HCMUS-CTF 2023

Hi, I’m Jinn As a reverser of the team, I try my best on 2 challenge: Is that crypto? and Go Mal!. These 2 challenges are fantastic and so cool. Btw, I also solve some other easy problems like Japanese, python is safe?, grind.Therefore, I will provide detailed explanations for 2 reverse challenges and briefly mention the remaining ones.

Rev + Crypto: Is this crypto? #

Challenge file: main

IDA .i64 file (renamed function, variable): main.i64

This challenge give me binary written in C++, but it seem clearly and without any strip, here is inside of main function:

As we can see, it take 2 input as name and favorite_word, check them before encrypt flag.txt.

If name and favorite_word are true, they will calculate key and IV from them like this:

key = SHA256(name)
IV = MD5(favorite_word)

They used these agrument for enc function, we easily confirm that function just a normal AES mode CBC.

So, in order to find name and favorite_word, we need to look deep into function check():

Btw, to identify some function like: base64, hash function, or something. We can use yara-findcrypt (an IDA plugin) to easily detect them.

We can see there are 2 array v8 and v9. Each array contain total 28 bytes and it compare with return value of function sus(name) or sus(word).

Look into that funtion, Im sure that is SHA224 (some sha224 constant is here) so, I have an idea:

Take v8 and v9 convert they to hex and using crack station to break these hash

import hashlib
v8 = [0]*7
v8[0] = "ACB7842B"
v8[1] = "5DFEBCAF"
v8[2] = "33801F1C"
v8[3] = "4F3FB333"
v8[4] = "A8F98777"
v8[5] = "CE40F926"
v8[6] = "EC339422"
v8 = [bytes.fromhex(i)[::-1] for i in v8] #little endian
print(b''.join(v8).hex())
#output: 2b84b7acafbcfe5d1c1f803333b33f4f7787f9a826f940ce229433ec

But there is no result:

I confuse in a time, I debug this file and type ‘a’ as name and check output of this hash, here is what I got:

print('sha224 name:', hashlib.sha224(b'a').hexdigest())
#output: "sha224 name: abd37534c7d9a2efb9465de931cd7055ffdb8879563ae98078d6d6d5"

and I saw a hash while debugging:

Look different, but when I compare this sha224 funtion and sha256 (sha224 differs from sha256 only in blocksize and constant) funtion given I see this:

Right side is sha256 function, and it has switch endian before return and sus function don’t. So we just need to change the endian of v8 and v9:

import hashlib
v8 = [0]*7
v8[0] = "ACB7842B"
v8[1] = "5DFEBCAF"
v8[2] = "33801F1C"
v8[3] = "4F3FB333"
v8[4] = "A8F98777"
v8[5] = "CE40F926"
v8[6] = "EC339422"
v8 = [bytes.fromhex(i) for i in v8] #big endian
print(b''.join(v8).hex())
#output: acb7842b5dfebcaf33801f1c4f3fb333a8f98777ce40f926ec339422

And we got name:

similar to favorite_word:

Then we have name and favorite_word, we easily decrypt the flag:

key = hashlib.sha256(b'recis').digest()
iv = hashlib.md5(b'cannibalization').digest()

aes = AES.new(key,AES.MODE_CBC,iv)
with open('flag.txt.enc','rb') as f:
    d = f.read()
    print(aes.decrypt(d))
#output: b'HCMUS-CTF{r_u_ready_for_fREddy?}\x10\x10\...'

Full script here: solve.py

Flag: b'HCMUS-CTF{r_u_ready_for_fREddy?}

Rev: Go Mal! #

Challenge file: server

IDA .i64 file (added breakpoint): server.i64

This program written in Golang, so it little hard, as a first time I see it, I’m clueless about this main funtion:V

After trying to run thi file, I see something:

Server have 2 while loop to listen something from client. We can see that server receive a hash and print out ~60 hash.

So, we start from string and find “flag”, I see it:

Reference to this string, I found the compare code:

and the main key:

Here is the code we focused:

To debug this file, we need to attach in stead of direct debug because golang has built-in anti-debug (perhaps). So we need to run ./server, using IDA attach option, add breakpoint and debug them.

