Ever wondered what happens when you insert a bill into a vending machine? [Janne] is back with his latest project: reverse engineering a banknote validator. Curious about how these common devices work, he searched for information but found few resources explaining their operation.
To learn more, [Janne] explored the security features that protect banknotes from counterfeiting. These can include microprinting, UV and IR inks, holograms, color-shifting coatings, watermarks, magnetic stripes, and specialty paper. These features not only deter fraud but also enable validators to quickly verify a bill’s authenticity.
[Janne] purchased several banknote validators to disassemble and compare. Despite varied exteriors, their core mechanisms were similar: systems to move the bill smoothly, a tape head to detect magnetic ink or security strips, and optical sensors to inspect visible, UV, and IR features. By reverse engineering the firmware of two devices, he uncovered their inner workings. There is a calibration procedure they use to normalize their readings, then it will analyze a bill through a sophisticated signal processing pipeline. If the data falls within a narrow acceptance range, the device authenticates the bill; otherwise, it rejects it.
Head over to his site to check out all the details he discovered while exploring these devices, as well as exploring the other cool projects he’s worked on in the past. Reverse engineering offers a unique window into technology Check out other projects we’ve featured showcasing this skill.
Seeing how Passports are made is also interesting.
https://youtu.be/bnKyw5-_E6o
I’ll post this here because it’s something I witnessed but haven’t been able to explain. This was 10 or 15 years ago, the company I worked at had just moved into a newly constructed headquarters where everything was new – including the printers and vending machines. I guess it was a snow day because the company owner brought his middle school aged daughter and a friend to work that day. I was the company geek so they’d come by my desk looking for stuff to play with. Everytime they got bored they’d come back by so eventually I thought of something to keep them busy enough so I could work – I gave them some small bills and introduced them to the company print room with a 20ft long industrial copier that dld everything a print store could offer. I knew this obviously wouldn’t work. The copier would get partially through the print then detect it was being used to copy a real US bill and abort the process. Every time they had an issue they’d come back by and I’d give them some possible solutions- simple things like folding the bills, flipping the paper, making copies of one portion at a time, etc. Eventually they surprisingly had some properly aligned, full size front and back bills but they were upset because they said the vending machine wouldn’t take them. I knew this would be the case so I wasn’t bothered by their attempts but I still needed something to send them away for a while on other task so I recommended wadding the fake bills up in a ball then feeding them to the machine (I’d noticed years before, like many other people, that crisp new bills are often rejected). About 15 minutes later I heard the girls running up the hall giggling then they came in my office and dropped about a dozen bags of Doritos on my desk, shouting “It worked!”. I was baffled but also horrified by the prospect of losing my job, so I confiscated the evidence and fake bills and swore them to secrecy.
I still have no idea how that actually worked on a modern vending machine. Any ideas?
I suspect that the bill sensors in some vending machines are not that discriminate unlike change machines. Hopefully you shredded the evidence. As I recall high end copiers embed in the picture a traceable serial or ID number.
There are still stories of dumb people making their own money. A person tried to pass a homemade 100K bill at at Walmart. They passed “GO” and went straight to jail. I decided to try to get change for a fake $20 at a small town gas station in the South….the old man at the counter gave me 2 Three’s and Fourteen…😁
Copies & printers add “yellow dots” to your printout, recording model, serial number, date, time etc. They’ve been doing that for a long time.
Money has little rings, usually yellow or green known as the ” EURion constellation” that copiers and some vending machines detect. These have also been around for a long times, at least 25 years.
What if you made a business card or something that had all the same security features as a bank note, and fools these validators, but on outward appearance does not look like or try to pass itself off as money at all? It’s obviously illegal to try to spend it as money, but would it be illegal to make?
Judge: If you’re not trying to counterfeit money, why does it have all the same features?
Me: I don’t want people making counterfeits of my business card.
I know, I know, I’ll see myself off to prison.
If identities can be stolen.
(How come all the comments look like they were AI generated)
Anyway.. that would be completely legal, but also would not work since size and spacing would be part of the security system and would prevent fooling the system surely.
But if the mechanism ignored size and spacing then it would be all legal since the law has specific wording.
Not that you can’t be completely legal and blameless andsoforth and be sure to avoid jail that way, there is too much evidence that that is not the case..
If you’re working on the same note acceptor shown in the image (looks like a CoinCo), I know of some companies that would be very grateful for updated bank note definitions for these. Currently, the manufacturer sends an flimsy encrypted programmer with a limited number of writes and charges $10 per upgrade. Definitely a pain point when you have an install base of thousands.
On the flip side, you can usually get these dirt cheap because they’re older and most operators switched to GBA validators. GBA released (for free) a set of images you can load on to a USB drive (and plug in to the validator) to teach the validator about new notes in each denomination.
As a hint, validators also do length/edge detection with visible light. Caused headaches for validator vendors when Australia released bank notes with a (mostly) clear plastic middle.