文件加密的问题

#include<iostream>
#include<iomanip>
#include<string>
#include<fstream>
using namespace std;


int    main()
{    string a,b,c;
    cout<<"please input you file name:";
    cin>>a;
    cout<<"please input your out file name:";
    cin>>b;
    cout<<"please input your file way(use Double Slash):";
    cin>>c;
    c=c+b;
    ifstream in(a);
    ofstream out(c);
    string s;
    getline(in,s);
    int i;
    for(i=0;s[i]!=0;i++)
    {
        s[i]=s[i]-97;

    }
    out<<s<<endl;
    cout<<"the file will output in "<<c<<"\n";
    system( "PAUSE ");