## Cryptology ePrint Archive: Report 2019/1317

Reverse Firewalls for Actively Secure MPCs

Suvradip Chakraborty and Stefan Dziembowski and Jesper Buus Nielsen

Abstract: Reverse firewalls were introduced at Eurocrypt 2015 by Mironov and Stephens-Davidowitz, as a method for protecting cryptographic protocols against attacks on the devices of the honest parties. In a nutshell: a reverse firewall is placed outside of a device and its goal is to sanitize'' the messages sent by it, in such a way that a malicious device cannot leak its secrets to the outside world. It is typically assumed that the cryptographic devices are attacked in a functionality-preserving way'' (i.e. informally speaking, the functionality of the protocol remains unchanged under this attacks). In their paper, Mironov and Stephens-Davidowitz construct a protocol for passively-secure two-party computations with firewalls, leaving extension of this result to stronger models as an open question. In this paper, we address this problem by constructing a protocol for secure computation with firewalls that has two main advantages over the original protocol from Eurocrypt 2015. Firstly, it is a multiparty computation protocol (i.e. it works for an arbitrary number $n$ of the parties, and not just for $2$). Secondly, it is secure in much stronger corruption settings, namely in the actively corruption model. More precisely: we consider an adversary that can fully corrupt up to $n-1$ parties, while the remaining parties are corrupt in a functionality-preserving way. Our core techniques are: malleable commitments and malleable non-interactive zero-knowledge, which in particular allow us to create a novel protocol for multiparty augmented coin-tossing into the well with reverse firewalls (that is based on a protocol of Lindell from Crypto 2001).

Category / Keywords: cryptographic protocols / Reverse Firewalls, Multi-party Computation, Coin-tossing, Broadcast

Date: received 13 Nov 2019

Contact author: suvradip1111 at gmail,stefan dziembowski@gmail com,jbn@cs au dk

Available format(s): PDF | BibTeX Citation

Short URL: ia.cr/2019/1317

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