Hsctf 2021
opisthocomus-hoazin
Solved by: Taz34
First opisthocomus-hoazin.py
import time
from Crypto.Util.number import *
flag = open('flag.txt','r').read()
p = getPrime(1024)
q = getPrime(1024)
e = 2**16+1
n=p*q
ct=[]
for ch in flag:
ct.append((ord(ch)^e)%n)
print(n)
print(e)
print(ct)
2nd is output.txt
15888457769674642859708800597310299725338251830976423740469342107745469667544014118426981955901595652146093596535042454720088489883832573612094938281276141337632202496209218136026441342435018861975571842724577501821204305185018320446993699281538507826943542962060000957702417455609633977888711896513101590291125131953317446916178315755142103529251195112400643488422928729091341969985567240235775120515891920824933965514217511971572242643456664322913133669621953247121022723513660621629349743664178128863766441389213302642916070154272811871674136669061719947615578346412919910075334517952880722801011983182804339339643
65537
[65639, 65645, 65632, 65638, 65658, 65653, 65609, 65584, 65650, 65630, 65640, 65634, 65586, 65630, 65634, 65651, 65586, 65589, 65644, 65630, 65640, 65588, 65630, 65618, 65646, 65630, 65607, 65651, 65646, 65627, 65586, 65647, 65630, 65640, 65571, 65612, 65630, 65649, 65651, 65586, 65653, 65621, 65656, 65630, 65618, 65652, 65651, 65636, 65630, 65640, 65621, 65574, 65650, 65630, 65589, 65634, 65653, 65652, 65632, 65584, 65645, 65656, 65630, 65635, 65586, 65647, 65605, 65640, 65647, 65606, 65630, 65644, 65624, 65630, 65588, 65649, 65585, 65614, 65647, 65660]
We can see the each vale in ct list is actually the n modulus(%) of the XOR (^) value of the ASCII values of the flag’s characters with e.
We can verify it here, by encoding first letter of the flag i.e. “f” and this is also the first letter in the ct list:
Now I have written a small script which encode out all the characters that can be used in the flag:
import string
chr_set = string.ascii_letters + string.digits + string.punctuation
e = 65537
n = 15888457769674642859708800597310299725338251830976423740469342107745469667544014118426981955901595652146093596535042454720088489883832573612094938281276141337632202496209218136026441342435018861975571842724577501821204305185018320446993699281538507826943542962060000957702417455609633977888711896513101590291125131953317446916178315755142103529251195112400643488422928729091341969985567240235775120515891920824933965514217511971572242643456664322913133669621953247121022723513660621629349743664178128863766441389213302642916070154272811871674136669061719947615578346412919910075334517952880722801011983182804339339643
for i in range(len(chr_set)):
x = chr_set[i] + ": " + str((ord(chr_set[i])^e)%n)
print(x)
I also made a lookup.txt dictionary with all the elements of the ct list Now i used grep to get the elements we want form the script output:
Now i used Sublime text to replace all the values with there specific letters to get the flag
flag{tH1s_ic3_cr34m_i5_So_FroZ3n_i"M_pr3tTy_Sure_iT's_4ctua1ly_b3nDinG_mY_5p0On}