Private Set Intersection (PSI)
What is Private Set Intersection (PSI)?
Private Set Intersection (PSI)A cryptographic protocol that lets two or more parties compute the intersection of their private sets while learning nothing about the elements that are not in common.
Private Set Intersection (PSI) is a specialized MPC protocol in which two or more parties hold private sets and wish to learn only their intersection — or only its size, or a function over it — without revealing the rest of their data. Modern PSI protocols use Oblivious Pseudorandom Functions, vectorized OPRF, or Diffie-Hellman based variants to scale to billions of elements with practical bandwidth. Applications include private contact discovery in messengers (Signal, WhatsApp), measurement of advertising conversions without sharing user identifiers (Google's Private Join and Compute, Apple's PCM), and threat-intelligence sharing where each party reveals only the IOCs they truly have in common.
● Examples
- 01
Signal discovering which of your phone contacts already use Signal without uploading your address book.
- 02
Two banks computing the count of customers they share without revealing their full customer lists.
● Frequently asked questions
What is Private Set Intersection (PSI)?
A cryptographic protocol that lets two or more parties compute the intersection of their private sets while learning nothing about the elements that are not in common. It belongs to the Cryptography category of cybersecurity.
What does Private Set Intersection (PSI) mean?
A cryptographic protocol that lets two or more parties compute the intersection of their private sets while learning nothing about the elements that are not in common.
How does Private Set Intersection (PSI) work?
Private Set Intersection (PSI) is a specialized MPC protocol in which two or more parties hold private sets and wish to learn only their intersection — or only its size, or a function over it — without revealing the rest of their data. Modern PSI protocols use Oblivious Pseudorandom Functions, vectorized OPRF, or Diffie-Hellman based variants to scale to billions of elements with practical bandwidth. Applications include private contact discovery in messengers (Signal, WhatsApp), measurement of advertising conversions without sharing user identifiers (Google's Private Join and Compute, Apple's PCM), and threat-intelligence sharing where each party reveals only the IOCs they truly have in common.
How do you defend against Private Set Intersection (PSI)?
Defences for Private Set Intersection (PSI) typically combine technical controls and operational practices, as detailed in the full definition above.
What are other names for Private Set Intersection (PSI)?
Common alternative names include: PSI, Set intersection protocol.
● Related terms
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Secure Multi-Party Computation (MPC)
A family of cryptographic protocols that lets several parties jointly compute a function over their private inputs while revealing nothing beyond the output.
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Homomorphic Encryption
An encryption scheme that allows computations to be performed directly on ciphertexts, producing encrypted results that match the operations on the underlying plaintexts.
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Zero-Knowledge Proof (ZKP)
A cryptographic protocol in which a prover convinces a verifier that a statement is true without revealing anything beyond the validity of the statement itself.
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Federated Learning
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Threshold Cryptography
A class of cryptographic schemes in which a secret key is split across n parties so that any t of them — but no smaller subset — can sign, decrypt, or perform any other key operation.
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Cryptography
The science of securing information through mathematical techniques that provide confidentiality, integrity, authenticity, and non-repudiation in the presence of adversaries.