Strongly Secure Universal Thresholdizer from Lattices

Title of the Talk: Strongly Secure Universal Thresholdizer from Lattices
Speaker: Dr. Anshu Yadav
Host Faculty: Dr.Maria Fransis
Date: Dec 17, 2024
Time: 11:00 am
Venue: C LH 9

Abstract: Threshold cryptography distributes a privileged operation, like signing in a signature scheme or decryption in a public key encryption scheme among multiple parties so that there is no single point of security failure. For example, in a t-out-of-n threshold signature scheme, each party is given a secret partial signing key using which it can generate a partial signature on any message m and then any t partial signatures can be combined to generate a valid signature on m. In 2018, Boneh et al. constructed the first lattice-based, non-interactive universal thresholdizer (UT) and used it to thresholdize any signature scheme (along with few other applications). They consider selective security where the adversary declares at most upto t-1 corrupt parties in the beginning of the game and is given their corresponding (partial) signing keys. However, during the rest of the game, the adversary is not allowed to get a partial signature from any honest party on the target message m, even if it corrupts less than t-1 parties in the beginning. In this talk, I will present a construction of threshold signature that improves upon Boneh et al’s construction to provide stronger security where the adversary is also allowed to query partial signature on m as long as it does not trivially let the adversary compute the final signature on m*. In the process, we also identified the desired stronger security property of the UT such that all the other thresholdized cryptographic primitives achieve stronger security in the similar sense. This is a joint work with Ehsan Ebrahimi from the University of Luxembourg which got accepted for publication at Asiacrypt, 2024. Towards the end of the talk, I will also briefly discuss my overall research interests and the areas/results that I have worked on.

Brief Bio: Anshu Yadav is currently a postdoctoral researcher at IST Austria in Prof. Krzysztof Pietrzak’s group. She received her PhD from IIT Madras, guided by Prof. Shweta Agrawal. Her research interests are in the area of theoretical cryptography in building various cryptographic primitives largely from post-quantum lattice-based cryptographic assumptions and some from classical assumptions as well. She has worked on problems related to threshold and blind signatures, and multi-input attribute-based and functional encryption systems.