Rating:

## TL;DR
1. Get the keystream where no permutation is applied.
2. Get the keystream where the only permutation is shift left in last round.
3. Xor those keystream to eliminate the addition of key at last step.
4. Calculate the inverse to recover the key.

Original writeup (https://sasdf.cf/ctf-tasks-writeup/writeup/2018/Hack.lu/crypto/escape/).