Cyber Apocalypse
2024
Dynastic
You find yourself trapped inside a sealed gas chamber, and suddenly, the air is pierced by the sound of a distorted voice played through a pre-recorded tape. Through this eerie transmission, you discover that within the next 15 minutes, this very chamber will be inundated with lethal hydrogen cyanide. As the tape’s message concludes, a sudden mechanical whirring fills the chamber, followed by the ominous ticking of a clock. You realise that each beat is one step closer to death. Darkness envelops you, your right hand restrained by handcuffs, and the exit door is locked. Your situation deteriorates as you realise that both the door and the handcuffs demand the same passcode to unlock. Panic is a luxury you cannot afford; swift action is imperative. As you explore your surroundings, your trembling fingers encounter a torch. Instantly, upon flipping the switch, the chamber is bathed in a dim glow, unveiling cryptic letters etched into the walls and a disturbing image of a Roman emperor drawn in blood. Decrypting the letters will provide you the key required to unlock the locks. Use the torch wisely as its battery is almost drained out!
Crypto
300 points
Walkthrough
We start by downloading the source files.
We are give a
source.py
file and an output.txt
file. It looks like the output is generated using the source code, so we start by analyzing the source code.from secret import FLAG
from random import randint
def to_identity_map(a):
return ord(a) - 0x41
def from_identity_map(a):
return chr(a % 26 + 0x41)
def encrypt(m):
c = ''
for i in range(len(m)):
ch = m[i]
if not ch.isalpha():
ech = ch
else:
chi = to_identity_map(ch)
ech = from_identity_map(chi + i)
c += ech
return c
with open('output.txt', 'w') as f:
f.write('Make sure you wrap the decrypted text with the HTB flag format :-]\n')
f.write(encrypt(FLAG))
It seems like the source code is some kind of caesar cipher. As it turns out it is trithemius cipher, which is a variant of the caesar cipher. The trithemius cipher is a substitution cipher where each letter is shifted by a fixed number of positions. The shift is not constant, but depends on the position of the letter in the plaintext. The shift is calculated using a function that takes the position of the letter as input.
We can write a small script to decrypt the output:
from random import randint
output = "DJF_CTA_SWYH_NPDKK_MBZ_QPHTIGPMZY_KRZSQE?!_ZL_CN_PGLIMCU_YU_KJODME_RYGZXL"
def to_identity_map(a):
return ord(a) - 0x41
def from_identity_map(a):
return chr(a % 26 + 0x41)
def decrypt(c):
m = ''
for i in range(len(c)):
ch = c[i]
if not ch.isalpha():
m += ch
else:
chi = to_identity_map(ch)
m += from_identity_map(chi - i)
return m
print("HTB{" + decrypt(output) + "}")
Running this script gives us the flag.
HTB{DID_YOU_KNOW_ABOUT_THE_TRITHEMIUS_CIPHER?!_IT_IS_SIMILAR_TO_CAESAR_CIPHER}
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