Finally, you encountered a hard reversing challenge that is 100% Open Source (OSREV). There is not even a binary for you to reverse.

However, the number of licensed copies is limited. We need to ship our top secret build key along with each copy. Plus recipients have to sign a non disclosure agreement before they can access the build key. The fewer people we give it to, the more secure the program is!

It looks like you have an unlicensed copy (no build key). Yikes!
得到的flag使用NSSCTF{}格式提交。