Hybrid

Platform: hackerlab | Category: Crypto | Type: Challenge | Difficulty: Medium | OS: Linux | Author: XESXOR | Date: 2026-04-06 | Status: Solved Techniques: zipcrypto_known_plaintext_attack, dictionary_password_cracking, hybrid_wordlist_attack, crc_based_password_verification, content_format_guessing

Summary

Task: Two password-protected ZIP archives using ZipCrypto Store encryption, each containing a part of the flag. Solution: Crack the second archive via dictionary attack (qwerty123), then use bkcrack known-plaintext attack on the first archive by guessing the content prefix from the second file's format.

Hybrid — Hackerlab

Description

На жёстком диске из коробки с барахолки вы нашли два зашифрованных архива. Вы открываете терминал и наливаете себе кофе. Похоже, ночь будет интересной.

English summary: On a hard drive from a flea market box, you found two encrypted archives. You open the terminal and pour yourself some coffee. Looks like tonight is going to be interesting.

Download: hybrid.zip containing:

• first.zip — password-protected, contains first.txt (32 bytes, ZipCrypto Store)
• second.zip — password-protected, contains second.txt (29 bytes, ZipCrypto Store) ‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

Analysis

Initial reconnaissance

Extracting hybrid.zip reveals two nested password-protected ZIP archives. Examining them with 7z l -slt:

first.zip → first.txt (32 bytes, Method: ZipCrypto Store)
second.zip → second.txt (29 bytes, Method: ZipCrypto Store)

Key observations:

1. ZipCrypto (traditional PKWARE encryption) — known to be weak, vulnerable to known-plaintext attacks
2. Store method (no compression) — critical for bkcrack, since the plaintext is stored as-is inside the encrypted stream
3. Small file sizes — 32 and 29 bytes respectively, suggesting short text content (likely flag parts) ‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

Attack strategy

The task name "Hybrid" hints at needing multiple attack approaches:

• Dictionary/hybrid attack for one archive (common password)
• Known-plaintext attack (bkcrack) for the other (strong password, but predictable content)

Solution

Step 1: Crack second.zip — Dictionary attack

Tried a hybrid wordlist approach combining dictionary words with common number suffixes. Used Python's zipfile module for accurate CRC-based verification (important because fcrackzip with ZipCrypto produces many false positives due to the 1-byte check): ‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

 python read-only • copy allowed Copy
#!/usr/bin/env python3
import zipfile

wordlist_path = "rockyou.txt"  # or custom hybrid wordlist
archive_path = "second.zip"

zf = zipfile.ZipFile(archive_path)
with open(wordlist_path, "r", encoding="latin-1") as f:
    for line in f:
        password = line.strip()
        try:
            zf.extractall(pwd=password.encode())
            print(f"[+] Password found: {password}")
            break
        except (RuntimeError, zipfile.BadZipFile):
            continue

Result: Password is qwerty123 ‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

Extracted content of second.txt:

Second part: REDACTED}

(29 bytes including trailing newline)

Step 2: Crack first.zip — bkcrack known-plaintext attack

The password for first.zip was NOT in rockyou.txt (tested top 500k entries) and not found via hybrid dictionary attack. However, since both archives use ZipCrypto Store, they are vulnerable to bkcrack known-plaintext attack.

Guessing the plaintext

From second.txt's format ("Second part: ..."), we can infer that first.txt starts with "First part: " — that's 12 bytes, exactly the minimum required for bkcrack. ‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

Create the known-plaintext file:

 bash read-only • copy allowed Copy
printf "First part: " > kp1.txt

Running bkcrack

 bash read-only • copy allowed Copy
bkcrack -C first.zip -c first.txt -p kp1.txt

After approximately 15 minutes of computation, bkcrack recovered the internal ZipCrypto keys:

[+] Keys: d5a9be9a 8c4f9531 6d49916b

Step 3: Decrypt first.txt

Using the recovered keys to decrypt without needing the actual password:

 bash read-only • copy allowed Copy
bkcrack -C first.zip -c first.txt -k d5a9be9a 8c4f9531 6d49916b -d first_decrypted.txt

Content of first_decrypted.txt:

First part: CODEBY{REDACTED

(32 bytes including trailing newline) ‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

Step 4: Combine the flag

First part:  CODEBY{REDACTED
Second part: REDACTED}
Full flag:   CODEBY{REDACTED}

The flag in leetspeak reads: "ARE YOU SURE IT'S A STEGA?" — a humorous hint that despite appearances (encrypted archives, hidden content), this was a cryptography challenge, not steganography. ‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