In plain words
Reading out a twenty-character Wi-Fi password to a guest guarantees three failed attempts. The QR code contains the network name and the password: the phone's camera reads it and immediately offers to connect. The calculation happens in your browser, the password is not sent anywhere.
How to use it
- Enter the exact network name, respecting upper and lower case, and its password.
- Check the QR code with your own phone before putting it on display.
- Print the card and leave it in the hallway, or show the image on a tablet.
Phones read this format natively: the iPhone camera since iOS 11, the camera or Wi-Fi settings on Android.
What the QR code contains
The QR code contains text in the WIFI: format recognised by every phone, like this:
WIFI:T:WPA;S:Home-Guests;P:password;;
T is the security type, S the network name, P the password, and H:true flags a hidden network. The characters \ ; , : " are escaped automatically.
A QR code is a password in clear text
Anyone can read the QR code with their phone and extract the password. Only display it for a guest network, isolated from your computers and your NAS. If the password changes, make a new card.
To create that guest network and isolate it, see Understanding VLANs. For a password that is both strong and readable on the card, use the Password generator in passphrase mode.