Locking shareware functions

• Download demo project -115 Kb

Introduction

If you're shareware programmer, you may need to lock some functions of your product from unauthorized access. Problem is that hackers can unlock these functions without your permission. :-) In this article I'll show how to lock shareware functions and to reduce of risk of a hacker cracking your product.

Background

Basic idea is that we call hidden shareware functions by its address, which we pass to user in key file. In this case, hacker can't crack the program because he cannot determine the necessary address of a protected function. In the provided demo project, the address of function is stored in an open kind ( pass.txt file), but nothing hinders you to encrypt this address by user's hard disk serial number, for example.

The required Steps

1. Let's find out the address of the function:

   // somewhere in program body our shareware function lives 
   void HiddenPaintFunc(DWORD pv,DWORD pp)
   
   {
       //...
   }
   
   
   BOOL CSharewareDialog::OnInitDialog()
   {
   //
   // let's find out its address. It can be done with debugger, 
   // or simply like this:
   
   CString strAddress;
   strAddress.Format("0x%X",HiddenPaintFunc);
   AfxMessageBox(strAddress);// this string need to be commented  
                             // after obtaining an address
   ...
   

2. Create a key file in hex editor and write an address to it ( don't forget about the return order - for example: 00401602 -> 02164000)
3. At last, instead of a customary function call, we write the following :

   void CSharewareDialog::INeedCallLockedFunction(CDC*pDC)
   {
                 FILE*pFile=NULL;
     pFile=fopen("C:\\pass.txt","r");// open key file
     if(pFile){
                 DWORD dwHidden=0;
     fread((void*)&dwHidden,4,4,pFile);
     fclose(pFile);
     if(dwHidden){
     try{
        DWORD dwParam=(DWORD)pDC;
        DWORD dwParam2=(DWORD)this;
       
       __asm{
       
       xor ebx,ebx
       mov ebx,dwParam // pass some parameters
       push ebx
       mov ebx,dwParam2
       push ebx
       call dwHidden    // address of function we've got from key file
                        // if key file is wrong - jump to anywhere
   
     }
   
   
   
     }
     catch(...){
   
       AfxMessageBox("You must register this program.");
   
     }
     }
     else
     {
   
           AfxMessageBox("You must register this program.");
   
     }
     }
      else
     {
   
           AfxMessageBox("You must register this program.");
   
     }       
   
   }
   

4. That's all.

Notes

It is necessary to keep track of the address of the shareware function, as it may be changed from build to build.

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)