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Paper: i-Hop Homomorphic Encryption and Rerandomizable Yao Circuits

Authors:
Craig Gentry
Shai Halevi
Vinod Vaikuntanathan
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URL: http://eprint.iacr.org/2010/145
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Abstract: Homomorphic encryption (HE) schemes enable computing functions on encrypted data, by means of a public $\Eval$ procedure that can be applied to ciphertexts. But the evaluated ciphertexts so generated may differ from freshly encrypted ones. This brings up the question of whether one can keep computing on evaluated ciphertexts. An \emph{$i$-hop} homomorphic encryption scheme is one where $\Eval$ can be called on its own output up to $i$~times, while still being able to decrypt the result. A \emph{multi-hop} homomorphic encryption is a scheme which is $i$-hop for all~$i$. In this work we study $i$-hop and multi-hop schemes in conjunction with the properties of function-privacy (i.e., $\Eval$'s output hides the function) and compactness (i.e., the output of $\Eval$ is short). We provide formal definitions and describe several constructions. First, we observe that "bootstrapping" techniques can be used to convert any (1-hop) homomorphic encryption scheme into an $i$-hop scheme for any~$i$, and the result inherits the function-privacy and/or compactness of the underlying scheme. However, if the underlying scheme is not compact (such as schemes derived from Yao circuits) then the complexity of the resulting $i$-hop scheme can be as high as $k^{O(i)}$. We then describe a specific DDH-based multi-hop homomorphic encryption scheme that does not suffer from this exponential blowup. Although not compact, this scheme has complexity linear in the size of the composed function, independently of the number of hops. The main technical ingredient in this solution is a \emph{re-randomizable} variant of the Yao circuits. Namely, given a garbled circuit, anyone can re-garble it in such a way that even the party that generated the original garbled circuit cannot recognize it. This construction may be of independent interest.
BibTeX
@misc{eprint-2010-23046,
  title={i-Hop Homomorphic Encryption and Rerandomizable Yao Circuits},
  booktitle={IACR Eprint archive},
  keywords={BHHO encryption, Compactness, Function Privacy, Homomorphic Encryption, Secure Two-party Computation, Oblivious  Transfer, Yao's Garbled Circuits},
  url={http://eprint.iacr.org/2010/145},
  note={extended abstract in CRYPTO 2010, this is the full version shaih@alum.mit.edu 14756 received 16 Mar 2010, last revised 27 May 2010},
  author={Craig Gentry and Shai Halevi and Vinod Vaikuntanathan},
  year=2010
}