International Association for Cryptologic Research

International Association
for Cryptologic Research


Paper: Rubato: Noisy Ciphers for Approximate Homomorphic Encryption

Jincheol Ha , KAIST, Daejeon, Korea
Seongkwang Kim , Samsung SDS, Seoul, Korea
Byeonghak Lee , KAIST, Daejeon, Korea
Jooyoung Lee , KAIST, Daejeon, Korea
Mincheol Son , KAIST, Daejeon, Korea
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Presentation: Slides
Conference: EUROCRYPT 2022
Abstract: A transciphering framework converts a symmetric ciphertext into a homomorphic ciphertext on the server-side, reducing computational and communication overload on the client-side. In Asiacrypt 2021, Cho et al. proposed the RtF framework that supports approximate computation. In this paper, we propose a family of noisy ciphers, dubbed Rubato, with a novel design strategy of introducing noise to a symmetric cipher of a low algebraic degree. With this strategy, the multiplicative complexity of the cipher is significantly reduced, compared to existing HE-friendly ciphers, without degrading the overall security. More precisely, given a moderate block size (16 to 64), Rubato enjoys a low multiplicative depth (2 to 5) and a small number of multiplications per encrypted word (2.1 to 6.25) at the cost of slightly larger ciphertext expansion (1.26 to 1.31). The security of Rubato is supported by comprehensive analysis including symmetric and LWE cryptanalysis. Compared to HERA within the RtF framework, client-side and server-side throughput is improved by 22.9% and 32.2%, respectively, at the cost of only 1.6% larger ciphertext expansion.
Video from EUROCRYPT 2022
  title={Rubato: Noisy Ciphers for Approximate Homomorphic Encryption},
  author={Jincheol Ha and Seongkwang Kim and Byeonghak Lee and Jooyoung Lee and Mincheol Son},