Exploring Fully Homomorphic Encryption

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2024-06-22 13:30:07

Fully homomorphic encryption has for a long time been considered one of the holy grails of cryptography. The promise of fully homomorphic encryption (FHE) is powerful: it is a type of encryption that allows a third party to perform computations on encrypted data, and get an encrypted result that they can hand back to whoever has the decryption key for the original data, without the third party being able to decrypt the data or the result themselves.

As a simple example, imagine that you have a set of emails, and you want to use a third party spam filter to check whether or not they are spam. The spam filter has a desire for privacy of their algorithm: either the spam filter provider wants to keep their source code closed, or the spam filter depends on a very large database that they do not want to reveal publicly as that would make attacking easier, or both. However, you care about the privacy of your data, and don't want to upload your unencrypted emails to a third party. So here's how you do it:

Fully homomorphic encryption has many applications, including in the blockchain space. One key example is that can be used to implement privacy-preserving light clients (the light client hands the server an encrypted index i, the server computes and returns data[0] * (i = 0) + data[1] * (i = 1) + ... + data[n] * (i = n), where data[i] is the i'th piece of data in a block or state along with its Merkle branch and (i = k) is an expression that returns 1 if i = k and otherwise 0; the light client gets the data it needs and the server learns nothing about what the light client asked).