ctf-writeup/BxMCTF 2023/MCV5U

MCV5U

Ryzon, who just finished his ICS5U class, forgets that he also needs to finish his final MCV5U assignment, which is due on the same day!

Unfortunately, all of Ryzon's brain cells are destroyed due to how scuffed ICS5U is, and he begs you, his friend, to help him finish this assignment for him.

The flag format for this problem is different from the rest, it is: ctf{[!-z]{6,128}} (the part inside the bracket can be any string of 6-128 printable ASCII characters, including letters, numbers, and symbols).

About the Challenge

We have been given a zip file that contains 3 files: Document 1.txt, Document 2.txt, and main.py. Here is the content of main.py

import math
import hashlib
import sys

SIZE = int(3e5)
VERIFY_KEY = "46e1b8845b40bc9d977b8932580ae44c"


def getSequence(A, B, n, m):

    ans = [0] * (n + m - 1)

    for x in range(n):
        for y in range(m):
            ans[x + y] += A[x] * B[y]

    return ans


A = [0] * SIZE
B = [0] * SIZE

document1 = open("Document 1.txt", "r")
nums1 = document1.readlines()

idx = 0

for num in nums1:
    A[idx] = int(num.strip())
    idx += 1

document2 = open("Document 2.txt", "r")
nums2 = document2.readlines()

idx = 0

for num in nums2:
    B[idx] = int(num.strip())
    idx += 1

sequence = getSequence(A, B, SIZE, SIZE)
val = 0

for num in sequence:
    val = (val + num)

val = str(val)
val_md5 = hashlib.md5(val.encode()).hexdigest()

if val_md5 != VERIFY_KEY:
    print("Wrong solution.")
    sys.exit(1)

key = str(hashlib.sha256(val.encode()).digest())
flag = "ctf{" + "".join(list([x for x in key if x.isalpha() or x.isnumeric()])) + "}"

print(flag)

This code calculates a sequence of numbers by performing element-wise multiplication between two lists of numbers read from Document 1.txt and Document 2.txt. It then calculates the sum of the resulting sequence, computes the MD5 hash of the sum, verifies it against a predefined key, and if it matches, generates a flag by taking the alphanumeric characters from the SHA256 hash of the sum.

The purpose of this challs is we need to recode the program using different approaches

How to Solve?

But in my case, I decided to run the program without doing any recode. Of course to obtain the flag, I need to wait 3-4 hours XD

ctf{bx8b2xdcx80x8bxafx90x16x0fxc9Cx87x99Gx8cx1dxb9x8exb4xfaLx93xcfx9dxcfyx13xb5Lxee}