01

Write-ups for TCTT2023/Programming/01

Flag pattern

CTT23{xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx}

Challenge Files

Decryptmessage.zip

Solution

1. After reading the encrypt.py file, we can see the flag is encrypted with Fernet and the key will be stored in the key.txt file, which we can see that there are many keys stored.

from cryptography.fernet import Fernet

fm = open("flag.txt", "r")
fs = open("secret.txt", "wb")
fk = open("key.txt", "wb")

message = fm.read()

# สร้างคีย์สำหรับการเข้ารหัส
key = Fernet.generate_key()
fk.write(key)

cipher_suite = Fernet(key)

# เข้ารหัสข้อความ
encrypted_message = cipher_suite.encrypt(message.encode())
fs.write(encrypted_message)

fm.close()
fs.close()
fk.close()

2. Therefore, we write a python script and read the key.txt file and take the keys one by one and decrypt them with secret in secret.txt by using Fernet algorithm.

from cryptography.fernet import Fernet

fs = open("secret.txt", "r")
fk = open("key.txt", "r")

keys = fk.read().split()
encrypted_data = fs.read()

for key in keys:
    try:
        cipher = Fernet(key)
        decrypted_data = cipher.decrypt(encrypted_data)
        print(decrypted_data.decode('utf-8'))
    except:
        pass

fs.close()
fk.close()

3. Run the python script.

python3 decrypt.py

4. After run the script, the result is CTT23{d9746e1b06807f6ed9d1ae666efaf7f7}