Resumen
Machine learning classification algorithms, such as decision trees and random forests, are commonly used in many applications. Clients who want to classify their data send them to a server that performs their inference using a trained model. The client must trust the server and provide the data in plaintext. Moreover, if the classification is done at a third-party cloud service, the model owner also needs to trust the cloud service. In this paper, we propose a protocol for privately evaluating decision trees. The protocol uses a novel private comparison function based on fully homomorphic encryption over the torus (TFHE) scheme and a programmable bootstrapping technique. Our comparison function for 32-bit and 64-bit integers is 26% faster than the naive TFHE implementation. The protocol is designed to be non-interactive and is less complex than the existing interactive protocols. Our experiment results show that our technique scales linearly with the depth of the decision tree and efficiently evaluates large decision trees on real datasets. Compared with the state of the art, ours is the only non-interactive protocol to evaluate a decision tree with high precision on encrypted parameters. The final download bandwidth is also 50% lower than the state of the art.