UTCTF 19 [ Regular Zips ]

To start with, UTCTF 2019 had pretty high scores for every challenge ie. The points were ranged from 100 to 2000.
And They were all fun to solve, But I personally liked Regular Zips as I learn a lot about Regular Expressions and ofcourse python’s awesomeness. XD

Download RegularZips.zip & problem.txt

Being a 600 pts challenge, it was somewhat a medium level challenge. Thanks to my friend @HimanshuKr who helped me with this challenge.

So I started with the given regular expression in the problem.txt which was provided to us. Also it was the same regex which was in the challenge description.
We were also given a zip which was obviously encrypted and we had to decrypt by brute-forcing it against some words that satisfy the regex.

I quickly used my google foo skills to search for a Regex library which can be used with python to generate those words.

Oh there’s a perfect library for our work => exrex

Install it using :

pip install exrex

And I shooted Sublime to make a wordlist and brute-force at the same time using python :

import exrex
import zipfile 
 
encZip = 'RegularZips.zip' 
wordlist = open('wordlist.txt','w')

wordlist.write('\n'.join(list(exrex.generate("^	7	y	RU[A-Z]KKx2 R4\d[a-z]B	N$"))))

password = None 
zf = zipfile.ZipFile(encZip) 

with open('wordlist.txt', 'r') as f: 
   for line in f.readlines(): 
         password = line.strip('\n') 
         try: 
               zf.extractall(pwd=password) 
               password = 'Password found: %s' % password 
               print password
         except: 
               pass 

And we got our password…
But Be careful with spaces and tabs !!

Password found: 	7	y	RUHKKx2 R47gB	N

But that’s not enough, we have a long journey ahead. lols..

So after extracting it we get another zipfile as archive.zip and a hint.txt file which contains another regex sadly.
So we can just write a script to extract all of the archives until we are left with none of them. I used zipfile library in python to workaround with zip files in python and added some colors to make the automation process more fun.

I created a new directory and placed this script along with RegularZips.zip and Problem.txt.

import os
import zipfile
import exrex


RED = lambda x: '\033[31m' + str(x)
RESET = lambda x: '\033[0m' + str(x)

def tryunzip(file, hint):
    with zipfile.ZipFile(file) as z:
        for pwd in exrex.generate(hint, limit=exrex.count(hint)):
            try:
                z.extractall(pwd=pwd.encode())
                print "Password Found : ",
                print RED(pwd)
                print RESET(' ') 
                if "flag.txt" in z.namelist():
                    print "---- Flag Found ----"
                    os.system('cat flag.txt') 
                return
            except:
                pass

def main():
    tryunzip('RegularZips.zip', open('problem.txt').read()[1:-1])
    i = 1
    while os.path.exists('archive.zip'):
        os.rename('archive.zip', 'archive'+str(i)+'.zip')
        tryunzip('archive'+str(i)+'.zip', open('hint.txt').read()[1:-1])
        print "Currently Processing : archive" + str(i)
        os.remove('archive'+str(i)+'.zip')
        i+=1

if __name__ == '__main__':
    main()

And To our Surprise …
There was a flag.txt in archive998.

The Flag was :

utflag{bean_pure_omission_production_rally}

Wasn’t it Sw33t.

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Till then Keep Escalating the Priveleges and Happy Hacking !!