Language: Python

Source code:

#!/usr/bin/env python3 # Generate serial for name 'SPb_CTF_2017' def check(name, serial): name = bytes(name, 'utf-8') if len(name) != 12: return False valid = [ int.from_bytes(name[:4], 'big'), int.from_bytes(name[4:8], 'big'), int.from_bytes(name[8:], 'big') ] valid[0] ^= valid[2] valid[2] ^= valid[0] valid[0] ^= valid[2] return serial == '{:08x}-{:08x}-{:08x}'.format(*valid) name = input('What is your name?\n> ') serial = input('... and your serial?\n> ') print('Your serial is {}'.format('valid!' if check(name, serial) else 'invalid...'))

Flag is the serial for name SPb_CTF_2017

How to run Python3:
1. Download Python 3.6.0 from python.org
2. Run in console: python task.py