On the Plausibility of Fully Homomorphic Encryption for RAMs

Ariel Hamlin, Justin Holmgren, Mor Weiss, Daniel Wichs

Research output: Chapter in Book/Report/Conference proceedingConference contributionpeer-review

4 Scopus citations


We initiate the study of fully homomorphic encryption for RAMs (RAM-FHE). This is a public-key encryption scheme where, given an encryption of a large database D, anybody can efficiently compute an encryption of P(D) for an arbitrary RAM program P. The running time over the encrypted data should be as close as possible to the worst case running time of P, which may be sub-linear in the data size. A central difficulty in constructing a RAM-FHE scheme is hiding the sequence of memory addresses accessed by P. This is particularly problematic because an adversary may homomorphically evaluate many programs over the same ciphertext, therefore effectively “rewinding” any mechanism for making memory accesses oblivious. We identify a necessary prerequisite towards constructing RAM-FHE that we call rewindable oblivious RAM (rewindable ORAM), which provides security even in this strong adversarial setting. We show how to construct rewindable ORAM using symmetric-key doubly efficient PIR (SK-DEPIR) (Canetti-Holmgren-Richelson, Boyle-Ishai-Pass-Wootters: TCC ’17). We then show how to use rewindable ORAM, along with virtual black-box (VBB) obfuscation for specific circuits, to construct RAM-FHE. The latter primitive can be heuristically instantiated using existing indistinguishability obfuscation candidates. Overall, we obtain a RAM-FHE scheme where the multiplicative overhead in running time is polylogarithmic in the database size N. Our basic scheme is single-hop, but we also extend it to obtain multi-hop RAM-FHE with overhead$$N^\epsilon $$ for arbitrarily small$$\epsilon >0$$. We view our work as the first evidence that RAM-FHE is likely to exist.

Original languageEnglish
Title of host publicationAdvances in Cryptology – CRYPTO 2019 - 39th Annual International Cryptology Conference, Proceedings
EditorsDaniele Micciancio, Alexandra Boldyreva
PublisherSpringer Verlag
Number of pages31
ISBN (Print)9783030269470
StatePublished - 2019
Externally publishedYes
Event39th Annual International Cryptology Conference, CRYPTO 2019 - Santa Barbara, United States
Duration: 18 Aug 201922 Aug 2019

Publication series

NameLecture Notes in Computer Science (including subseries Lecture Notes in Artificial Intelligence and Lecture Notes in Bioinformatics)
Volume11692 LNCS
ISSN (Print)0302-9743
ISSN (Electronic)1611-3349


Conference39th Annual International Cryptology Conference, CRYPTO 2019
Country/TerritoryUnited States
CitySanta Barbara

Bibliographical note

Publisher Copyright:
© 2019, International Association for Cryptologic Research.


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