International Association for Cryptologic Research

International Association
for Cryptologic Research


Suvradip Chakraborty


Witness Maps and Applications 📺
Suvradip Chakraborty Manoj Prabhakaran Daniel Wichs
We introduce the notion of Witness Maps as a cryptographic notion of a proof system. A Unique Witness Map (UWM) deterministically maps all witnesses for an $$mathbf {NP}$$ statement to a single representative witness, resulting in a computationally sound, deterministic-prover, non-interactive witness independent proof system. A relaxation of UWM, called Compact Witness Map (CWM), maps all the witnesses to a small number of witnesses, resulting in a “lossy” deterministic-prover, non-interactive proof-system. We also define a Dual Mode Witness Map (DMWM) which adds an “extractable” mode to a CWM. Our main construction is a DMWM for all $$mathbf {NP}$$ relations, assuming sub-exponentially secure indistinguishability obfuscation ( $${imathcal {O}}$$ ), along with standard cryptographic assumptions. The DMWM construction relies on a CWM and a new primitive called Cumulative All-Lossy-But-One Trapdoor Functions (C-ALBO-TDF), both of which are in turn instantiated based on $${imathcal {O}}$$ and other primitives. Our instantiation of a CWM is in fact a UWM; in turn, we show that a UWM implies Witness Encryption. Along the way to constructing UWM and C-ALBO-TDF, we also construct, from standard assumptions, Puncturable Digital Signatures and a new primitive called Cumulative Lossy Trapdoor Functions (C-LTDF). The former improves up on a construction of Bellare et al. (Eurocrypt 2016), who relied on sub-exponentially secure $${imathcal {O}}$$ and sub-exponentially secure OWF. As an application of our constructions, we show how to use a DMWM to construct the first leakage and tamper-resilient signatures with a deterministic signer , thereby solving a decade old open problem posed by Katz and Vaikunthanathan (Asiacrypt 2009), by Boyle, Segev and Wichs (Eurocrypt 2011), as well as by Faonio and Venturi (Asiacrypt 2016). Our construction achieves the optimal leakage rate of $$1 - o(1)$$ .
Reverse Firewalls for Actively Secure MPCs 📺
Reverse firewalls were introduced at Eurocrypt 2015 by Miro-nov 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 \emph{multi}party 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 \emph{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).