project sekai 2023

Project Sekai 2023/Cryptography/Noisy CRC/chall.py

@@ -0,0 +1,45 @@
+import secrets
+from Crypto.Util.number import *
+from Crypto.Cipher import AES
+from hashlib import sha256
+
+from flag import FLAG
+
+def getCRC16(msg, gen_poly):
+	assert (1 << 16) <= gen_poly < (1 << 17)  # check if deg = 16
+	msglen = msg.bit_length()
+
+	msg <<= 16
+	for i in range(msglen - 1, -1, -1):
+		if (msg >> (i + 16)) & 1:
+			msg ^= (gen_poly << i)
+
+	return msg
+
+def oracle(secret, gen_poly):
+	res = [secrets.randbits(16) for _ in range(3)] 
+	res[secrets.randbelow(3)] = getCRC16(secret, gen_poly)
+	return res
+
+
+def main():
+	key = secrets.randbits(512)
+	cipher = AES.new(sha256(long_to_bytes(key)).digest()[:16], AES.MODE_CTR, nonce=b"12345678")
+	enc_flag = cipher.encrypt(FLAG)
+	print(f"Encrypted flag: {enc_flag.hex()}")
+
+	used = set({})
+
+	while True:
+		gen_poly = int(input("Give me your generator polynomial: "))
+		assert (1 << 16) <= gen_poly < (1 << 17)  # check if deg = 16
+
+		if gen_poly in used:
+			print("No cheating")
+			exit(1)
+
+		used.add(gen_poly)
+
+		print(oracle(key, gen_poly))
+
+main()