch+=97;