# AIS3 Pre-exam/MFC 2023 Write up
> [name= WHY]

## Misc
+ Welcome
打開welcome.pdf檔案，將各頁的英數符號寫下，即為flag。
+ Robot
1. 於terminal 輸入`nc chals1.ais3.org 12348`
2. 回答每一題的答案
3. 最後得到flag

## Crypto
+ Fernet
起初看到題目進行了encrypt，
我運用python進行decrypt，即獲得flag。
程式碼如下。

import os
import base64
from cryptography.fernet import Fernet
from Crypto.Hash import SHA256
from Crypto.Protocol.KDF import PBKDF2
from secret import *

def encrypt(plaintext, password):
    salt = os.urandom(16)  
    key = PBKDF2(password.encode(), salt, 32, count=1000, hmac_hash_module=SHA256)  
    f = Fernet(base64.urlsafe_b64encode(key))  
    ciphertext = f.encrypt(plaintext.encode())  
    return base64.b64encode(salt + ciphertext).decode()

def decrypt(ciphertext, password):
    ciphertext = base64.b64decode(ciphertext)
    salt = ciphertext[:16]
    ciphertext = ciphertext[16:]
    key = PBKDF2(password.encode(), salt, 32, count=1000, hmac_hash_module=SHA256)
    f = Fernet(base64.urlsafe_b64encode(key))
    plaintext = f.decrypt(ciphertext)
    return plaintext.decode()

# 題目txt file中提供的資訊
leak_password = 'mysecretpassword'
ciphertext = "iAkZMT9sfXIjD3yIpw0ldGdBQUFBQUJrVzAwb0pUTUdFbzJYeU0tTGQ4OUUzQXZhaU9HMmlOaC1PcnFqRUIzX0xtZXg0MTh1TXFNYjBLXzVBOVA3a0FaenZqOU1sNGhBcHR3Z21RTTdmN1dQUkcxZ1JaOGZLQ0E0WmVMSjZQTXN3Z252VWRtdXlaVW1fZ0pzV0xsaUM5VjR1ZHdj"

plaintext = decrypt(ciphertext, leak_password)
print("Decrypted data:", plaintext)

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###### tags: `Side-Project`