Tags: engineering reverse 

Rating: 5.0

We figured out that this binary had an encrypt function which was obfuscated
with the [obfy obfuscator](https://github.com/fritzone/obfy).
Having spent a couple of hours trying to manually reverse the operations we
found that the loop body seemed simple. After doing so, we finally saw that
the `verify_key` function strcmp the output of the encryption function to a
hardcoded string.

disassemble verify_key(char*)
Dump of assembler code for function _Z10verify_keyPc:
0x0000555555556059 <+0>: push rbp
0x000055555555605a <+1>: mov rbp,rsp
0x000055555555605d <+4>: sub rsp,0x20
0x0000555555556061 <+8>: mov QWORD PTR [rbp-0x18],rdi
0x0000555555556065 <+12>: mov rax,QWORD PTR [rbp-0x18]
0x0000555555556069 <+16>: mov rdi,rax
0x000055555555606c <+19>: call 0x555555555420 <strlen@plt>
0x0000555555556071 <+24>: cmp rax,0x9
0x0000555555556075 <+28>: jbe 0x555555556089 <verify_key(char*)+48>
0x0000555555556077 <+30>: mov rax,QWORD PTR [rbp-0x18]
0x000055555555607b <+34>: mov rdi,rax
0x000055555555607e <+37>: call 0x555555555420 <strlen@plt>
0x0000555555556083 <+42>: cmp rax,0x40
0x0000555555556087 <+46>: jbe 0x555555556090 <verify_key(char*)+55>
0x0000555555556089 <+48>: mov eax,0x0
0x000055555555608e <+53>: jmp 0x5555555560c3 <verify_key(char*)+106>
0x0000555555556090 <+55>: mov rax,QWORD PTR [rbp-0x18]
0x0000555555556094 <+59>: mov rdi,rax
0x0000555555556097 <+62>: call 0x555555555cb1 <enc(char const*)>
0x000055555555609c <+67>: mov QWORD PTR [rbp-0x10],rax
0x00005555555560a0 <+71>: lea rax,[rip+0x1b59] # 0x555555557c00
0x00005555555560a7 <+78>: mov QWORD PTR [rbp-0x8],rax
0x00005555555560ab <+82>: mov rdx,QWORD PTR [rbp-0x10]
0x00005555555560af <+86>: mov rax,QWORD PTR [rbp-0x8]
0x00005555555560b3 <+90>: mov rsi,rdx
0x00005555555560b6 <+93>: mov rdi,rax
=> 0x00005555555560b9 <+96>: call 0x555555555490 <strcmp@plt>
0x00005555555560be <+101>: test eax,eax
0x00005555555560c0 <+103>: sete al
0x00005555555560c3 <+106>: leave
0x00005555555560c4 <+107>: ret
End of assembler dump.

One thing we also discovered was that there was a minimum length needed for our
input. So, breaking at the strcmp line we ran the following.

``` gdb
$ break *verify_key+96
Breakpoint 1 at 0x20b9

$ commands
Type commands for breakpoint(s) 1, one per line.
End with a line saying just "end".
>echo "RDI: \n"
>x/4xg $rdi
>echo "RSI: \n"
>x/4xg $rsi
gdb-peda$ r
Starting program: /home/gh0s1/Documents/CTF/CTFWriteups/2019/TamuCTF/Rev/obfuscaxor/obfuscaxor

Please Enter a product key to continue:
"0x555555557c00: 0x81d3c7ab9cff9eae 0xae8def9d8afbeee7
0x555555557c10: 0x0000000000000000 0x20657361656c500a
"0x55555576e280: 0xaeffec9faeffec9f 0x000000000000009f
0x55555576e290: 0x0000000000000000 0x0000000000000000

It was suspected that this was a straight XOR so, to test we compared the
output to our input and retested.

def toAns(got, expected):
... return (chr(got ^ ord("A") ^ expected))

Starting program: /home/gh0s1/Documents/CTF/CTFWriteups/2019/TamuCTF/Rev/obfuscaxor/obfuscaxor

Please Enter a product key to continue:
"0x555555557c00: 0x81d3c7ab9cff9eae 0xae8def9d8afbeee7
0x555555557c10: 0x0000000000000000 0x20657361656c500a
"0x55555576e280: 0xaeffec9faeff9eae 0x000000000000009f
0x55555576e290: 0x0000000000000000 0x0000000000000000

Looks like we are in the right path. Continuing down we got the flag:

``` bash
echo "p3Asujmn9CEeCB3A" | nc rev.tamuctf.com 7224

Please Enter a product key to continue:

Original writeup (https://github.com/balbassam/CTFWriteups/blob/master/2019/TamuCTF/Rev/obfuscaxor/solution.md).