Private Set Intersection
Find what two parties have in common — without either side revealing the rest of their data.
Private Set Intersection (PSI) is a cryptographic protocol that lets two or more parties compute the intersection of their private sets — which records they both have — without either party learning anything about the elements that aren’t in the intersection.
It’s a narrower, faster relative of general secure multi-party computation, purpose-built for exactly one question — “what do we have in common?” — which makes it practical at the scale most cross-institution matching problems actually need: fraud lists, audience overlap, contact discovery, and shared watchlists.
How It Works
Private Set Intersection in four steps
Encode Sets
Each party cryptographically encodes its own set locally, without exposing raw values.
Exchange
Encoded sets are exchanged through a cryptographic protocol that leaks nothing about non-matching elements.
Compute Intersection
The intersection is computed blindly — neither party can reconstruct the other’s full set from the exchange.
Reveal Only Matches
Only the matching elements (or a count) are revealed — everything else stays private to its original owner.
Real-World Applications
Where PSI shows up in production
Cross-Institution Fraud Matching
Banks or telcos check whether a customer appears on another institution’s fraud list without exposing full customer databases to each other.
Ad Measurement & Audience Overlap
Measure campaign overlap between an advertiser and a publisher’s user base without either side sharing raw user lists.
Private Contact Discovery
Apps find which of your contacts are already users without ever uploading your address book in the clear.
Vendor & Sanctions Screening
Check whether a vendor appears on multiple sanctions or watchlists without pooling private vendor databases.