In the first view, we think we should hack the python random.randint function (It's possible!), but this challenge is a lot easier! in provided file, we see the service will set the python random seed in start. That's it, we have the exact time, we can set our seed equal to server seed and generate exact random number. This is possible with a simple bruteforce!

Original writeup (https://github.com/RaaCT0R/CTF-Challenges/blob/master/cryptography/nactf2020/random-number-generator/WRITEUP.md).