HTB BabyEncryption writeup

chall.py

import string
from secret import MSG

def encryption(msg):
    ct = []
    for char in msg:
        ct.append((123 * char + 18) % 256)
    return bytes(ct)

ct = encryption(MSG)
f = open('./msg.enc','w')
f.write(ct.hex())
f.close()

msg.enc

6e0a9372ec49a3f6930ed8723f9df6f6720ed8d89dc4937222ec7214d89d1e0e352ce0aa6ec82bf622227bb70e7fb7352249b7d893c493d8539dec8fb7935d490e7f9d22ec89b7a322ec8fd80e7f8921

sol.py

from Crypto.Util.number import *

ct = '6e0a9372ec49a3f6930ed8723f9df6f6720ed8d89dc4937222ec7214d89d1e0e352ce0aa6ec82bf622227bb70e7fb7352249b7d893c493d8539dec8fb7935d490e7f9d22ec89b7a322ec8fd80e7f8921'

def decryption(ct):
    pt = []
    for char in ct:
        pt.append((char - 18) * inverse(123, 256) % 256)
    return bytes(pt)

print(decryption(bytes.fromhex(ct)))

주어진 코드를 보고 암호문을 대상으로 거꾸로 실행하면 된다.

 

output

b'Th3 nucl34r w1ll 4rr1v3 0n fr1d4y.\nHTB{l00k_47_y0u_r3v3rs1ng_3qu4710n5_c0ngr475}'

HTB{l00k_47_y0u_r3v3rs1ng_3qu4710n5_c0ngr475}