Demystifying OTPs: the logic behind the offline generation of tokens

Hello there! Another evening, on my way back home, I decided to check the mailbox. I don&rsquo;t mean my email inbox, but the old-school actual box where the postman puts the physical letters. And to my great surprise, I found an envelope there with something inside! While opening it, I spent a few moments hoping that it&rsquo;s the decades delayed letter from Hogwarts. But then I had to get back…