ctf2019_cofee_break.py

import sys
from Crypto.Cipher import AES
import base64

def decrypt(key, et):
    text = ''
    for i in range(len(et)):
        for t in range(0x7e):
            if (chr((((t - 0x20) + (ord(key[i % len(key)]) - 0x20)) % (0x7e - 0x20 + 1)) + 0x20) == et[i]):
                text += chr(t)
    return text


def solve(cipher_text):
    encode_key = "SECCON"
    aes_symmetric_key = "seccon2019"

    cipher = AES.new(aes_symmetric_key + chr(0x00) *
                     (16 - (len(aes_symmetric_key) % 16)), AES.MODE_ECB)
    b64_decode = base64.b64decode(cipher_text)
    et = cipher.decrypt(b64_decode)
    print(decrypt(encode_key, et.decode('ascii')))


text = sys.argv[1]
solve(text)