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Towards Practical Transciphering for FHE with Setup Independent of the Plaintext Space
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| Abstract: | Fully Homomorphic Encryption (FHE) is a powerful tool to achieve non-interactive privacy preserving protocols with optimal computation/communication complexity. However, the main disadvantage is that the actual communication cost (bandwidth) is high due to the large size of FHE ciphertexts. As a solution, a technique called transciphering (also known as Hybrid Homomorphic Encryption) was introduced to achieve almost optimal bandwidth for such protocols. However, all existing works require clients to fix a precision for the messages or a mathematical structure for the message space beforehand. It results in unwanted constraints on the plaintext size or underlying structure of FHE based applications. In this article, we introduce a new approach for transciphering which does not require fixed message precision decided by the client, for the first time. In more detail, a client uses any kind of FHE-friendly symmetric cipher for $\{0,1\}$ to send its input data encrypted bit-by-bit, then the server can choose a precision $p$ depending on the application and homomorphically transforms the encrypted bits into FHE ciphertexts encrypting integers in $\mathbb{Z}_p$. To illustrate our new technique, we evaluate a transciphering using FiLIP cipher and adapt the most practical homomorphic evaluation technique [CCS'22] to keep the practical latency. As a result, our proof-of-concept implementation for $p$ from $2^2$ to $2^8$ takes only from $13$ ms to $137$ ms. |
BibTeX
@article{cic-2024-34111,
title={Towards Practical Transciphering for FHE with Setup Independent of the Plaintext Space},
journal={cic},
publisher={International Association for Cryptologic Research},
volume={1, Issue 1},
url={https://cic.iacr.org//p/1/1/20},
doi={10.62056/anxrxrxqi},
author={Pierrick Méaux and Jeongeun Park and Hilder V. L. Pereira},
year=2024
}