Nuit du Hack Quals 2015 - Bpythonastic - Forensic Challenge

CTFing at Easter time was super challenging. Endless family commitments, using my iPhone to browse challenges while at the dinner table and thinking through how to attack them when I can finally get to a PC. Fortunately NDH seemed to go for long enough for me to at least try a few of the challenges, which I did!

I’ve decided to document Bpythonastic for the reason that it was worth 300 points and not everyone got it, surprising because it was a trivial challenge.

The challenge, like many at NDH2k15 was just a link to a file. Which when you download it is a 81Mb file that extracts a 1.4GB raw file:

root@mankrik:~/ndh/bpy# ls -lah  
total 1.5G  
drwxr-xr-x 2 root root 4.0K Apr 5 12:37 .  
drwxr-xr-x 15 root root 4.0K Apr 5 10:38 ..  
-rw-r--r-- 1 root root 81M Apr 3 03:10 Bpythonastic.tar.gz  
-rw-r--r-- 1 root root 1.4G Mar 20 03:32 chall.raw  

file reports it as a ELF file. It might be a memory dump I suppose.

root@mankrik:~/ndh/bpy# file chall.raw   
chall.raw: ELF 64-bit LSB core file x86-64, version 1 (SYSV)  

Let’s just use strings and look for a flag.

root@mankrik:~/ndh/bpy# strings chall.raw | grep -c flag  
2893  

Ok 2893 instances of the word flag, thats a few but we’re looking for something related to Python I guess from the name of the challenge so I’ll widen the context of the grep to show a little before and after and review them quickly.

Before long I spot this python code in the strings.

 >>> from chall import *  
 >>> flag=Challenge()  
 >>> flag=base64.b64encode(pickle.dumps(flag))  
 >>> print flag  
 KGljaGFsbApDaGFsbGVuZ2UKcDAKKGRwMQpTJ2ZsYWcnCnAyClMnYTYzOWEyMWE0YTc0NzAzZmEwNDJk  
 NjE3NjgxYWE0NGJiNGQxYzA4YmM1ZmJmN2VmZDZiNDU1ODJiMGYwZDQwMycKcDMKc1MnaWQnCnA0Ckkw  
 CnNTJ2F1dGhvcicKcDUKUydZZ2dkcmFzaWwnCnA2CnNiLg==  

When I decode the base64 string I see this:

(ichall  
Challenge  
p0  
(dp1  
S'flag'  
p2  
S'a639a21a4a74703fa042d617681aa44bb4d1c08bc5fbf7efd6b45582b0f0d403'  
p3  
sS'id'  
p4  
I0  
sS'author'  
p5  
S'Yggdrasil'  
p6  
sb.  

Ok so that looks right, the flag is hashed though and we need the raw value. So we need to find what is generating this hash.

We look a little further in the strings output and find this:

     self.author="Yggdrasil"  
     self.flag=hashlib.sha256("Yougotit").hexdigest()  
 import chall  
 flag=Challenge()  

Which I conclude is probably the code that generates the above code, so the flag is probably “Yougotit”. I submitted it and it was correct.

So 300 points for using strings and grep. Nice.