[Writeup] Asis 2019 Quals - A Delicious Soup

In this challenge there is a encrypted file called flag.txt which contains the scrambled flag.

The encryption process works as follows

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def encord(msg, perm, l):  for _ in xrange(l):   msg = encrypt(msg, perm)  return msg   W, l = 7, random.randint(0, 1337) perm = range(W) random.shuffle(perm)

l contains a random number - the number of times the function will be applied to the message and a shuffled perm array.

To solve this challenge first we need to construct a decrypt function
and then bruteforce the two random variables.
For bruteforce the fastest strategy I could come up with was to first iterate over perm and then for that perm apply the decrypt function 1337 times.

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import random import itertools     def decrypt(enc,perm):         W = len(perm)         msg = "-"*len(enc)         for j in xrange(0,len(enc),W):                 for k in xrange(W):                         try:                                 m = enc[k+j]                         except: #unknown bug                                 pass                         msg = msg[:j+perm[k]] + m + msg[j+perm[k]+1:]                msg = msg[::-1][0] + msg[:len(msg)-1]         m = ""         w = len(msg)/2         for i in range(len(msg)/2):                 m += msg[i]+msg[w+i]         msg = m         msg = msg[::-1][0] + msg[:len(msg)-1]         return msg.strip(".")   f = open("flag.enc","r") enc_c = f.read() print("Decrypting",enc_c)           W = 7 l = 1337 for perm in itertools.permutations(range(W)):         enc = str(enc_c)         for _ in xrange(l):                 enc = decrypt(enc,perm)                 if enc.startswith("ASIS"):                         print(enc)

Bruteforce time ~ 2 mins