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.