The binary is similar to that of Purchases.
However, there is no `flag` function and we have to get the shell by ourselves.

[writeup](https://ptr-yudai.hatenablog.com/entry/2019/04/25/141422#Binary-160pts-Returns)

Original writeup (https://ptr-yudai.hatenablog.com/entry/2019/04/25/141422#Binary-160pts-Returns).