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September 28, 2020, 12:00 PM
Ivan Damgård: Information-Theoretically Secure MPC against Mixed Dynamic Adversaries

[Monday, September 28, 2020 at 12:00 PM in] Abstract
In this work we consider information-theoretically secure Multiparty Computation (MPC) against an adversary who can corrupt tp parties passively, ta parties actively, and can make tf parties fail-stop. With perfect security, it is known that every function can be computed securely if and only if 3ta+2tp+tf
In this work we consider instead a dynamic adversary. In this setting, the goal is to design a single protocol that is secure, no matter which set of corruption thresholds (ta,tp,tf) is chosen by the adversary as long as the inequality is respected. Thus, a dynamic adversary can choose a corruption strategy after seeing the protocol and so is clearly stronger than a standard adversary.

The feasibility and round complexity of dynamically secure protocols has been considered before for computational security. In this work we consider for the first time the information theoretic threshold case. For statistical security we show that dynamic secure function evaluation (SFE) is possible if 2ta + 2tp + tf < n, but any dynamically secure protocol must use Ω(n) rounds. We also show that general reactive MPC is possible if we assume in addition that 2ta + 2tf ≤ n. For perfect security we show that both dynamic SFE and verifiable secret sharing with guaranteed output delivery (G.O.D.) are impossible for 3ta+2tp+tf

(Joint work with Daniel Escudero and Divya Ravi.)

Bio: Ivan Damgård was born in 1956 and got his PhD in 1988 from Aarhus University, where he has also been professor since 1995. ID became a fellow of the IACR in 2010 and received the RSA conference award for excellence in mathematics in 2015. In 2018 he received the Villum Kann Rasmussen annual award and became member of the Danish Royal Society for sciences and letters. He was the editor in chief of Journal of Cryptology 2014-2016. He has been the supervisor for 20+ postdocs and 35 PhD students. ID has authored 170+ peer reviewed scientific papers that received more than 21000 citations in total. He is a co-founder of spin-off companies Cryptomathic, Partisia and Sepior.