  We present a cryptographic primitive called WibsonTree designed to preserve
users' privacy by allowing them to demonstrate predicates on their personal
attributes, without revealing the values of those attributes. We suppose that
there are three types of agents --buyers, sellers and notaries-- who interact
in a decentralized privacy-preserving data marketplace (dPDM) such as the
Wibson marketplace. We introduce the WibsonTree protocol as an efficient
cryptographic primitive that enables the exchange of private information while
preserving the seller's privacy. Using our primitive, a data seller can
efficiently prove that he/she belongs to the target audience of a buyer's data
request, without revealing any additional information.
