# IWCTF 2016 - Reversing - File Checker - 60 pts

> Description: My friend sent me this file. He told that if I manage to reverse it, I'll have access to all his devices. My misfortune that I don't know anything about reversing :/
> Attachment: [rev60.zip](rev60.zip)

# Write-up

$ file filechecker
filechecker: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked,
interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux 2.6.32, BuildID[sha1]=564c7e61a18251b57f8c2c3dc205ed3a5b35cca6,

Executing the binary reveals, that it fails on opening '.password'

$ ltrace ./filechecker
__libc_start_main(0x400666, 1, 0x7ffc12dfa718, 0x400850 <unfinished ...>
fopen(".password", "r") = 0
printf("Fatal error: File does not exist"...) = 32
Fatal error: File does not exist+++ exited (status 1) +++

Creating the file and input some testing flag like "IW{abcdefg}" makes it run, but it just tells us, that we have the wrong chars.

Time to have a look at the disassembly. It shows, that it opens and read the file and then calls a test function for every character.

000000000040079d mov rbp, rsp
00000000004007a0 mov dword [ss:rbp+var_44], edi
00000000004007a3 mov qword [ss:rbp+var_50], rsi
00000000004007a7 mov dword [ss:rbp+var_40], 0x12ee
00000000004007ae mov dword [ss:rbp+var_3C], 0x12e0
00000000004007b5 mov dword [ss:rbp+var_38], 0x12bc
00000000004007bc mov dword [ss:rbp+var_34], 0x12f1
00000000004007c3 mov dword [ss:rbp+var_30], 0x12ee
00000000004007ca mov dword [ss:rbp+var_2C], 0x12eb
00000000004007d1 mov dword [ss:rbp+var_28], 0x12f2
00000000004007d8 mov dword [ss:rbp+var_24], 0x12d8
00000000004007df mov dword [ss:rbp+var_20], 0x12f4
00000000004007e6 mov dword [ss:rbp+var_1C], 0x12ef
00000000004007ed mov dword [ss:rbp+var_18], 0x12d2
00000000004007f4 mov dword [ss:rbp+var_14], 0x12f4
00000000004007fb mov dword [ss:rbp+var_10], 0x12ec
0000000000400802 mov dword [ss:rbp+var_C], 0x12d6
0000000000400809 mov dword [ss:rbp+var_8], 0x12ba
0000000000400810 mov eax, dword [ss:rbp+var_44]
0000000000400813 cdqe
0000000000400815 mov edx, dword [ss:rbp+rax*4+var_40]
0000000000400819 mov rax, qword [ss:rbp+var_50]
000000000040081d mov eax, dword [ds:rax]
000000000040081f lea ecx, dword [ds:rdx+rax]
0000000000400822 mov edx, 0x354ac933
0000000000400827 mov eax, ecx
0000000000400829 imul edx
000000000040082b sar edx, 0xa
000000000040082e mov eax, ecx
0000000000400830 sar eax, 0x1f
0000000000400833 sub edx, eax
0000000000400835 mov eax, edx
0000000000400837 imul eax, eax, 0x1337
000000000040083d sub ecx, eax
000000000040083f mov eax, ecx
0000000000400841 mov rdx, qword [ss:rbp+var_50]
0000000000400845 mov dword [ds:rdx], eax
0000000000400847 nop
0000000000400848 pop rbp
0000000000400849 ret

Hopper gives the following pseudo code for this:

function sub_40079c {
rcx = *(int32_t *)(rbp + sign_extend_32(arg0) * 0x4 + 0xffffffffffffffc0) + *(int32_t *)arg1;
rax = rcx - ((SAR(rcx * 0x354ac933, 0xa)) - (SAR(rcx, 0x1f))) * 0x1337;
*(int32_t *)arg1 = rax;
return rax;

Though quite some shifting and calculations there, this boils down to:

`result = 0x1337 - (0x12ee + curchar)`

So in the end, every character gets summed with the value at the same index in the array `[0x12ee, 0x12e0, 0x12bc, 0x12f1, 0x12ee, 0x12eb, 0x12f2, 0x12d8, 0x12f4, 0x12ef, 0x12d2, 0x12f4, 0x12ec, 0x12d6, 0x12ba]`, which should result in `0x1337` to pass the validation.


coll = [ 0x12ee, 0x12e0, 0x12bc, 0x12f1, 0x12ee, 0x12eb, 0x12f2, 0x12d8, 0x12f4, 0x12ef, 0x12d2, 0x12f4, 0x12ec, 0x12d6, 0x12ba ]

result = ""

for val in coll:
res = 0x1337 - val
result += chr(res)

print result

This will calculate the password:

$ python solver.py

Original writeup (https://github.com/Kileak/CTF/tree/master/2016/internetwache/rev/filechecker).