#IrisCTF2024
from json import JSONDecodeError, loads, dumps
from binascii import hexlify, unhexlify
from Crypto.Cipher import AES
from Crypto.Random import get_random_bytes
with open("flag") as f:
flag = f.readline()
key = get_random_bytes(16)
def encrypt(plaintext: bytes) -> (bytes, bytes):
iv = get_random_bytes(16)
aes = AES.new(key, AES.MODE_CBC, iv)
print("IV:", hexlify(iv).decode())
return iv, aes.encrypt(plaintext)
def decrypt(ciphertext: bytes, iv: bytes) -> bytes:
aes = AES.new(key, AES.MODE_CBC, iv)
return aes.decrypt(ciphertext)
def create_command(message: str) -> (str, str):
payload = {"from": "guest", "act": "echo", "msg": message}
payload = dumps(payload).encode()
while len(payload) % 16 != 0:
payload += b'\x00'
iv, payload = encrypt(payload)
return hexlify(iv).decode('utf-8'), hexlify(payload).decode('utf-8')
def run_command(iv: bytes, command: str):
try:
iv = unhexlify(iv)
command = unhexlify(command)
command = decrypt(command, iv)
while command.endswith(b'\x00') and len(command) > 0:
command = command[:-1]
except:
print("Failed to decrypt")
return
try:
command = command.decode()
command = loads(command)
except UnicodeDecodeError:
print(f"Failed to decode UTF-8: {hexlify(command).decode('UTF-8')}")
return
except JSONDecodeError:
print(f"Failed to decode JSON: {command}")
return
match command["act"]:
case "echo":
msg = command['msg']
print(f"You received the following message: {msg}")
case "flag":
if command["from"] == "admin":
print(f"Congratulations! The flag is: {flag}")
else:
print("You don't have permissions to perform this action")
case action:
print(f"Invalid action {action}")
def show_prompt():
print("-" * 75)
print("1. Create command")
print("2. Run command")
print("3. Exit")
print("-" * 75)
try:
sel = input("> ")
sel = int(sel)
match sel:
case 1:
msg = input("Please enter your message: ")
iv, command = create_command(msg)
print(f"IV: {iv}")
print(f"Command: {command}")
case 2:
iv = input("IV: ")
command = input("Command: ")
run_command(iv, command)
case 3:
exit(0)
case _:
print("Invalid selection")
return
except ValueError:
print("Invalid selection")
except:
print("Unknown error")
if __name__ == "__main__":
while True:
show_prompt()
