Part of the appeal of the artist Banksy is his/her/their anonymity. Despite continuing announcements of Banksy's identity being revealed, no one is reallysure who Banksy is (and to further add to the confusion, a well-known time traveler recently hinted that she was - or maybe wasn't - Banksy). Regardless,collectors of art by an anonymous creator face a difficult problem of authenticating any individual artwork. For a public piece of graffiti, Banksy's officialwebsite will confirm or deny whether it was painted by the artist.

But how do you know that the print or painting you're buying is the real deal? Gallery authenticity certificates can be as readily forged as paintings. Soin 2009, Banksy launched Pest Control, a not-for-profit company to sell and authenticate artworks. Basically, you submit images of the print or artwork, and ifit is deemed authentic, for £65, you receive a certificate of authenticity, which includes half of a "Di faced tenner", a torn in half fake £10 notecreated by Banksy with Lady Diana's face on it (an example can be seen here). The half-note has a handwritten ID number on it which can be matched to the otherhalf of the note held by Pest Control.

This offers a fairly clever authentication system:

Even if you could somehow duplicate the exact tear in the middle of the note, you still don't have the private key held by Pest Control. When a work issold, one submits a change of ownership request which requires an image of the authentication certificate. Further, Pest Control then contacts the person theyhave listed as the current owner of the work for confirmation.

It's a smart piece of cryptographic engineering, although as one commenter points out, not all that original. Chirographs have been used to determineauthenticity dating back to Medieval times. Sometimes, old ideas (even really old ideas) still have value.

To read the complete article, see:
Art Authentication: Banksy's Chirograph System(https://scholarlykitchen.sspnet.org/2019/04/05/art-authentication-banksys-chirograph-system/)

Chirograph (https://en.wikipedia.org/wiki/Chirograph)
Indenture (https://www.dictionary.com/browse/indentured)

To read the earlier E-Sylum articles, see:
BRITISH MUSEUM ACQUIRES BANKSY £10 NOTE (https://www.coinbooks.org/v22/esylum_v22n05a33.html)
IN LETTERS WE TRUST: BANKNOTE TYPOGRAPHY(https://www.coinbooks.org/v22/esylum_v22n08a15.html)
NEWMAN HISTORIC AMERICAN FISCAL PAPER : Lot 89390: New-York 6% Six Month Loan Certificate(https://www.coinbooks.org/v20/esylum_v20n17a24.html)