Message Authentication Code with Fast Verification over Encrypted Data and Applications

In common data analytic scenarios, data is produced by a multitude
of data producers (e.g., medical clinics), stored and maintained by
some data keeper (e.g., a centralized repository), and substantial
benefit can be gained from making data accessible to a variety
of data consumers (e.g., researchers); however, making cleartext
data accessible poses a privacy threat and may infringe on privacy
regulation. Computing over data encrypted by fully homomorphic
encryption (HE) enables providing privacy guarantee together with
data mining utility. To ensure that correct insights are extracted, it
is essential to guarantee data authenticity. In this work we present
an authenticity proof for encrypted data:
(1) As a central tool we show how to modify a classical MAC
based on universal hashing to introduce the first MAC with
fast homomorphic verification over the reals (7.37 microseconds amortized runtime).
(2) We then utilize our MAC for guaranteeing data authenticity,
for data provided by an untrusted data keeper in HE encrypted form. We implemented our solution, demonstrating
substantial efficiency improvements over the prior art (Chatel
et al. USENIX’21): improving the proof size and generation
time by over 104×.
To demonstrate the usefulness of our homomorphic verification in
realistic systems we implemented it in AWS EC2 with S3 storage,
demonstrating it achieves practical performance for fetching and
authenticating HE ciphertexts, as well as smooth integration with
subsequent homomorphic evaluation of decision tree models.