Rating:

# Cereal Killer

![Category](http://img.shields.io/badge/Category-Reverse_Engineering-orange?style=for-the-badge) ![Points](http://img.shields.io/badge/Points-50-brightgreen?style=for-the-badge)

## Details

> ![image](https://user-images.githubusercontent.com/73170900/137821256-df7966fe-b006-4d14-8b35-a0eab0b2d12f.png)
>
> spookyboi is really into Serial Killers. He loves to watch Mindhunter on NetFlix. He can also SLAY a bowl of his favorite cereal.
>
> (Choose either the Windows or Linux binaries to analyze...)
>
> [RE01 (Windows)](https://tinyurl.com/wz2rt9y5)
>
> SHA1: 3674964b75d2894f297470be8a39802af40314b3
>
> [RE01 (Linux)](https://tinyurl.com/sj75xk2r)
>
> SHA1: af2392a43e3571fa43be6a48a3a83131f549013c
---

To solve this challenge we can use `gdb` (which i like to use with [pwndbg plug-in](https://github.com/pwndbg/pwndbg))

```assembly
❯ gdb deadface_re01.bin
GNU gdb (GDB) 11.1
Copyright (C) 2021 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
Type "show copying" and "show warranty" for details.
This GDB was configured as "x86_64-pc-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<https://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
<http://www.gnu.org/software/gdb/documentation/>.

For help, type "help".
Type "apropos word" to search for commands related to "word"...
pwndbg: loaded 190 commands. Type pwndbg [filter] for a list.
pwndbg: created $rebase, $ida gdb functions (can be used with print/break)
Reading symbols from deadface_re01.bin...
(No debugging symbols found in deadface_re01.bin)
pwndbg>
```

Running the program we see how it works we see the below;

```assembly
pwndbg> run
Starting program: /home/jaxigt/Downloads/deadface_re01.bin
ERROR: Could not find ELF base!
What is the best and sp00kiest breakfast cereal?
Please enter the passphrase: 123
notflag{you-guessed-it---this-is-not-the-flag}
[Inferior 1 (process 6160) exited normally]
```

Next we dissamble the main fucntion to try and understand what's hapenning.

```assembly
pwndbg> disassemble main
Dump of assembler code for function main:
0x00005555555550c0 <+0>: endbr64
0x00005555555550c4 <+4>: push rbx
0x00005555555550c5 <+5>: lea rcx,[rip+0xf3c] # 0x555555556008
0x00005555555550cc <+12>: sub rsp,0x230
0x00005555555550d3 <+19>: mov rax,QWORD PTR fs:0x28
0x00005555555550dc <+28>: mov QWORD PTR [rsp+0x228],rax
0x00005555555550e4 <+36>: xor eax,eax
0x00005555555550e6 <+38>: cs nop WORD PTR [rax+rax*1+0x0]
0x00005555555550f0 <+48>: movzx edx,BYTE PTR [rcx+rax*1]
0x00005555555550f4 <+52>: test dl,dl
0x00005555555550f6 <+54>: je 0x5555555550fe <main+62>
0x00005555555550f8 <+56>: xor edx,0x5a
0x00005555555550fb <+59>: mov BYTE PTR [rsp+rax*1],dl
0x00005555555550fe <+62>: add rax,0x1
0x0000555555555102 <+66>: cmp rax,0x1f
0x0000555555555106 <+70>: jne 0x5555555550f0 <main+48>
0x0000555555555108 <+72>: lea rdi,[rip+0xf19] # 0x555555556028
0x000055555555510f <+79>: mov BYTE PTR [rsp+0x1e],0x0
0x0000555555555114 <+84>: lea rbx,[rsp+0x20]
0x0000555555555119 <+89>: call 0x555555555080 <[email protected]>
0x000055555555511e <+94>: lea rdx,[rip+0xf6a] # 0x55555555608f
0x0000555555555125 <+101>: lea rsi,[rip+0xf81] # 0x5555555560ad
0x000055555555512c <+108>: xor eax,eax
0x000055555555512e <+110>: mov edi,0x1
0x0000555555555133 <+115>: call 0x5555555550a0 <[email protected]>
0x0000555555555138 <+120>: mov rsi,rbx
0x000055555555513b <+123>: lea rdi,[rip+0xf6b] # 0x5555555560ad
0x0000555555555142 <+130>: xor eax,eax
0x0000555555555144 <+132>: call 0x5555555550b0 <[email protected]>
0x0000555555555149 <+137>: movabs rax,0x68632d746e753063
0x0000555555555153 <+147>: cmp QWORD PTR [rsp+0x20],rax
0x0000555555555158 <+152>: je 0x555555555184 <main+196>
0x000055555555515a <+154>: lea rdi,[rip+0xeff] # 0x555555556060
0x0000555555555161 <+161>: call 0x555555555080 <[email protected]>
0x0000555555555166 <+166>: mov rax,QWORD PTR [rsp+0x228]
0x000055555555516e <+174>: xor rax,QWORD PTR fs:0x28
0x0000555555555177 <+183>: jne 0x55555555519f <main+223>
0x0000555555555179 <+185>: add rsp,0x230
0x0000555555555180 <+192>: xor eax,eax
0x0000555555555182 <+194>: pop rbx
0x0000555555555183 <+195>: ret
0x0000555555555184 <+196>: cmp DWORD PTR [rsp+0x28],0x6c756330
0x000055555555518c <+204>: jne 0x55555555515a <main+154>
0x000055555555518e <+206>: cmp WORD PTR [rbx+0xc],0x61
0x0000555555555193 <+211>: jne 0x55555555515a <main+154>
0x0000555555555195 <+213>: mov rdi,rsp
0x0000555555555198 <+216>: call 0x555555555080 <[email protected]>
0x000055555555519d <+221>: jmp 0x555555555166 <main+166>
0x000055555555519f <+223>: call 0x555555555090 <[email protected]>
End of assembler dump.
```

