Encrypting a file in win API

Question

hi I have to write a windows api code that encrypts a file by adding three to each character.

so I wrote this now its not doing anything ... where i go wronge

    #include "stdafx.h"
    #include <windows.h>

int _tmain(int argc, _TCHAR* argv[])
{
    HANDLE filein,fileout;

    filein=CreateFile
    (L"d:\\test.txt",GENERIC_READ,0,NULL,OPEN_ALWAYS,FILE_ATTRIBUTE_NORMAL,NULL);

    fileout=CreateFile
    (L"d:\\test.txt",GENERIC_WRITE,0,NULL,CREATE_ALWAYS,FILE_ATTRIBUTE_NORMAL,NULL);

    DWORD really; //later this will be used to store how many bytes I succeed to read
    do
    {
        BYTE x[1024];  //the buffer the thing Im using to read in
        ReadFile(filein,x,1024,&really,NULL);

        for(int i=0 ; i<really ; i++)
        {
            x[i]= (x[i]+3) % 256; 
        }

        DWORD really2;
        WriteFile(fileout,x,really,&really2,NULL);

    }while(really==1024);

    CloseHandle(filein);
    CloseHandle(fileout);

    return 0;
}

and if Im right how can i know its ok

Answer 1

First, you can't overwrite a file that's in use. You will need to use different pathnames for your input and output, and then rename files at the end.