winmain reversing the assembly to c code

Question

this is I got so far

#include <windows.h>

int WinMain(HINSTANCE hlnst,HINSTANCE hPrev, LPSTR lpCmd,int nShow){

    int k =0; 
    int j= 0 ;  
    MessageBox(0,"crackname","",0);

    int i;
    for (i=0; *(lpCmd+i)  ; i++) {
    }
    while (i < j) {
        j = *(lpCmd + i);//j = ECX *lpCmd = EDX
        *(lpCmd) = i + *(lpCmd);
    }
    MessageBox(0,"success","",0);
    return 0;
}

I'm stuck with

  LEA ECX,DWORD PTR DS:[EAX+EDX-2F] 

anyone give me some hint to solve this question? is this represent the pointer?

Answer 1

Although the complete picture of your assembly cannot be reconstructed, as there is something missing at the bottom, it can be said that your C re-coding is correct in the first (and simpler) loop, but wrong in the second loop.

First loop: This is simply a strlen of the command line parameter, being stored in the local variable i.

Second loop: Calculation of k from each character of the command line (presumably, because the rest of the loop is missing), taking into account the old k value. Probably that algo is part of your crackme.

Also, your "while" comparison of the 2nd loop is wrong.

The re-compiled code as I see it could look like follows (probably correct until the last statement of the second loop, the rest remained your secret):

 int WinMain(_In_ HINSTANCE hInstance, 
    HINSTANCE hPrevInstance,
    LPTSTR    lpCmd,            //ebp+10h
    int       nCmdShow)
{
    int j = 0;      //ebp-4
    int i = 0;      //ebp-8
    MessageBox(0, "crackname", "", 0);
    int k = 0;      //ebp-c

    //you got this correct
    //i contains the string length at lpCmd, i.e. the command line parameter length
    for (i = 0; *(lpCmd + i); i++) {
    }

    //iterate through all elements of the parameter and calculate sth.
    //the algo calculates k and probably j is incremented. 
    //However, the rest of the loop is missing in your assembler code
    while (j < i) {
        k = *(lpCmd + j)+k-0x2f; //this contains the lea statement
        //from here, your code is missing
        j++;
    }
    //MessageBox(0, "success", "", 0);
    //return 0;
}