International Association for Cryptologic Research

International Association
for Cryptologic Research


Paper: Faster Homomorphic Function Evaluation Using Non-integral Base Encoding

Charlotte Bonte
Carl Bootland
Joppe W. Bos
Wouter Castryck
Ilia Iliashenko
Frederik Vercauteren
DOI: 10.1007/978-3-319-66787-4_28
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Conference: CHES 2017
Abstract: In this paper we present an encoding method for real numbers tailored for homomorphic function evaluation. The choice of the degree of the polynomial modulus used in all popular somewhat homomorphic encryption schemes is dominated by security considerations, while with the current encoding techniques the correctness requirement allows for much smaller values. We introduce a generic encoding method using expansions with respect to a non-integral base, which exploits this large degree at the benefit of reducing the growth of the coefficients when performing homomorphic operations. This allows one to choose a smaller plaintext coefficient modulus which results in a significant reduction of the running time. We illustrate our approach by applying this encoding in the setting of homomorphic electricity load forecasting for the smart grid which results in a speed-up by a factor 13 compared to previous work, where encoding was done using balanced ternary expansions.
  title={Faster Homomorphic Function Evaluation Using Non-integral Base Encoding},
  booktitle={Cryptographic Hardware and Embedded Systems – CHES 2017},
  series={Lecture Notes in Computer Science},
  author={Charlotte Bonte and Carl Bootland and Joppe W. Bos and Wouter Castryck and Ilia Iliashenko and Frederik Vercauteren},