Lior Rotem (Theory Seminar)

Accountability in Threshold Cryptography

Lior Rotem (Stanford University)
Monday, June 3rd 2024, 2-3pm


Threshold cryptography has been fundamental to the construction of secure distributed protocols over the past three decades. Alas, the use of threshold cryptography typically comes at the expense of accountability; if secret information is shared among multiple parties, we might not know who to blame if this information is leaked or misused. 
In this talk, I will focus on three recent works that show that this is not inherent -- we can in fact have accountable threshold cryptosystems. 

Most of the talk will be dedicated to accountability in secret sharing. Suppose Alice uses a t-out-of-n secret sharing to store her secret key on n servers, and a less-than-t subset of the servers decides to offer the shares they have for sale. In this case, Alice should be able to hold them accountable, or else nothing prevents them from selling her shares. A secret sharing scheme that allows Alice to trace the leak back to the corrupted servers is called a traceable secret sharing scheme. I will present practical constructions for traceable secret sharing, based on the natural and widely used secret sharing schemes of Shamir and Blakley. 

I will also touch on two related works: one deals with accountability in threshold decryption, with applications to encrypted mempools, sealed-bid auctions, and e-voting. The second work deals with threshold signatures and shows, for the first time, how to combine two important properties: accountability and immunity to perpetual leakage via a proactive refresh protocol. 

The talk is based on joint works with Dan Boneh and Aditi Partap.


Speaker information:

Lior Rotem is a postdoctoral scholar at the Applied Cryptography Group at Stanford University, hosted by Prof. Dan Boneh. He obtained his Ph.D. in Computer Science from the Hebrew University of Jerusalem in 2022.
Lior’s work has appeared in top conferences such as CRYPTO, EUROCRYPT, TCC, and FC. He is the recipient of prestigious fellowships and awards, including the Rothschild Postdoctoral Fellowship, the Protocol Labs Postdoctoral Fellowship, the Adams Doctoral Fellowship of the Israel Academy of Sciences and Humanities, and the Hebrew University’s Excellence in Research Award.