When street artist Banksy first started selling screen prints online, they became a popular target for forgers looking to make a few bucks duping unsuspecting fans. It quickly got out of hand and must have been infuriating for both Banksy and his followers.

Normally this is where galleries begin to flex their authority and declare which prints are authentic and which are fake. Galleries sometimes print certificates to reassure customers that the large lumps of cash they are parting with is for the real thing, not one of those dirty fakes doing the rounds. I don’t have large lumps of cash, so I’ve never seen one, but I imagine they say something like this:

The enclosed screen print is totally by Banksy. By authority of some gallery corporation.

But in Banky’s case, the whole issue of authenticity was compounded by his anonymity, which of course is part of his appeal. But it does make establishing the provenance of many prints difficult at best. Plus different galleries would have created their own certificates and competed to be the ultimate Banksy authority. I don’t think it would have taken long before the forgers simply moved onto faking the certificates themselves.

So Banksy created a not-for-profit company, Pest Control, to sell and authenticate his works. The process is fiendishly clever, as Will Ellsworth-Jones writes in his book ‘Banksy: The Man Behind the Wall’:

Now, for £65 you can get your Banksy print authenticated. And just to keep the whole thing as jokey as possible, the authentication certificate has stapled to it half a ‘Di faced tenner’, a £10 note faked by Banksy with Lady Diana’s face on it. The tenner has a handwritten ID number on it which can be matched to the number on the other half of the of the note held by Pest Control.

To save you a bit of googling, here is one of the certificates I found online:

Picture of a Banksy authentiation certificate from the not-for-profit company Pest Control.

Hell yeah! Someone with a bit of cryptography nous cooked up this flipping sweet authentication system. Let’s dig into it a bit:

It all starts off with a fairly bog standard gallery style certificate. Details of the work, the authenticating agency, a bit of embossing and a large impressive signature at the bottom. Exactly the sort of things that can be easily copied by someone on a mission to create the perfect fake.

That torn-in-half banknote though? Never mind signatures, embossing or wax seals. The Di Faced Tenner is doing all the authentication heavy lifting here.

The tear is what uniquely separates the private key, the half of the note kept secret under lock and key at Pest Control, with the public key. The public key is the half of the note attached to the authentication certificate which gets passed on with the print, and allows its authenticity to be easily verified.

We have no idea what has been written on Pest Control’s private half of the note. Which means it can’t be easily recreated, and that empowers Pest Control to keep the authoritative list of who currently owns each authenticated Banksy work.

It’s not perfect, but I don’t know how somebody could recreate the exact same tear in the note. Certainly not by hand, but maybe with a laser cutter or something? But it wouldn’t change the private key held by Pest Control.

Let’s say someone did manage to recreate a convincing copy of a print, certificate and the public half of the matching Di Faced Tenner. And you want to purchase this thing that looks like an authenticated print, so you head over to Pest Control’s website and make a change of ownership request. They contact who they have on file as the current owner of the work: ‘Hey, are you going to give your print to Joe Bloggs?’. Naturally the owner, unaware of the forgery and has no intention of selling their current print is going to be confused and reply ‘WTF are you talking about?

Can an information system be art? Because, like I said, it’s flipping sweet, and all executed in Banksy’s trademark tongue in cheek style. This whole authentication process would easily be my favourite artwork by Banksy.

P.S. Will Ellsworth-Jones’ book ‘Bansky: The Man Behind the Wall’ is an alright read. I give it three out of five stars.