Rating:

It uses the CBC-MAC protocol.
We can cause a collision by repeating the message blocks and changing the IV.

[writeup](https://ptr-yudai.hatenablog.com/entry/2019/04/25/141422#Crypto-220pts-Mac-Forgery)

Original writeup (https://ptr-yudai.hatenablog.com/entry/2019/04/25/141422#Crypto-220pts-Mac-Forgery).