International Association for Cryptologic Research

International Association
for Cryptologic Research


Paper: Better Concrete Security for Half-Gates Garbling (in the Multi-Instance Setting)

Chun Guo , School of Cyber Science and Technology, Shandong University
Jonathan Katz , George Mason University
Xiao Wang , Northwestern University
Chenkai Weng , Northwestern University
Yu Yu , Shanghai Jiao Tong University
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Presentation: Slides
Conference: CRYPTO 2020
Abstract: We study the concrete security of high-performance implementations of half-gates garbling, which all rely on (hardware-accelerated) AES. We find that current instantiations using k-bit wire labels can be completely broken—in the sense that the circuit evaluator learns all the inputs of the circuit garbler—in time O(2k/C), where C is the total number of (non-free) gates that are garbled, possibly across multiple independent executions. The attack can be applied to existing circuit-garbling libraries using k = 80 when C ≈ $10^9$, and would require 267 machine-months and cost about $3500 to implement on the Google Cloud Platform. Since the attack can be entirely parallelized, the attack could be carried out in about a month using ≈ 250 machines. With this as our motivation, we seek a way to instantiate the hash function in the half-gates scheme so as to achieve better concrete security. We present a construction based on AES that achieves optimal security in the single-instance setting (when only a single circuit is garbled). We also show how to modify the half-gates scheme so that its concrete security does not degrade in the multi-instance setting. Our modified scheme is as efficient as prior work in networks with up to 2 Gbps bandwidth.
Video from CRYPTO 2020
  title={Better Concrete Security for Half-Gates Garbling (in the Multi-Instance Setting)},
  author={Chun Guo and Jonathan Katz and Xiao Wang and Chenkai Weng and Yu Yu},