Advances in Cryptology – EUROCRYPT 2012

Volume 7237 of the series Lecture Notes in Computer Science pp 465-482

Fully Homomorphic Encryption with Polylog Overhead

  • Craig GentryAffiliated withLancaster UniversityIBM T.J. Watson Research Center
  • , Shai HaleviAffiliated withLancaster UniversityIBM T.J. Watson Research Center
  • , Nigel P. SmartAffiliated withLancaster UniversityDept. Computer Science, University of Bristol

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We show that homomorphic evaluation of (wide enough) arithmetic circuits can be accomplished with only polylogarithmic overhead. Namely, we present a construction of fully homomorphic encryption (FHE) schemes that for security parameter λ can evaluate any width-Ω(λ) circuit with t gates in time \(t\cdot \mbox{polylog}(\lambda )\).

To get low overhead, we use the recent batch homomorphic evaluation techniques of Smart-Vercauteren and Brakerski-Gentry-Vaikuntanathan, who showed that homomorphic operations can be applied to “packed” ciphertexts that encrypt vectors of plaintext elements. In this work, we introduce permuting/routing techniques to move plaintext elements across these vectors efficiently. Hence, we are able to implement general arithmetic circuit in a batched fashion without ever needing to “unpack” the plaintext vectors.

We also introduce some other optimizations that can speed up homomorphic evaluation in certain cases. For example, we show how to use the Frobenius map to raise plaintext elements to powers of p at the “cost” of a linear operation.