UNIMOD Writeup

Solved By : Avantika

In this challenge we are given the following script along with the cipher text

unimod.py

import random

flag = open('flag.txt', 'r').read()
ct = ''
k = random.randrange(0,0xFFFD)
for c in flag:
    ct += chr((ord(c) + k) % 0xFFFD)

open('out', 'w').write(ct)

output.txt

饇饍饂饈饜餕饆餗餙饅餒餗饂餗餒饃饄餓饆饂餘餓饅餖饇餚餘餒餔餕餕饆餙餕饇餒餒饞飫

Looking at the script we can see that a random integer is being assigned to the variable k , to find this integer, we can do a simple trick, as we know that first letter of flag, is going to be f i.e 102 in ASCII chart , we can brute-force to find k, here is the brute-force script

ct = '饇饍饂饈饜餕饆餗餙饅餒餗饂餗餒饃饄餓饆饂餘餓饅餖饇餚餘餒餔餕餕饆餙餕饇餒餒饞飫'

for k in range(0, 65533):
    goal = ct[0]
    goal = ord(goal)
    num = (102 - k) % 65533
    if goal == num:
        print(k)
        break

Script simple takes the first character of cipher-text, gets the ASCII value to and assigns that to goalthen just “reverses” what the unimod.py script did

Next part is pretty easy, we just perform the same operation again on the cipher-text and we would get the flag

k = 26396
for char in ct:
    print(chr((ord(char) + k) % 65533), end='')

This prints out our flag.