subtitle_decryptor.py

from Crypto import Random
from Crypto.Cipher import AES
import base64
from hashlib import md5

def pad(data):
    length = 16 - (len(data) % 16)
    return data + (chr(length)*length).encode()

def unpad(data):
    return data[:-(data[-1] if type(data[-1]) == int else ord(data[-1]))]

def bytes_to_key(data, salt, output=48):
    assert len(salt) == 8, len(salt)
    data += salt
    key = md5(data).digest()
    final_key = key
    while len(final_key) < output:
        key = md5(key + data).digest()
        final_key += key
    return final_key[:output]

def encrypt(message, passphrase):
    salt = Random.new().read(8)
    key_iv = bytes_to_key(passphrase, salt, 32+16)
    key = key_iv[:32]
    iv = key_iv[32:]
    aes = AES.new(key, AES.MODE_CBC, iv)
    return base64.b64encode(b"Salted__" + salt + aes.encrypt(pad(message)))

def decrypt(encrypted:bytes, passphrase:bytes):
    encrypted = base64.b64decode(encrypted)
    encrypted = pad(encrypted)
    assert encrypted[0:8] == b"Salted__"
    salt = encrypted[8:16]
    key_iv = bytes_to_key(passphrase, salt, 32+16)
    key = key_iv[:32]
    iv = key_iv[32:]
    aes = AES.new(key, AES.MODE_CBC, iv)
    return unpad(aes.decrypt(encrypted[16:]))

def decrypt_subtitle(encryptedSubtitle:str):
    encryptedSubtitle = bytes(encryptedSubtitle, 'utf-8')
    key = encryptedSubtitle[0:8][::-1]
    data = encryptedSubtitle[8:-5]
    encrypted = data
    return decrypt(encrypted, key)