Rating:

The file app.py contains an implementation of Diffie-Hellman (DHx class), with fingerprinting too.
Assuming Alice, Bob and Carol have private keys a, b and c respectively, the following desrbies the key-exchange scheme:
1. Alice sends g^a (mod p) to Bob.
2. Bob raises by b, generating g^ab (mod p) and sends that to Carol.
3. Carol receives, raises by b and keeps that as the secret: g^abc (mod p).

If we denote this chain as A --> B --> C then similar chains happen to get everyone synced to the same secet: B --> C --> A and C --> A --> B.

As the name suggests, the attacker is a MiTM (man-in-the-middle) and can interfere with all comms, but there is a catch: after all of the exchanges, everyone compares their secrets (as fingerprinting), and Alice will only send the encrypted flag if this check passes.
This is kind of equivalent to the QR code option inside WhatsApp to ensure key exchange hasn't been tampered.

So, as an attacker, I didn't find any way to break the d-log problem, but I do note that the key exchange has a weekness, as the receiving party still trusts the sending party to follow the "correct" key exchange schema.
For example, if as an attacker I send to Carol some number x instead of g^ab (mod p), then Carol blindly trusts it and calculates the secret x^c (mod p). There is a limitation, however, as the code checks that the input is stricly larger than 1 and strictly smaller than p.

Therefore, I have decided to supply x = -1 = p-1 (mod p). Note that raising -1 to any power results in either 1 or -1 (which is again, p-1). So, supplying Carol, for instacne, with p-1 results in her saving the joint key as either 1 or p-1, randomly.
This needs to be done to each party, and each of them "randomly" generates either 1 or p-1.
What are the chances of all parties to agree on a key? Well, there are 2^3=8 possibilities and only in 2 of them there is an agreement, so the chances are 2/8 = 0.25%, which is pretty good odds.

After a successful run, we only need to guess the shared secret out of 2 possibilities (1 or p-1), which is easy to do.

The solution is therefore:
python
from pwn import *

import hashlib
from Crypto.Cipher import AES
from Crypto.Util.number import long_to_bytes

minus_one = p-1

output = ''
while True:

print('Starting attempt...')
conn = remote('crypto1.q21.ctfsecurinets.com', 1337)

for i in range(3):
conn.recvline()
conn.recvuntil(': ')
conn.sendline(str(minus_one))
conn.recvline()
conn.recvuntil(': ')
conn.sendline(str(minus_one))

conn.recvline().decode('ascii') # "Alice says"
output = conn.recvline().decode('ascii')
if 'ABORT MISSION' in output:
print('Attempt failed, rerying...')
continue
print('Success %s' % (output,))
break

crypt_bytes = bytes.fromhex(output.strip())
iv = crypt_bytes[:16]
encrypted = crypt_bytes[16:]

# Try key=p-1
key = hashlib.sha1(long_to_bytes(minus_one)).digest()[:16]
print(AES.new(key, AES.MODE_CBC, iv).decrypt(encrypted))

# Try key=1
key = hashlib.sha1(long_to_bytes(1)).digest()[:16]
print(AES.new(key, AES.MODE_CBC, iv).decrypt(encrypted))


Original writeup (https://thegoonies.github.io/2021/03/21/securinetctf-2021-mitm/).