I debugged many time and I got something, let me explain:

This function receive a hash and get the current time:

After that, in a while loop, rely on the timestamp, it calculate something and save in int64buf.array

They using 8 bytes of int64buf.array to calculate the hmac hash (with the main key given)

Then, this hash converted to hex and compare with our input. If they are same, it will print flag.

So, the problem is how it int64.array was caculated? In the time I debug this file, I see this array in this format:

byte1 + byte2 + b"Vd" + b"\x00"*4

b"Vd" may different in this time, you need to debug and get another one

So, we just need to find 2 bytes, and it must be same with one of 60 hash given.

I used this script to testing something:

sus = """70f181860de673250e2548df1fe51014473af59022866ea9c9db8bbdbd963798"""
main_key = b'Bj7tSK6L4E8tmVebTzH0O0ylb1dTcdpahryyGi2of3q3TLXJxeNYdeUFveFehbOWqrjFQAxV4EF9Rb4c'
for i in range(255):
    for j in range(255):
        array = bytes([100,75]) +b'Vd' + b'\x00'*4
        signature = hmac.new(main_key,msg=array,digestmod=hashlib.sha256).digest()
        if signature.hex() in sus:
            print(i,j,bytes([i,j]))
            break
#output: 45 75 b'-K'

with sus is one of hash I got from 60 hash server print out.

As we can see, it’s just 2 number and after I try with 60 hash in a time I see j permanent and i is 60 consecutive numbers.

So, to get flag, we need to choose new_j near and greater than j (example: 76 maybe) and i is random in range [0,255]

array = bytes([100,76]) +b'Vd' + b'\x00'*4
signature = hmac.new(main_key,msg=array,digestmod=hashlib.sha256).digest()
while True:
    r = remote("go-mal.chall.ctf.blackpinker.com", 443, ssl=True)
    r.send(signature.hex().encode())
    rec = r.recv(1024)
    if b"{" in rec:
        print(rec)
        break
    # r.interactive()
    r.close()

Just wait several minute, and I get flag:

Flag: HCMUS-CTF{1_us3_t1mest4Mp_W1tH_k3y_T0_4UTHENT1c4t3d_dATA}

PWN: Python is safe? #

Code:

#!/usr/bin/env python3

from ctypes import CDLL, c_buffer
libc = CDLL('/lib/x86_64-linux-gnu/libc.so.6')
buf1 = c_buffer(512)
buf2 = c_buffer(512)
libc.gets(buf1)
if b'HCMUS-CTF' in bytes(buf2):
    print(open('./flag.txt', 'r').read())

It’s just bufferover flow (buf1) by using gets().

Calc Payload:

python3 -c 'print("a"*612 + "HCMUS-CTF")'

Grind #

This one little guessing, in the first time, as descsription given, I try using this query:

select DISTINCT uid,name from ranking where (uid like "23______" or uid Like "24______" )and points > 900000000 and points < 1000000000
INTERSECT
select DISTINCT uid,name from 'data-64-final'.ranking where (uid like "23______" or uid Like "24______" ) and rank  > 5000 
and name REGEXP '^[a-zA-Z0-9]+$'
ORDER BY name ASC

23 or 24 mean uid right before 2019

Nothing interested

So, at the last time, I see my query has wrong point condition, so I changed this query like this:

select * from "data-64-final".ranking as df where df.uid in
(select day2.uid from 'data-64-day2'.ranking as day2, ranking as day3 where (day3.uid like "23______" or day3.uid Like "24______" )and (day3.points - day2.points between 900000000 and 1000000000) and day2.uid = day3.uid
INTERSECT
select fn.uid from 'data-64-final'.ranking as fn where (fn.uid like "23______" or fn.uid Like "24______" ) and rank  > 5000)

And here is result:

As we can see that user name ζ(2) is sussy and this is: Zeta function

Flag:HCMUS-CTF{23983477-1.6449340668-2391789368-9614}