Theory is where cryptography starts — asking which security goals can be achieved and under what assumptions. In our section, we study the foundational limits and possibilities of cryptography: proving lower bounds, designing protocols in minimal or novel trust models, and building the basic tools on which future cryptographic systems will depend.

Some of our work is highly applied, but we also actively explore directions that are, for now, purely theoretical, and that’s by design. Much of the foundational work on secure multiparty computation (MPC, see below) carried out in Aarhus during the 1980s, including by members of our group, was initially considered purely theoretical. Today, MPC powers privacy-preserving applications and fuels innovation in startups both in Aarhus and around the world.

We believe that some of the theoretical directions we pursue today may have a similar impact in 20–30 years. Two current examples are:

• The YOSO model (“You Only Speak Once”), which designs MPC protocols where each party speaks only once. This enables MPC in highly decentralized settings, such as blockchains (see below).
• Pseudorandom Correlation Generators (PCGs), which generate large volumes of correlated randomness from small seeds, without interaction. PCGs can replace expensive cryptographic setup phases in many protocols, including MPC and zero-knowledge — with lightweight, non-interactive preprocessing, offering a powerful new paradigm for scalable secure computation.

We also work on proving lower bounds that show the inherent limitations of cryptographic tasks — for example, how much communication is fundamentally required to resist certain classes of attacks. This includes lower bounds for key primitives like Oblivious RAM (ORAM), where we study the tradeoffs between bandwidth, locality, and security, helping define what “efficient” can realistically mean in privacy-preserving systems. 

Our research contributes to building the foundations of secure, private, and trustworthy digital systems. By investigating fundamental cryptographic questions today, we help enable the technologies of tomorrow — from secure communication and privacy-preserving data sharing to decentralized applications. The theoretical advances made by our group have already shaped real-world innovations, and we aim to continue driving long-term impact in areas that protect users, empower secure collaboration, and support privacy in an increasingly digital society.