# Pwnable.kr crypto1 writeup
ihciah@gmail.com

In this challenge we can input username and password, then the server return an encrypted string of `{username}-{password}-{cookie}` in which the `cookie` is not known.

Since `CBC` is used, each 32 word in encrypted string is encrypted by last block and 16 word in original string.

So let's enter `"A"*16` as username, and enter different password, the first 32 word of the encrypted data is the same.

So we can brute-force the cookie through trying each bit of cookie.

Case A:
`Username = "-"*13 + x`
`Password = ""`
So `String = "-"*15 + x`

Case B:
`Username = "-"*13`
`Password = ""`
So `String = "-"*15 + cookie`

We can change `x` and compare first, second, third... 32 bit.

Write a simple script to exploit it:
```
you_will_never_guess_this_sugar_honey_salt_cookie
```

Calculate `PW` through `hashlib.sha256("admin"+"you_will_never_guess_this_sugar_honey_salt_cookie").hexdigest()`

Input username `admin` and password to get the flag.