#bsidesnoidactf
from Crypto.Cipher import AES
import os
with open('flag.txt') as f:
FLAG = f.read()
menu = r"""
/===== MENU =====\
| |
| [M] MAC Gen |
| [A] AUTH |
| |
\================/
"""
def MAC(data, check=False):
assert len(data) % 16 == 0, "Invalid Input"
if check:
assert data != secret_msg, "Not Allowed!!!"
cipher = AES.new(key, AES.MODE_CBC, iv)
tag = cipher.encrypt(data)[-16:]
return tag.hex()
def AUTH(tag):
if tag == secret_tag:
print("[-] Successfully Verified!\n[-] Details:", FLAG)
else:
print("[-] Verification Flaied !!!")
if __name__ == "__main__":
iv = os.urandom(16)
key = os.urandom(16)
secret_msg = os.urandom(48)
secret_tag = MAC(secret_msg)
print(f"[+] Forbidden msg: {secret_msg.hex()}")
try:
for _ in range(3):
print(menu)
ch = input("[?] Choice: ").strip().upper()
if ch == 'M':
data = input("[+] Enter plaintext(hex): ").strip()
tag = MAC(bytes.fromhex(data), check=True)
print("[-] Generated tag:", tag)
print("[-] iv:", iv.hex())
elif ch == 'A':
tag = input("[+] Enter your tag to verify: ").strip()
AUTH(tag)
else:
print("[!] Invalid Choice")
exit()
except Exception as e:
print(":( Oops!", e)
print("Terminating Session!")
MAC CBCモード
from ptrlib import Socket, xor
sock = Socket("nc 34.121.95.29 1338")
msg = bytes.fromhex(sock.recvlineafter("msg: ").decode())
sock.sendlineafter("Choice: ", "M")
sock.sendlineafter("plaintext(hex): ", msg[:32].hex())
c2 = bytes.fromhex(sock.recvlineafter("tag: ").decode())
iv = bytes.fromhex(sock.recvlineafter("iv: ").decode())
payload = xor(xor(iv, c2), msg[32:])
sock.sendlineafter("Choice: ", "M")
sock.sendlineafter("plaintext(hex): ", payload.hex())
tag = bytes.fromhex(sock.recvlineafter("tag: ").decode())
sock.sendlineafter("Choice: ", "A")
sock.sendlineafter("verify: ", tag.hex())
sock.interactive()