We can see here that there is a `cmp` (compare) funtion called.

```assembly
0x0000555555555153 <+147>: cmp QWORD PTR [rsp+0x20],rax
```

Lets set a break point at that compare funtion.

```
pwndbg> break *0x0000555555555153
Breakpoint 1 at 0x555555555153
```
~

Now we run again...

```assembly
pwndbg> run
Starting program: /home/jaxigt/Downloads/deadface_re01.bin
What is the best and sp00kiest breakfast cereal?
Please enter the passphrase: 123

Breakpoint 1, 0x0000555555555153 in main ()
LEGEND: STACK | HEAP | CODE | DATA | RWX | RODATA
───────────────────────────────────────────────────────────────────────[ REGISTERS ]────────────────────────────────────────────────────────────────────────
RAX 0x68632d746e753063 ('c0unt-ch')
RBX 0x7fffffffd7f0 ◂— 0x34000333231 /* '123' */
RCX 0x0
RDX 0x0
RDI 0x7fffffffd290 ◂— 0x0
RSI 0xa
R8 0x73
R9 0x7ffff7f7ca60 (main_arena+96) —▸ 0x555555559ab0 ◂— 0x0
R10 0x77
R11 0x0
R12 0x5555555551b0 (_start) ◂— endbr64
R13 0x0
R14 0x0
R15 0x0
RBP 0x0
RSP 0x7fffffffd7d0 ◂— 'flag{c0unt-ch0cula-cereal-FTW}'
RIP 0x555555555153 (main+147) ◂— cmp qword ptr [rsp + 0x20], rax
─────────────────────────────────────────────────────────────────────────[ DISASM ]─────────────────────────────────────────────────────────────────────────
► 0x555555555153 <main+147> cmp qword ptr [rsp + 0x20], rax
0x555555555158 <main+152> je main+196 <main+196>

0x55555555515a <main+154> lea rdi, [rip + 0xeff]
0x555555555161 <main+161> call [email protected] <[email protected]>

0x555555555166 <main+166> mov rax, qword ptr [rsp + 0x228]
0x55555555516e <main+174> xor rax, qword ptr fs:[0x28]
0x555555555177 <main+183> jne main+223 <main+223>

0x555555555179 <main+185> add rsp, 0x230
0x555555555180 <main+192> xor eax, eax
0x555555555182 <main+194> pop rbx
0x555555555183 <main+195> ret
─────────────────────────────────────────────────────────────────────────[ STACK ]──────────────────────────────────────────────────────────────────────────
00:0000│ rsp 0x7fffffffd7d0 ◂— 'flag{c0unt-ch0cula-cereal-FTW}'
01:0008│ 0x7fffffffd7d8 ◂— 'nt-ch0cula-cereal-FTW}'
02:0010│ 0x7fffffffd7e0 ◂— 'la-cereal-FTW}'
03:0018│ 0x7fffffffd7e8 ◂— 0x7d5754462d6c /* 'l-FTW}' */
04:0020│ rbx 0x7fffffffd7f0 ◂— 0x34000333231 /* '123' */
05:0028│ 0x7fffffffd7f8 ◂— 0x34000000340
... ↓ 2 skipped
───────────────────────────────────────────────────────────────────────[ BACKTRACE ]────────────────────────────────────────────────────────────────────────
► f 0 0x555555555153 main+147
f 1 0x7ffff7de3b25 __libc_start_main+213
f 2 0x5555555551de _start+46
────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
pwndbg>
```

Now, when we hit the break point that we set, take note of the line showing the value in the top of the stack at that point!

```assembly
00:0000│ rsp 0x7fffffffd7d0 ◂— 'flag{c0unt-ch0cula-cereal-FTW}'
```

## flag{c0unt-ch0cula-cereal-FTW}

Original writeup (https://github.com/CTSecUK/DEADFACE_CTF_2021/blob/main/Write-ups/Reverse%20Engineering/Cereal%20Killer%20(50%20Points).md).