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| import os
from Crypto.Cipher import AES
from Crypto.Util.Padding import pad
flag = os.environ.get("FLAG", "ctf4b{dummy_flag}")
iv = os.urandom(16)
key = os.urandom(16)
challenge = os.urandom(16 * 6)
ENC_TICKET = 3
DEC_TICKET = 3
GOLDEN_TICKET = 0
def menu() -> int:
print("Your tickets:")
if ENC_TICKET > 0:
print(f"{ENC_TICKET} encryption ticket(s)")
if DEC_TICKET > 0:
print(f"{DEC_TICKET} decryption ticket(s)")
if GOLDEN_TICKET > 0:
print(f"{GOLDEN_TICKET} golden ticket(s)")
print()
print(f"1. Encrypt")
print(f"2. Decrypt")
print(f"3. Get ticket")
print(f"4. Get flag")
print(f"5. Quit")
while True:
i = int(input("> "))
if 1 <= i <= 5:
return i
print("Invalid input!")
def consume_ticket(enc: int = 0, dec: int = 0, golden: int = 0):
global ENC_TICKET, DEC_TICKET, GOLDEN_TICKET
if ENC_TICKET < enc or DEC_TICKET < dec or GOLDEN_TICKET < golden:
print("Not enough tickets.")
exit(1)
ENC_TICKET -= enc
DEC_TICKET -= dec
GOLDEN_TICKET -= golden
while True:
i = menu()
if i == 1:
consume_ticket(enc=1)
pt = bytes.fromhex(input("pt> "))
if len(pt) > 16:
print("Input must not be longer than 16 bytes.")
continue
cipher = AES.new(key, AES.MODE_CBC, iv=iv)
print(f"ct:", cipher.encrypt(pad(pt, 16)).hex())
if i == 2:
consume_ticket(dec=1)
ct = bytes.fromhex(input("ct> "))
if len(ct) > 16:
print("Input must not be longer than 16 bytes.")
continue
cipher = AES.new(key, AES.MODE_CBC, iv=iv)
print("pt:", cipher.decrypt(pad(ct, 16)).hex())
if i == 3:
print("challenge:", challenge.hex())
answer = bytes.fromhex(input("answer> "))
if len(answer) != len(challenge) + 16:
print("Wrong length.")
continue
cipher = AES.new(key, AES.MODE_CBC, iv=answer[:16])
if cipher.decrypt(answer[16:]) == challenge:
print("Correct!")
GOLDEN_TICKET += 1337
else:
print("Wrong :(")
if i == 4:
consume_ticket(golden=1)
print("flag:", flag)
if i == 5:
print("Bye!")
exit(0)
|