publications

2026

1. SecurityAnalysis2026

   A Taxonomy of Security Analysis of Blockchain Layer 2 Scalability Solutions and Future Directions

   Adrià Torralba-Agell and Cristina Pérez-Solà

   Blockchain: Research and Applications, 2026

   Abs DOI PDF

   Blockchains face significant scalability limitations that currently prevent their mass adoption, despite their contribution to the transformation across various sectors. Layer 2 (L2) solutions emerge as a promising avenue to alleviate these limitations, offering scalability enhancements while trying to retain the core security guarantees of underlying blockchains. However, despite their increasing prominence, the security assumptions underpinning Layer 2 solutions have not been sufficiently scrutinized. Addressing this gap, this paper contributes a rigorous examination of the security assumptions inherent in the most used blockchain Layer 2 scalability solutions (Payment Channel Networks, Optimistic Rollups, Zero-Knowledge Rollups, Validium, and Volition), by compiling and specifying all security assumptions that permissionless blockchains rely on, as well as, the additional security assumptions that Layer 2 solutions currently need on top of the inherited ones. Our analysis reveals that Layer 2 solutions need additional security assumptions beyond those already established by their underlying blockchains (Layer 1, or L1). By delineating these assumptions and their implications, our research facilitates a deeper understanding of the security challenges in blockchain scalability solutions.

2. RECSI2026

   A Taxonomy and Security Analysis of Rollup Architectures in Ethereum Blockchain

   Adrià Torralba-Agell, Muoi Tran, and Cristina Pérez-Solà

   In XIX Reunión Española sobre Criptologı́a y Seguridad de la Información, 2026

   Abs

   Layer 2 rollups improve Ethereum’s scalability by executing transactions off-chain while relying on on-chain verification for finality. Despite their growing importance with increasingly high stakes, rollups have received limited systematic security analysis. In this paper, we present a taxonomy of rollup architectures and examine their associated security threats. We identify three main architectural patterns, primarily determined by the design of the sequencer, a critical component that bridges off-chain transaction execution with on-chain finality. Our analysis also reveals inherent trade-offs between throughput, decentralization, and resilience, demonstrating that no single architecture simultaneously optimizes all these properties.

3. Journal2026

   ZK-Rollups on the Wild: A Public Dataset for the Polygon zkEVM

   Adrià Torralba-Agell, Cristina Pérez-Solà, and Leonardo Bautista-Gómez

   In Under Journal Review, 2026

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   Permisionless blockchains provide transparency, but converting raw on-chain traces into research-ready assets still demands specialized infrastructure, highly specialized low-level technical expertise, and non-trivial cost. These frictions are especially evident in Layer 2 environments (where Polygon zkEVM is an example), due to its added complexity on top of an already complex system. To lower this barrier, we assembled and released the corpus of Polygon zkEVM activity from its genesis up until 2024, extracted from a synced node and indexed into analysis-friendly tables. In this work, we describe the collection and transformation pipeline carried out over the Polygon zkEVM data, and showcase a representative analysis on user behavior patterns. We conclude with open questions and paths for future work. Moreover, we publish our code, notebooks and dataset to promote reproducibility, independent access to the data, and community contributions.

2024

1. AFT2024

   Analyzing and Benchmarking ZK-Rollups

   Stefanos Chaliasos, Itamar Reif, Adrià Torralba-Agell, Jens Ernstberger, Assimakis Kattis, and Benjamin Livshits

   In Advances in Financial Technologies, 2024

   Abs DOI PDF Code

   As blockchain technology continues to transform the realm of digital transactions, scalability has emerged as a critical issue. This challenge has spurred the creation of innovative solutions, particularly Layer 2 scalability techniques like rollups. Among these, ZK-Rollups are notable for employing ZeroKnowledge Proofs to facilitate prompt on-chain transaction verification, thereby improving scalability and efficiency without sacrificing security. Nevertheless, the intrinsic complexity of ZK-Rollups has hindered an exhaustive evaluation of their efficiency, economic impact, and performance. This paper offers a theoretical and empirical examination aimed at comprehending and evaluating ZK-Rollups, with particular attention to ZK-EVMs. We conduct a qualitative analysis to break down the costs linked to ZK-Rollups and scrutinize the design choices of well-known implementations. Confronting the inherent difficulties in benchmarking such intricate systems, we introduce a systematic methodology for their assessment, applying our method to two prominent ZK-Rollups: Polygon zkEVM and zkSync Era. Our research provides initial findings that illuminate trade-offs and areas for enhancement in ZK-Rollup implementations, delivering valuable insights for future research, development, and deployment of these systems.

