[HV22.18] Santa’s Nice List

December 18, 2022 1 minute read

Santa stored this years “Nice List” in an encrypted zip archive. His mind occupied with christmas madness made him forget the password. Luckily one of the elves wrote down the SHA-1 hash of the password Santa used.

xxxxxx69792b677e3e4c7a6d78545c205c4e5e26

Can you help Santa access the list and make those kids happy?

Solution

So, we got a zip file nice-list.zip which is password protected. We don’t know a plaintext in it, so plaintext attacks are not feasible. Since the zip is AES encrypted (file returns nice-list.zip: Zip archive data, at least v5.1 to extract, compression method=AES Encrypted), other attacks I know are also not suited. Let’s do some research.

While researching, I stumbled over this interesting article called An encrypted ZIP file can have two correct passwords — here’s why which uses the same preconditions and I noticed that the known bytes of the SHA-1 were all ASCII Printable.

The remainder was quite straightforward. I generated a wordlist of all the possible combinations (3 printable characters + known bytes in ascii) with this (very ugly) script:

import string

printables = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ!"#$%&\'()*+,-./:;<=>?@[\\]^_`{|}~ '

hashes = []
for i in printables:
    for j in printables:
        for k in printables:
            hashes.append(
                f"{i.encode('utf-8').hex()}{j.encode('utf-8').hex()}{k.encode('utf-8').hex()}69792b677e3e4c7a6d78545c205c4e5e26")

print(len(printables))
print(len(hashes))

file = open('./wordlist.txt', 'w')
for hash in hashes:
    file.writelines(bytes.fromhex(hash).decode('UTF-8') + "\n")

The script took about 5 seconds to create a wordlist with ~857k entries.

I then used john to crack the password. Extract the hash with zip2john nice-list.zip > nice-list.hash and crack it with john --wordlist ./wordlist.txt nice-list.hash. On my azure VM running Kali, this took a second and gave me the correct password 4Ltiy+g~>LzmxT\ \N^&. I used that with 7z e nice-list.zip (unzip doesn’t support the zip file version) to get the flag HV22{HAVING_FUN_WITH_CHOSEN_PREFIX_PBKDF2_HMAC_COLLISIONS_nzvwuj}

Daniel Henschel

Solution