Paper 2019/1168

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

Chun Guo, Jonathan Katz, Xiao Wang, Chenkai Weng, and Yu Yu


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 $O(2^k/C)$, 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 $C \approx 10^9$, and would require 267 machine-months and cost about USD 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 $\approx 250$ 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.

Available format(s)
Cryptographic protocols
Publication info
Preprint. MINOR revision.
garbled circuitideal cipher modelconcrete security
Contact author(s)
wangxiao @ cs northwestern edu
2019-10-08: received
Short URL
Creative Commons Attribution


      author = {Chun Guo and Jonathan Katz and Xiao Wang and Chenkai Weng and Yu Yu},
      title = {Better Concrete Security for Half-Gates Garbling (in the Multi-Instance Setting)},
      howpublished = {Cryptology ePrint Archive, Paper 2019/1168},
      year = {2019},
      note = {\url{}},
      url = {}
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