2. RECSI2024

   Unmasking the Illusion: The Shortcomings of “Zero-Knowledge" Rollups in Achieving Privacy

   Adrià Torralba-Agell, Ghazaleh Keshavarzkalhori, Cristina Pérez-Solà, David Megías, and Jordi Herrera-Joancomartí

   In RECSI, 2024

   Abs DOI PDF

   The rise of Layer 2 (L2) solutions, including Payment Channel Networks, sidechains, and rollups, has aimed to tackle the scalability challenges of Layer 1 (L1) blockchains like Bitcoin and Ethereum. Among these, Zero-Knowledge Rollups (ZK-Rollups) have emerged as a compelling solution by utilizing zkSNARKs to bundle multiple transactions, thereby enhancing throughput and reducing costs. However, despite their technical sophistication, ZK-Rollups do not inherently provide transaction privacy, a common misconception given the “Zero-Knowledge" nomenclature. This paper explores the privacy limitations of ZK-Rollups, emphasizing the need for privacy-preserving features that align with the expectations set by their name. We also review the strategies being developed by various projects to address these limitations. Furthermore, we propose the community begin adapting other names for the technology, such as “Verifiable Rollup" (verRollup), “Incrementally Verifiable Computation Rollup" (ivcRollup), or “Succinct Rollup" (sucRollup) that better represent the current capabilities of rollups. This work contributes to the ongoing discussion on achieving a balance between efficiency, scalability, and privacy in blockchain technologies.

2023

1. InfVis’23

   Visualisation of hierarchical multivariate data: Categorisation and case study on hate speech

   Ecem Kavaz, Anna Puig, Inmaculada Rodrı́guez, Reyes Chacón, David De-La-Paz, Adrià Torralba-Agell, Montserrat Nofre, and Mariona Taule

   Journal of Information Visualization, 2023

   Abs DOI PDF Code

   Multivariate hierarchical data has an important role in many applications. To find the best visualisation that best fits a concrete data is crucial to explore and understand the relationships between the data. This paper proposes a categorisation – Elongated and Compact – of hierarchical data based on the inner shapes of the hierarchies, that is the connectivity degree of the internal nodes, the number of nodes, etc, that can be applied to any hierarchical data. Based on this taxonomy, we explore implicit and explicit layouts – Tree, Circle Packing, Force and Radial – to provide users with a complete view of the data. We hypothesise that Tree and Circle Packing fit with Elongated structures, and Force and Radial fit with Compact ones. In addition, we cluster multivariate features to embed them in the hierarchical layouts. Especially, we propose two different glyphs –one-by-one and all-in-one, and we bet for the one-by-one glyphs as the most suitable for showing the distribution of several features along with the hierarchical structures. To validate our hypotheses, we conducted a user study with 35 participants using a hate speech annotated corpus. This corpus comes from 4359 comments posted in online Spanish newspapers. The results indicated that users preferred the Tree layout over the other three layouts (Circle, Force, Radial) with both types of structures (EC and CC). However, when we focused the analysis only on Radial and Force layouts, both of them scored significantly higher with Compact than with Elongated data. Moreover, participants scored the one-by-one glyph higher than the all-in-one glyph, but the difference was not significant.

2022

1. RECSI2022

   A Comparison of Layer 2 Techniques for Scaling Blockchains

   Adrià Torralba-Agell and Cristina Pérez-Solà

   In RECSI, 2022

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   Since the creation of Bitcoin, back in 2009, many other cryptocurrencies have appear, and its usage has been growing year after year. With this huge popularity, doubts about the ability of blockchains to become worldwide payment systems (or even universal mediums for general decentralised systems) begin to arise and, with them, solutions started to be explored. In this paper we first explain the blockchain scalability problem and then present a brief review and a comparison among some of the state-of-the-art techniques that are used to scale blockchains on Layer 2 (or off-chain), analysing properties related to their Usability, Security and Cost.

2021

1. TFM2021

   Twitter engagement model for RecSys Challenge 2021

   Adrià Torralba-Agell, Marcos Moreno Blanco, Santi Seguí, and Pere Gilabert Roca

   Universitat de Barcelona, 2021

   Abs DOI PDF Code

   Recommendation systems is an interesting and wide field of research and it is present in a huge amount of different areas in our daily life. The RecSys ACM conference is the most important conference in the recommendation area and every year they organise a competition: the RecSys Challenge. The work presented here aims to solve the RecSys 2021 Challenge which consists of giving a probability to two Twitter users that interact. In this project we have worked in the development of a model which uses the power of Gradient Boosting Trees to combine multiple hand-crafted features in an aim to represent the interaction between the users. Our team reached the 14th place in the overall challenge leaderboard and is placed between the 7th and the 9th place in terms of Like overall performance.

2020

1. TFG2020

   Iteration of holomorphic functions and visualisation of fractals

   Adrià Torralba-Agell, Anna Puig Puig, and Núria Fagella Rabionet

   Universitat de Barcelona, 2020

   Abs DOI PDF Code

   When a holomorphic function is iterated, it generates a dynamical system on the complex plane. In this project we describe both the local and global theory of the different orbits of holomorphic functions, focusing on the polynomial families. We present the necessary results leading to two algorithms to draw both Julia sets and Mandelbrot (and Multibrot) set: the Escape Algorithm and Henriksen Algortihm. In addition, we present the development of an interactive application, made with Unity, that allows us to visualise fractals on the complex plane -rendered using the aforementioned algorithms- and a generalisation of them over a 3-dimensional space, directly on a website.