Question

So I have been solving this crackme today. I managed to locate and understand the serial generating routine except for a few last instructions. I decided to write a keygen in assembly for the first time. Everything was going nicely until I came to the last few instructions of the serial routine. I’m using MASM and Intel assembly (Intel, AT&T, how do you call these?) This is my current code:

.386
.model flat,stdcall
option casemap:none

include     \masm32\include\windows.inc
include     \masm32\include\kernel32.inc
include     \masm32\include\user32.inc
includelib  \masm32\lib\kernel32.lib
includelib  \masm32\lib\user32.lib

DlgProc     proto :DWORD,:DWORD,:DWORD,:DWORD
SerialCalc  proto :DWORD

.data
EnterText   db      "...enter a name...",0
temp        db      "temp",0
Format      db      "%i-x019871",0

.data?
NameBuffer      db          100 dup(?)
SerialBuffer    db          150 dup(?)
SerialLength    dd          ?
hInstance       HINSTANCE   ?

.const
IDC_NAME            equ     1002
IDC_SERIAL          equ     1003
IDC_GENERATE        equ     1004
IDC_NAMELABEL       equ     1005
IDC_SERIALLABEL     equ     1006
IDD_MAIN            equ     1001

.code
start:

    invoke GetModuleHandle, NULL
    mov hInstance,eax
    invoke DialogBoxParam, hInstance, IDD_MAIN, NULL, addr DlgProc, NULL
    invoke ExitProcess, 0

DlgProc proc hWnd:HWND, uMsg:UINT, wParam:WPARAM, lParam:LPARAM
    .if uMsg == WM_INITDIALOG
        invoke GetDlgItem,hWnd,IDC_NAME ;get IDC_NAME
        invoke SetFocus,eax             ;focus on it
    .elseif uMsg == WM_COMMAND
        mov eax, wParam ;wParam = control that issued the WM_COMMAND message
        .if ax == IDC_NAME ;if it was the name box
            shr eax, 16 ;shift right and get more info?
            .if ax == EN_CHANGE ;if the text was changed
                invoke GetDlgItemText, hWnd, IDC_NAME, addr NameBuffer, 100 ;get text
                invoke lstrlen, addr NameBuffer ;get length
                mov SerialLength, eax ;move length into var
                .if eax == 0 ;if length is 0
                    invoke SetDlgItemTextA, hWnd, IDC_SERIAL, addr EnterText ;"...enter a name..."
                .elseif eax > 0 ;if length is bigger than 0
                    invoke SerialCalc, hWnd ;calc
                    invoke SetDlgItemTextA, hWnd, IDC_SERIAL, addr SerialBuffer ;"serial"
                .endif
            .endif
        .endif
    .elseif uMsg == WM_CLOSE
        invoke EndDialog, hWnd, 0
    .endif

    xor eax,eax
    ret
DlgProc endp

SerialCalc proc hWnd:HWND
    ;push ecx allocate space for 1 local variable; i was trying to do something with local variables, but I failed
    mov edx, SerialLength
    imul edx, edx, 875CDh
    mov eax, 51EB851Fh
    mul edx
    mov eax, edx
    shr eax, 5h
    imul eax, eax, -370h
    xor edx, edx ;mov edx, 0
    ;problems start here; I took this code from a solution i found
    ;push edx
    ;push eax
    ;fild qword ptr [esp]
    ;add esp, 8
    ;fstp real8 ptr [SerialBuffer]
            ;more stuff should come here sprintf etc.. but since I haven't solved my main problem yet I decided not to rush
SerialCalc endp

end start

And this is the actual serial routine in the program itself:

MOV EDX,EAX
IMUL EDX,EDX,875CD
MOV EAX,51EB851F
MUL EDX
MOV EAX,EDX
SHR EAX,5
IMUL EAX,EAX,-370
MOV EDX,0
PUSH EDX                                                            ; ||format = NULL
PUSH EAX                                                            ; ||s = FE8BC1A0
FILD QWORD PTR SS:[ESP]                                             ; ||
LEA ESP,DWORD PTR SS:[ESP+8]                                        ; ||
FSTP QWORD PTR SS:[EBP-410]                                         ; ||
FLD QWORD PTR SS:[EBP-410]                                          ; ||
FSTP QWORD PTR SS:[ESP+8]                                           ; ||
MOV DWORD PTR SS:[ESP+4],Crackme_.00401469                          ; ||ASCII "%i-x019871"
LEA EAX,[LOCAL.194]                                                 ; ||
MOV DWORD PTR SS:[ESP],EAX                                          ; ||
CALL <JMP.&msvcrt.sprintf>                                          ; |\sprintf
LEA EAX,[LOCAL.194]                                                 ; |
MOV DWORD PTR SS:[ESP+4],EAX                                        ; |
LEA EAX,[LOCAL.130]                                                 ; |
MOV DWORD PTR SS:[ESP],EAX                                          ; |
CALL <JMP.&msvcrt.strcmp>                                           ; \strcmp

The routine calculates the serial which finishes in EAX, pushes it onto the stack and then, as far as I understand, uses FILD to push it onto the FPU stack, FSPT to take it off of the FPU and put it into EBP-410, FLD to push EBP-410 onto the FPU again and finally, uses the FSTP to store it into ESP+8 as a parameter for sprintf. The sprint and strcmp don’t really matter here but I included them anyway so you can get a better understanding of what’s going on.

By the way, this LOCAL 194. is the place where the formatted string will be placed.

I searched the Internet and found these instructions’ descriptions but haven’t found any actual examples or material that could help me transfer this to my keygen.

So the final question is: how do I transfer this to my keygen? I always get the “Program has stopped working” message or nothing shows up in the serial box. The commented stuff in my SerialCalc routine is the part which I ripped off of another guy’s solution just to try if it’ll work, but unfortunately, it didn’t.

Tell me if you need more details about the problem or any extra information.

I apologize for my noobiness!

Thanks in advance,
Tuntuni.

Answer 1

I finally got it! It seems I haven’t balanced the stack correctly after the instructions or something. Anyway, I used Visual Studio to write inline assembly and finally got it work. Thanks for reading.