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Buckeye CTF 2024 reduce_recycle write-up

Mariusz Bartosik
dog with tools

In this CTF challenge, man’s best friend becomes a CTF player’s best asset! Tasked with breaking into an encrypted archive hiding a valuable flag, we’ll see how a dog, armed with some clever forensics skills, can fetch us just the right tool for the job.

Category: Forensics

Points: 269

Solves: 38 out of 648 teams

Author: corgo

Challenge:

I forgot the randomly generated 12-character password I used to encrypt these files…. is there anything you can do to help me get my flag back??

dogs_wearing_tools.zip important_flags.7z

The challenge provides two password-protected archives:

  1. dogs_wearing_tools.zip containing 4 .png files.
  2. important_flags.7z containing flag.txt.

Both archives are encrypted with a 12-character random password. A brute-force approach is impractical due to the password’s length, and common wordlists do not contain the password.

Step 1: Cracking the ZIP File

To solve this, we can leverage the fact that the ZIP archive (dogs_wearing_tools.zip) uses the ZipCrypto encryption, which is weak. Additionally, since the .png files are stored uncompressed, we know they start with a predictable PNG header.

Analyzing the ZIP Structure

Using bkcrack, we can view the contents of dogs_wearing_tools.zip:

bkcrack -L dogs_wearing_tools.zip
bkcrack 1.7.0 - 2024-05-26
Archive: dogs_wearing_tools.zip
Index Encryption Compression CRC32    Uncompressed  Packed size Name
----- ---------- ----------- -------- ------------ ------------ -----
    0 ZipCrypto  Store       346673b4      1817550      1817562 1.png
    1 ZipCrypto  Store       0fe18ee0      1830967      1830979 2.png
    2 ZipCrypto  Store       9c62018f        94416        94428 3.png
    3 ZipCrypto  Store       ba690f9b      1210542      1210554 4.png

Leveraging Known Plaintext

To break ZipCrypto, bkcrack requires 12 bytes of known plaintext. PNG files start with the following hex header:

89 50 4E 47 0D 0A 1A 0A 00 00 00 0D 49 48 44 52 (‰PNG   IHDR)

Using this information, we can attempt to recover the decryption keys with:

bkcrack -C dogs_wearing_tools.zip -c 3.png -x 0 89504E470D0A1A0A0000000D49484452
bkcrack 1.7.0 - 2024-05-26
[20:03:10] Z reduction using 9 bytes of known plaintext
100.0 % (9 / 9)
[20:03:10] Attack on 744092 Z values at index 6
Keys: adf73413 6f6130e7 0cfbc537
69.1 % (514274 / 744092)
Found a solution. Stopping.
You may resume the attack with the option: --continue-attack 514274
[20:08:32] Keys
adf73413 6f6130e7 0cfbc537

This gives us the keys:

Keys: adf73413 6f6130e7 0cfbc537

Decrypting the ZIP Archive

Now, we can decrypt the archive and save it as an unencrypted version:

bkcrack -C dogs_wearing_tools.zip -k adf73413 6f6130e7 0cfbc537 -D dogs_with_no_password.zip
bkcrack 1.7.0 - 2024-05-26
[20:10:38] Writing decrypted archive dogs_with_no_password.zip
100.0 % (4 / 4)

The decrypted ZIP file, dogs_with_no_password.zip, contains images of cute dogs. While they didn’t fetch the flag for us, they certainly brought some tools to help us tackle the CTF challenge!

Step 2: Recovering the Password

We still need the password for important_flags.7z. Using bkcrack again with the recovered keys, we can brute-force the original password.

bkcrack -k adf73413 6f6130e7 0cfbc537 --bruteforce ?p --length 12
bkcrack 1.7.0 - 2024-05-26
[20:17:14] Recovering password
length 12...
Password: 2n3Ad3&ZxDvV
19.9 % (1796 / 9025)
Found a solution. Stopping.
You may resume the password recovery with the option: --continue-recovery 327555202020
[20:17:23] Password
as bytes: 32 6e 33 41 64 33 26 5a 78 44 76 56
as text: 2n3Ad3&ZxDvV

After a short process, the password is revealed as:

Password: 2n3Ad3&ZxDvV

Step 3: Extracting the Flag

Now that we have the password, we can extract flag.txt from important_flags.7z. The contents of flag.txt reveal the flag:

bctf{wH1ch_d0g_w4s_youR_FaVOr1t3}

Note: you may want to check the official solution as it takes slighty different approach to finding the password.

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