Tool
The Last Slide
A QR code, a contact card, and scan statistics nobody else sees: why I built qr4prez.
The talk ends. A few people walk up to the stage, phones in hand. Pleasantries are exchanged, then comes the awkward part: spelling out an email address over the crowd noise, hunting for a LinkedIn profile among a dozen namesakes, handing over a business card that will end its life at the bottom of a jacket pocket. I lived that scene after every talk for years.
qr4prez was born from that scene. It is a small web tool: you fill in a form once and get a QR code to put on your last slide. The audience scans it with the phone camera and your contact card lands straight in their address book, photo, title and links included. As a bonus, a private page counts the scans: how many, when, and from which city.
The tool lives at deraison.ai/qr4prez. It is free, bilingual, and there is no account to create.
Motivation
The paper business card has two flaws. The first is well known: it gets lost. The second bothered me more: it tells you nothing. Once handed out, there is no way to know whether it was ever used, whether someone dug it up three weeks later, or whether it survived the washing machine.
The digital answer has existed for a long time: the vCard, a standardized text file every phone knows how to read. It holds the name, phone number, email, employer, even the photo. The QR code is the bridge: an image the camera turns into a link, and that link opens the vCard. Neither brick is new; what was missing was an assembly I actually wanted to use.
I wanted three things. A single gesture for the audience: no app to install, nothing to fill in on their side, just the camera. Feedback for me: a way to know whether that last slide actually earns its place. And control over the data: everything hosted on my own server, with no third-party service collecting my audience's contact details, no cookies, no tracker.
How it works
It starts with a form: name, job title, email, phone, a square photo. To make the wait less administrative, the empty fields suggest a different personality each time: Marie Curie, Grace Hopper, Nina Simone, Alan Turing… These suggestions stay dimmed examples; they are never submitted on your behalf.
The form: five required fields, one square photo, and a suggested personality to set an example.
The photo taught me the project's humility lesson. Some iOS versions silently drop the entire photo from a vCard when the JPEG contains a technical comment inserted by the server's image library. No error message: the contact arrives, faceless. So the photo is decoded, resized to 400 × 400, re-encoded cleanly, and the offending comment is stripped byte by byte from the final file. Nobody will ever see that work, which is exactly the point.
Each card then receives an address that cannot be guessed. The idea rests on a hash function, a mathematical grinder that turns any text into a fixed-size fingerprint, so unpredictable that changing one letter of the text scrambles the whole fingerprint. qr4prez hashes the first name, last name and submission instant with SHA-256, then keeps the first 16 hexadecimal characters of the fingerprint, which is 64 bits. The number of possible addresses reads:
Eighteen billion billion. The risk worth checking carries a lovely name, the birthday paradox: in a group, two people share a birthday far more often than intuition suggests, and likewise two cards could land on the same address. The probability of a collision among \(n\) cards is bounded by:
With one million cards, that is about \(2.7 \times 10^{-8}\): three chances in a hundred million. The address also carries no personal information: neither name nor date appears in it, only the fingerprint survives.
On the audience side, the gesture stays minimal. The phone scans the QR, opens the card's address, the server serves the vCard and then records the visit: country, approximate city (IP-based geolocation, accurate to the city center at best, never GPS), device type, operating system. The statistics page, visible only to the card's owner, shows a world map of scans, four counters, and the detail of every visit.
This one works: scan it and you land on qr4prez.
Two sobriety details matter to me. Scans arriving within a minute of creation are ignored: that is you checking your own QR, not a new contact. And IPv6 addresses are truncated on display, because a statistics page does not need to expose a full address to say that a scan came from Lyon.
Conclusion
My talks now end with a QR code. The after-talk line still forms, thankfully: it is where the substance gets discussed, objections raised, ideas traded. The exchange of contact details, meanwhile, takes two seconds and no longer mangles a single email address.
If you give talks, lectures or defenses, try it: deraison.ai/qr4prez. Fill in the form once, put the QR on your last slide, and watch, afterwards, the scans draw the map of your audience.