HackTheBox Gonna Lift Em All Writeup

Explore the basics of cybersecurity in the Gonna Lift Em All Challenge on Hack The Box. This very-easy-level Challenge introduces encryption reversal and file handling concepts in a clear and accessible way, perfect for beginners.

Provided Output

output.txt

p = 163924920994230253637901818188432016168244271739612329857589126113342762280179217681751572174802922903476854156324228497960403054780444742311082033470378692771947296079573091561798164949003989592245623978327019668789826246878280613414312438425787726549209707561194579292492350868953301012702750092281807657719
g = 97407673851268146184804267386115296213106535602908738837573109808033224187746927894605766365039669844761355888387043653015559933298433068597707383843814893442087063136640943475006105673619942401850890433169719970841218851182254280222787630139143746993351533776324254770080289574521452767936507196421481076841
h = 7771801879117000288817915415260102060832587957130098985489551063161695391373720317596178655146834967333192201720460001561670355858493084613455139466487717364432242890680666229302181326080340061384604634749443972114930849979067572441792867514664636574923631540074373758015873624100768698622048136552173788916
(c1, c2) = (83194887666722435308945316429939841668109985194860518882743309895332330525232854733374220834562004665371728589040849388337869965962272329974327341953512030547150987478914221697662859702721549751949905379177524490596978865458493461926865553151329446008396048857775620413257603550197735539508582063967332954541, 46980139827823872709797876525359718565495105542826335055296195898993549717497706297570900140303523646691120660896057591142474133027314700072754720423416473219145616105901315902667461002549138134613137623172629251106773324834864521095329972962212429468236356687505826351839310216384806147074454773818037349470)

Provided Script

chal.py

from Crypto.Util.number import bytes_to_long, getPrime
import random

FLAG = b'HTB{???????????????????????????????????????????????????}'

def gen_params():
    p = getPrime(1024)
    g = random.randint(2, p - 2)
    x = random.randint(2, p - 2)
    h = pow(g, x, p)
    return (p, g, h), x

def encrypt(pubkey):
    p, g, h = pubkey
    m = bytes_to_long(FLAG)
    y = random.randint(2, p - 2)
    s = pow(h, y, p)
    return (g * y % p, m * s % p)

def main():
    pubkey, _ = gen_params()
    c1, c2 = encrypt(pubkey)
    with open('out.txt', 'w') as f:
        f.write(
            f'p = {pubkey[0]}\ng = {pubkey[1]}\nh = {pubkey[2]}\n(c1, c2) = ({c1}, {c2})\n'
        )

if __name__ == "__main__":
    main()

Proof of Concept (PoC)

from Crypto.Util.number import long_to_bytes, inverse

p = 163924920994230253637901818188432016168244271739612329857589126113342762280179217681751572174802922903476854156324228497960403054780444742311082033470378692771947296079573091561798164949003989592245623978327019668789826246878280613414312438425787726549209707561194579292492350868953301012702750092281807657719
g = 97407673851268146184804267386115296213106535602908738837573109808033224187746927894605766365039669844761355888387043653015559933298433068597707383843814893442087063136640943475006105673619942401850890433169719970841218851182254280222787630139143746993351533776324254770080289574521452767936507196421481076841
h = 7771801879117000288817915415260102060832587957130098985489551063161695391373720317596178655146834967333192201720460001561670355858493084613455139466487717364432242890680666229302181326080340061384604634749443972114930849979067572441792867514664636574923631540074373758015873624100768698622048136552173788916
c1 = 83194887666722435308945316429939841668109985194860518882743309895332330525232854733374220834562004665371728589040849388337869965962272329974327341953512030547150987478914221697662859702721549751949905379177524490596978865458493461926865553151329446008396048857775620413257603550197735539508582063967332954541
c2 = 46980139827823872709797876525359718565495105542826335055296195898993549717497706297570900140303523646691120660896057591142474133027314700072754720423416473219145616105901315902667461002549138134613137623172629251106773324834864521095329972962212429468236356687505826351839310216384806147074454773818037349470

g_inv = inverse(g, p)
y = (c1 * g_inv) % p
s = pow(h, y, p)
s_inv = inverse(s, p)
m = (c2 * s_inv) % p
flag = long_to_bytes(m)
print(flag.decode())

Summary

Gonna Lift Em All on Hack The Box is a very easy cryptography challenge centered around reversing an ElGamal encryption process. Participants are provided with encryption parameters and ciphertext, and the task involves deriving the plaintext by reversing modular arithmetic operations. Using Python’s Crypto.Util.number library, the challenge demonstrates concepts like modular inverses and exponentiation, making it a beginner-friendly introduction to cryptographic analysis and decryption techniques.