A while ago, Google created a two-factor authentication scheme called U2F. The general idea is as follows. You have a little USB dongle that you can register with sites. When you enable U2F on a site, the dongle gives that site a public key and a key handle, and that dongle is the only device that can use that key handle to sign things with the corresponding public key. So from then on, websites can hand over the key handle and a challenge, and if they get back a valid signature they know that whoever’s logging in has the dongle you used to sign up.
A few days ago, someone released a U2F implementation based on a Teensy microcontroller and it made Hackaday – and they added a disclaimer warning that doing anything related to crypto by yourself is a bad idea and that using XOR encryption for the key handle is insecure. There was a bit of a pushback against this in the comments. After all, so reasoned the Hackday commenters, security isn’t that hard and XOR isn’t a big deal, right? Surely this warning is just insulting, gratuitous negativity from the Hackaday editors.
Spoiler alert: XOR is in fact horribly insecure in this application, and this code is broken and should not be used by anyone.