This is ‘server.py’
import hashlib
from typing import List
class CustomMAC:
def __init__(self):
self._internal_state = b""
def update(self, message: bytes) -> None:
if not self._internal_state:
self._internal_state = self.get_key() + message
else:
self._internal_state += message
def get_key(self) -> bytes:
return open("key.txt", "rb").read().strip()
def digest(self) -> bytes:
return hashlib.sha256(self._internal_state).digest()[:8]
class TokenManager:
def __init__(self):
self._mac = CustomMAC()
self._seen_tokens: List[bytes] = []
def verify_and_store(self, message: bytes, token: bytes) -> bool:
self._mac = CustomMAC()
self._mac.update(message)
expected_token = self._mac.digest()
if token != expected_token:
print(f"Invalid token! Expected token: {expected_token.hex()}")
return False
if token in self._seen_tokens:
print("Token already used!")
return False
self._seen_tokens.append(token)
return True
def main():
print("Welcome to the Token Verification Challenge!")
print("============================================")
print("Rules:")
print("1. Submit message-token pairs")
print("2. Each token must be valid for its message")
print("3. You cannot reuse tokens")
print("4. Get 64 valid tokens accepted to win!")
print("\nFormat: <hex-encoded-message> <hex-encoded-token>")
print("Example: 48656c6c6f 1234567890abcdef")
manager = TokenManager()
successes = 0
for i in range(128):
try:
print(f"\nAttempt {i+1}/128")
print("Enter your message and token: ", end='')
user_input = input().strip().split()
if len(user_input) != 2:
print("Invalid input format!")
continue
message = bytes.fromhex(user_input[0])
token = bytes.fromhex(user_input[1])
if manager.verify_and_store(message, token):
successes += 1
print(f"Success! {successes}/64 valid tokens verified")
if successes >= 64:
print("\nCongratulations! You beat the challenge!")
with open("flag.txt", "r") as f:
print(f.read().strip())
break
except Exception as e:
print(f"Error: {str(e)}")
continue
if successes < 64:
print("\nChallenge failed! Not enough valid tokens.")
if __name__ == "__main__":
main()
Since there is 128 attempt, we can success 64 times.
flag is: flag{C0ngr4ts_0n_f1nd1ng_Th1s_H4sh_c0ll1s10ns_N0w_G0_h4v3_4_D0ubl3_Ch33s3_Burg3r}