This challenge is an almost-exact copycat of VolgaCTF Quals 2015 – CPKC.
You can find an excellent writeup by team More Smoked Leet Chicken here: [http://mslc.ctf.su/wp/volgactf-quals-2015-cpkc-crypto-400-writeup/](http://mslc.ctf.su/wp/volgactf-quals-2015-cpkc-crypto-400-writeup/)
The same code can be used to solve this chall.
[solve.sage](https://github.com/CTF-STeam/ctf-writeups/blob/master/2020/ISITDTU%20Quals/Encrypt/solve.sage) [task3_sol.py](https://github.com/CTF-STeam/ctf-writeups/blob/master/2020/ISITDTU%20Quals/Encrypt/task3_sol_final.py)
Flag: `ISITDTU{NTRU_3ncrypti0n_alg0rithm_3asy!!!!!!!!}`