Private set intersection (PSI)

Private Set Intersection (PSI) is a powerful cryptographic technique that allows two parties to compute the intersection of their data — without exposing their raw data to the other party.

What is Private set intersection?

PSI is a privacy-preserving technique that allows for two parties to compare their sets of data points and compute the intersection of their data, without giving up on their individual data privacy. The result is a third data set with only the elements both parties have in common.

A real-world use case of PSI would be Apple’s use of this technique in Password Monitoring, as well as the company’s recently announced Expanded Protections for Children.

What is a common application of private set intersection?

For sensitive data, PSI can be used to remove privacy barriers inhibiting data sharing or increase security to existing services. 

One specific encryption form, which can be used in constructing a PSI protocol — is homomorphic encryption. This is a technique in which computations can be performed on encrypted messages without any access to the non-encrypted message. 

Homomorphic encryption is any highly regulated industry’s best friend, because it lets analysts compute results on encrypted data without the hassle of sharing the secret key with any cloud service provider, since only the analyst can decrypt the output.

That said, keep in mind that homomorphic encryption is only one of many possible encryption techniques, and that PSI can work with other techniques too. 

Key takeaways

  • PSI is a privacy-preserving technique that allows for two parties to compare their sets of data points and compute the intersection of their data, without giving up on their individual data privacy. 
  • The result of this cryptographic technique is a third data set with only the elements that both involved parties have in common.
  • For sensitive data, PSI can be used to remove privacy barriers inhibiting data sharing or increase security to existing services. 

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