Banksy's anonymity makes it hard to authenticate his pieces and prints, so Banksy has created a nonprofit called "Pest Control" that issues certificates of authenticity: you send them an alleged Banksy print and £65 and if they agree that it's authentic, they'll return it with a certificate that has a torn-in-half "Di-faced" fake banknote with Lady Diana's face on it, with a handwritten ID number across the bill.
As Clinton Freeman points out, this is a great piece of cryptographic engineering: faking a Banksy cert involves matching the tear precisely, and also requires that a would-be counterfeiter know what was written on the other half of the note, which is stored at Pest Control and is not made available.
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.
How Banksy Authenticates His Work
(via Evil Mad Scientist Labs)