chal.py

from Crypto.Cipher import DES
from Crypto.Util.Padding import pad, unpad
from hashlib import md5

IV = b'ABCDEFGH'
KEY = b'DES KEY!'

HASHSEP = b'=++!MD5!++='
SECRET  = b'SECRET123'

def encode(s: bytes) -> bytes:
    cipher = DES.new(KEY, DES.MODE_CBC, IV)
    payload = s + HASHSEP + md5(s).hexdigest().encode()
    return cipher.encrypt(pad(payload, 8, style='pkcs7'))

token = encode(SECRET)
print(f'token=' + token.hex())

cipher = DES.new(KEY, DES.MODE_CBC, IV)
token_dec = unpad(cipher.decrypt(token), 8, style='pkcs7')
print(f'+ {token_dec=}')

def decode(s: bytes) -> bool:
    cipher = DES.new(KEY, DES.MODE_CBC, IV)
    try:
        dec = cipher.decrypt(s)
    except Exception as e:
        print(f'+ exception: {e}')
        return False
    print(f'+ {dec.hex()=}')
    try:
        up = unpad(dec, 8, style='pkcs7')
    except Exception as e:
        print(f'+ exception: {e}')
        return False
    if HASHSEP not in up:
        print(f'+ no hashsep found')
        return False
    message, message_hash = up.split(HASHSEP, 1)
    expected = md5(message).hexdigest().encode()
    print(f'+ {message=} {message_hash=} {expected=}')
    if expected != message_hash:
        print(f'+ md5 mismatch')
        return False
    return True 

while True:
    s = input('> ').strip()
    if s == SECRET.decode():
        print('You Win!')
        break
    try:
        d = bytes.fromhex(s)
    except Exception as e:
        print(e)
    else:
        print(decode(d))