C++ Simple Algorithm Not Working

/*
 * Copyright 2004 Caphyon LTD. All rights reserved.
 *
 * mailto: eng@caphyon.com
 * http://www.caphyon.com
 *
 */
 
// Serial Validation DLL.cpp : Defines the entry point for the DLL application.
//
 
#include "stdafx.h"
 
#define PIDKEY_LENGTH 100
#define DIGIT(a)  ((a) - '0')
 
BOOL APIENTRY DllMain(HANDLE /*hModule*/, DWORD  /*ul_reason_for_call*/,
                      LPVOID /*lpReserved*/)
{
  return TRUE;
}
 
#ifndef _DEBUG
BOOL WINAPI __DllMainCRTStartup(HINSTANCE hModule, DWORD ul_reason_for_call,
                                LPVOID lpReserved)
{
  return DllMain(hModule, ul_reason_for_call, lpReserved);
}
#endif
 
UINT __stdcall ValidateSerial(MSIHANDLE hInstall)
{
  TCHAR szPidKey[PIDKEY_LENGTH];
  DWORD dwLen = sizeof(szPidKey) / sizeof(szPidKey[0]);
 
  ///retrieve the text entered by the user
  UINT res = ::MsiGetProperty(hInstall, _T("PIDKEY"), szPidKey, &dwLen);
  if(res != ERROR_SUCCESS)
  {
    //fail the installation
    return 1;
  }
 
  bool snIsValid = true;
  //validate the text from szPidKey according to your algorithm
  //put the result in snIsValid
 
  int x = DIGIT(szPidKey[0]);
  int y = DIGIT(szPidKey[1]);
  int z = DIGIT(szPidKey[2]);
  int xyz = (x + y+ z);
  int left = ((((xyz * xyz)+ 7) * 13 ));
  //std::string Mystring = left;
  //std::string left = Mystring.substr(0, 4); 
  int right = DIGIT(szPidKey[4]);
  right = right & DIGIT(szPidKey[5]);
  right = right & DIGIT(szPidKey[6]);
  right = right & DIGIT(szPidKey[7]);
  snIsValid = (left != 0) && (right != 0) && (left == right);
 
  TCHAR * serialValid = NULL;
  if(snIsValid)
    serialValid = _T("TRUE"); 
  else
  {
    //eventually say something to the user
    ::MessageBox(0, _T("Invalid Serial"), _T("Message"), MB_ICONSTOP);
 
    serialValid = _T("FALSE");
  }
 
  res = ::MsiSetProperty(hInstall, _T("SERIAL_VALIDATION"), serialValid);
  if(res != ERROR_SUCCESS)
  {
    //fail the installation
    return 1;
  }
 
  //the validation succeeded - even the serial is wrong
  //if the SERIAL_VALIDATION was set to FALSE the installation will not continue
  return 0;
}