INS'HACK CTF 2018 - Web Crypt0r part 2 - Extend the hash - RushB%

Readme.md

1. Guessed integrity check : user : hash : b64_encoded_info in cookie user

2. info in YAML format

3. Some YAML parsers allow to redefine the same key, that replaces the old value

4. We have a hint : /src-code/ contains a swp file, original file was removed. It's easy to extract from it the src code of the web app. Of course it's a double-key permissive YAML parser

5. A bad salt algorithm is used : sha256(passkey + salt + info)

6. Salt length : passkey.len == 20 and salt.len == 10, we could consider a single salt of 30 bytes and extend the hash

7. Pwned :>

web-crypto2.py

import base64
import subprocess
import sys

cookie = sys.argv[1].split(':')
user = cookie[0]
base_sign = cookie[1]
info = base64.decodestring(cookie[2])

to_append = "\ninsa_coins: 600\n"

file = open('base_info.txt', 'w')
file.write(info)
file.close()

result = subprocess.check_output([
    './hash_extender',
    '--file=base_info.txt',
    '-s', base_sign, '-f', 'sha256', '-l', '30',
    '-a', to_append,
    '--out-data-format', 'raw',
    '--out-signature-format', 'hex',
    '-q'
])

new_hash = result[:64]
new_user_infos = result[64:]

new_cookie = user + ':' + new_hash + ':' + base64.b64encode(new_user_infos)

print("New cookie is : " + new_cookie)