International Association for Cryptologic Research

International Association
for Cryptologic Research


Towards Practical Transciphering for FHE with Setup Independent of the Plaintext Space

Pierrick Méaux , University of Luxembourg
Jeongeun Park , Norwegian University of Science and Technology (NTNU)
Hilder V. L. Pereira , University of Campinas (UNICAMP)
DOI: 10.62056/anxrxrxqi
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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.

  title={Towards Practical Transciphering for FHE with Setup Independent of the Plaintext Space},
  publisher={International Association for Cryptologic Research},
  volume={1, Issue 1},
  author={Pierrick Méaux and Jeongeun Park and Hilder V. L. Pereira},