> For the complete documentation index, see [llms.txt](https://n2l-cysec.gitbook.io/notes/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://n2l-cysec.gitbook.io/notes/writeup-ctfs/2024/cr3-ctf-2024.md).
# CR3 CTF 2024
| Chall | Category | Total Solved |
| ------------------------------------------------------ | -------- | ------------ |
| [#getting-closer-dang](#getting-closer-dang "mention") | crypto | 46 Solved |
| [#donut](#donut "mention") | forensic | 36 Solved |
| jscripting | web | 13 Solves |
This only me + n2l team writeup ( because i also play in another team called Huntik ) . im not played in N2L so N2L got 119th place.
## getting-closer-dang
{% file src="/files/yjK02mYx7GjmabSeAeIE" %}
chall.py
```python
import os, json, random, math
from Crypto.Cipher import AES
from Crypto.Util.Padding import pad
from Crypto.Util.number import getPrime, GCD
from hashlib import sha256
#from secret import FLAG
def get_prod():
return math.prod([random.choice(pool) for _ in range(3)])
FLAG = b'cr3{???????????????????????????????}'
N = getPrime(512)
pool = [getPrime(9) for _ in range(10)]
a, b = [get_prod() for _ in range(2)]
g = GCD(N**a - 1, N**b - 1) # pros of having a quantum computer ^-^
key = sha256(str(g).encode()).digest()[:16]
iv = os.urandom(16)
cipher = AES.new(key, AES.MODE_CBC, iv)
ct = cipher.encrypt(pad(FLAG, 16))
out = {'iv': iv.hex(), 'ct': ct.hex()}
with open('output.txt', 'w') as f:
f.write(f'{N = }\n')
f.write(f'{out = }')
```
program enkripsi aes cbc sederhana di mana, pada variabel g terdapat penghitungan GCD antara (N^a)-1 dengan (N^b)-1 output.txt
```python
N = 13185232652915309492470700885494158416479364343127310426787872363041960044885500175769971795951432028221543122753022991035176378747918784016983155565886369
out = {'iv': '6f69ac380715dbf9b00ef32ca8c204bb', 'ct': 'e654237a76d61d3bf97b315af1c2a517797b34b8eca270dbc8132dda1f425065b7b84690d4c21cdaf2ab17c2876738ed'}
```
mekanisme enkripsi:
1. Generate N prime random sebesar 512 bit
2. Generate nilai array pool
3. Menghitung faktor secara random yang terdapat pada array pool dan di simpan pada variabel a dan b
4. Menghitung GCD(FPB) dari (N^a) - 1 dengan (N^b) - 1)
5. Menyimpan hasil perhitungan GCD kemudian dijadikan sebagai key untuk mengencrypt
mekanisme decrypt menurut gwhj: Merecover nilai a,b -> tidak mungkin karena tidak disediakan nilai g, karena hal tersebut saya menemukan opsi lain yakni penghitungan nilai g dari GCD akan sama dengan persamaan berikut (N^GCD(a,b))-1, karena hal tersebut sama maka tinggal bruteforce saja nilai GCD(a,b)
solver.py
```python
import os, json, random, math
from Crypto.Cipher import AES
from Crypto.Util.Padding import pad
from Crypto.Util.number import getPrime, GCD
from hashlib import sha256
import sys
sys.set_int_max_str_digits(0)
#from secret import FLAG
N = 13185232652915309492470700885494158416479364343127310426787872363041960044885500175769971795951432028221543122753022991035176378747918784016983155565886369
iv = b'oi\xac8\x07\x15\xdb\xf9\xb0\x0e\xf3,\xa8\xc2\x04\xbb'
ct = b'\xe6T#zv\xd6\x1d;\xf9{1Z\xf1\xc2\xa5\x17y{4\xb8\xec\xa2p\xdb\xc8\x13-\xda\x1fBPe\xb7\xb8F\x90\xd4\xc2\x1c\xda\xf2\xab\x17\xc2\x87g8\xed'
g = ((N**(439))-1)
#g = [((N**i)-1) for i in range(1000)]
key = sha256(str(g).encode()).digest()[:16]
#key = [sha256(str(g).encode()).digest()[:16] for g in g]
#iv = os.urandom(16)
cipher = AES.new(key, AES.MODE_CBC, iv)
#cipher = [AES.new(key, AES.MODE_CBC, iv) for key in key]
fl = cipher.decrypt(ct)
#fl = [i.decrypt(ct) for i in cipher]
#ct = cipher.encrypt(pad(FLAG, 16))
#out = {'iv': iv.hex(), 'ct': ct.hex()}
print(f"""
g = {g}
key = {key}
N = {N}
flag = {fl.decode('latin-1')}""")
```
by : gr3yr4t
## donut
{% file src="/files/LMHwf0yPXx1z6xFUEhB6" %}
chall source
{% endfile %}
after we extract it we can see there is a .git files and my terminal shows it git
we can see we in branch master
bet the git seems broken after i use command log or status
so i directly use grep or strings like this:
```bash
cd .git
grep -roa 'flag'
```
and we can see in the head we can strings to see the log of commit
```bash
grep -roa 'flag'
output :
index:flag
HEAD:flag
hooks/fsmonitor-watchman.sample:flag
logs/HEAD:flag
logs/HEAD:flag
logs/HEAD:flag
COMMIT_EDITMSG:flag
```
```
strings logs/HEAD
e1cf8e13bfe7f2b9522642a878368677621349b1 e1cf8e13bfe7f2b9522642a878368677621349b1 dxqwww 1713541575 +0300 checkout: moving from master to flag
e1cf8e13bfe7f2b9522642a878368677621349b1 2a461812df0b80851da28cf6e69f3a2d84129b01 dxqwww 1713541687 +0300 commit: added flag
2a461812df0b80851da28cf6e69f3a2d84129b01 e1cf8e13bfe7f2b9522642a878368677621349b1 dxqwww 1713541710 +0300 checkout: moving from flag to master
```
we can see the hash of the added flag we directly use git show
```bash
git show 2a461812df0b80851da28cf6e69f3a2d84129b01
```
cr3{w3\_4r3\_411\_10v3\_d0n7T5!1
## jscripting
{% file src="/files/9IRTLOtFlMrMD6COvCDe" %}
referr to this [Escaping nodejs vm](/notes/cheatsheet/web-exploitation/escaping-nodejs-vm.md#bypassing-force-return-and-proccess-null)
cr3{W0vv53Rs\_1n5q3c70R\_G4dg3T!!1}