Announcement: Public PhD defense

On June 17th, 2024, I will defend my PhD thesis titled “Rapid prototyping and deployment of privacy-enhancing technologies” at the Vrije Universiteit Brussel. My promotors are prof. dr. ir. Kris Steenhaut and prof. dr. An Braeken, and my jury further consists out of prof. dr. Jan Tobias Mühlberg, dr. ing. Jorn Lapon, prof. dr. Ann Dooms, prof. dr. ir. Frederik Temmermans, prof. dr. Bart Jansen, and prof. dr. ir. Wendy Meulebroeck.

Practical details

The defense will take place at the Vrije Universiteit Brussel, Campus Etterbeek, in the Promotiezaal D.2.01 at 🗺️ 10:00 CEST.

The defense will be in English, and will be tailored to a general audience. The defense will be followed by a reception (probably at ‘t Complex) around early noon. If you cannot attend the defense and reception, I expect many people to stay at the Complex after 18:00. If you would like to physically attend the defense or the reception, please register online on

The event will additionally be live-streamed via Microsoft Teams. If you would like to receive the link to the live stream, either register online, or let me know via any other channel, e.g. through a message via Signal.

PhD cover


Since its inception, the internet has quickly become a public service utility. The combination of its commercial exploitation, and the rather intimate nature of how humans actively use the internet, gives rise to some paradoxical situations.

As a citizen of Belgium, I would probably not expect to give my name and phone number to a company in the United States to talk to my brother, 50 km up north. However, for over two billion people, this is their rather paradoxical reality: the company Meta, owning WhatsApp, collects and stores these data for their users. This cherry-picked scenario stands example for a wider trend in the industry.

Cryptographers have worked on several privacy-enhancing technologies (PETs). These PETs aim to minimize the amount of personal data to fulfill a service for users. Although these technologies exist on paper, several practical issues arise. These practicalities are the subject of this thesis.

One practical issue is the performance. PETs, especially those that run on end-user devices, should both be fast and require little bandwidth. We study how implementation details may lead to significant speedups or bandwidth savings. Specifically, we devise a zero-knowledge proof (ZKP) tailored to electronic road pricing (ERP). ERP is a privacy-sensitive topic, and our ERP system achieves some notable performance improvements over preexisting proposals.

A second practical issue is the challenging nature of implementing PETs. We present “Circuitree” and “Whisperfish”, to study how to bring PETs to an actual application. Circuitree is a high-level framework to tailor ZKPs to specific scenarios, using a bespoke logic programming language. The language is designed such that the resulting ZKP is highly efficient.

Whisperfish is effectively a reimplementation of the Signal instant messaging client, and allows to present in detail how Signal deploys their PETs to users. All ideas put forward in this thesis were evaluated by means of their implementation in the Rust programming language.


These publications have lead to the work in this thesis:

De Smet, R., Blancquaert, R., Godden, T., Steenhaut, K., & Braeken, A. (2024).Armed with Faster Crypto: Optimizing Elliptic Curve Cryptography for ARM Processors. Sensors, 24(3), Article 3.

De Smet, R., Steenhaut, K., & Braeken, A. (2023). Private Electronic Road Pricing using Bulletproofs with Vector Commitments. IEEE Transactions on Dependable and Secure Computing, 1–13.

Godden, T., De Smet, R., Debruyne, C., Vandervelden, T., Steenhaut, K., & Braeken, A. (2022). Circuitree: A Datalog Reasoner in Zero-Knowledge. IEEE Access, 10, 21384–21396.

Vandervelden, T., De Smet, R., Steenhaut, K., & Braeken, A. (2022). SHA3 and Keccak variants computation speeds on constrained devices. Future Generation Computer Systems, 128, 28–35.

I would like to thank my co-authors for their contributions to these articles. I would also like to thank Matti, direc85, for his contributions to Whisperfish.