crackable

Deskripsi

Author: rui

did you know that md5 is not supposed to be used? yep cause it's crackable

nc ctf.ukmpcc.org 42069

Kita diberikan source code dari server. Intinya server akan meminta dua buah string (hex) yang hasil MD5 nya sama sehingga membuktikan adanya collision pada MD5.

from Crypto.Util.number import *

flag = "YOKOSO_KIRAKIRA_DOKIDOKI_MOCHIMOCHI_PUYOPUYO_WAKUWAKU_WASHOI"
N = 0x11b4c225f4dc385553faef71fd12d7a7d3731eebb47d01df2bd9c06cc95d67d933a3867dc3ef17547ae5a969dbc985489a3e835ddb0f8e1eb82b2cb84a5b168f74a808a0b7accbc1513cd416a5e8a4055a2823e192bdbe5da3583b11b0a1697a5a47

def able(s):
    a = inverse(e, N)
    b = pow(a, 2, N)
    return (pow(e, s, N) + pow(a, s, N) + pow(b, s, N)) % N

def solv(nbit):
    pbit = nbit // 3
    p, q, r = [getPrime(pbit) for _ in range(3)]
    return p*q*r, able((q * r) % (1 << pbit))

m = bytes_to_long(flag.encode())
e = 65537
n, u = solv(1536)
c = pow(m, e, n)

print(f"{n = }\n{u = }\n{c = }")

Contoh collision MD5 paling sederhana ada di https://www.mscs.dal.ca/~selinger/md5collision/. Berikutnya tinggal submit saja ke server dan flag didapatkan.

Flag: TCF2024{HALOOooOO👋_I'm_Emu🤩_Otori🤗_Emu😍_is_meaning😋_smile😁_wonderhoyyyy✨🎉🧨🎁🎢}