Helger Lipmaa

Research Background

I have been an active cryptography researcher since around 1997. My first research topic was time-stamping --- combined with authenticated data structures and accumulators. I have researched many cryptographic topics, from differential cryptanalysis to MPC. However, since 2001, I have been active in zero-knowledge research. I wrote my first SNARK paper in 2008, where we showed that one can achieve (inefficient) SNARKs under knowledge assumptions. My 2003 paper on using Diophantine equations and groups of unknown order and my 2012 paper on trustless accumulators inspired well-known SNARKs like DARK. My 2012 paper on SNARKs was the last "theoretical" step before GGPR13 and Pinocchio made SNARKs practical. In 2017, I got the best (or best-3) paper award in Asiacrypt for our paper that defined and constructed subversion-resistant SNARKs. In 2022, I had one paper in which I generalized Groth16-like SNARKs to many settings, and another in which we proposed the most communication-efficient known updatable SNARK, Vampire. In 2023, we proposed AGM, a more realistic version of AGM. In 2024, we showed that one can prove KZG-based zk-SNARKs are knowledge-sound in the random oracle model without relying on non-falsifiable assumptions. In 2024, I also proposed Polymath, which has a shorter argument size than the famous Groth16 zk-SNARK. In 2025, we proved that Plonk (one of the most important zk-SNARKs) is secure in the random oracle model under falsifiable assumptions. In 2026, we proposed a new technique to prove the knowledge soundness of (KZG-based) zk-SNARKs solely from binding properties. In addition, we proposed Cyclo, a new and efficient lattice-based folding scheme.

News

See the news section in the research group page.

Teaching

• Zero-Knowledge Proofs (Autumn 2025)

• Cryptographic Protocols (Autumn 2018)

Current Ph.D. Students

• Erki Külaots (MSc 2024, University of Tartu, supervisor Toomas Krips)

• Urmas Luhaäär (MSc 2024)

• Pritam Pal (MSc 2024, Indian Statistical Institute, supervisor Bimal Kumar Roy)

Previous Ph.D. Students

• Sven Laur (Ph.D. 2008, now an associate professor at the University of Tartu) - google scholar 

• Bingsheng Zhang (Ph.D. 2012,  now a professor at the Zhejiang University) - google scholar

• Prastudy Fauzi (Ph.D. 2017, now a research fellow at the Nanjiang Technical University) - google scholar 

• Rafik Chaabouni (Ph.D. 2017, now a cyber security architect at the Swiss Armed Forces) - google scholar 

• Janno Siim (Ph.D. 2020, now an assistant professor at the University of Tartu) - google scholar 

• Karim Baghery (Ph.D. 2020, now a postdoc at the KU Leuven) - google scholar 

• Behzad Abdolmaleki (Ph.D. 2020, now an assistant professor at the University of Sheffield) - google scholar 

• Arne-Tobias Ødegaard (Ph.D. 2022, now a postdoc at the NSM) - google scholar

• Roberto Parisella (Ph.D. 2023, now a postdoc at the Simula UiB) - google scholar

Previous Honorary Ph.D. Students

(People who had a different supervisor but who spent a considerable amount of time in my research group as a Ph.D. student)

• Michal Zajac (now a head of cryptography research at Nethermind) - google scholar

• Toomas Krips (now an assistant professor at University of Tartu) - google scholar