Lila: Decentralized Build Reproducibility Monitoring for the Functional Package Management Model

Technical Summary: Lila – Decentralized Build Reproducibility Monitoring

Problem Statement
While reproducible builds (R-B) offer a principled method for ensuring software supply chain integrity by verifying bitwise equivalence between source code and binaries, their large-scale adoption faces two primary hurdles. First, there has historically been uncertainty regarding the feasibility of achieving high reproducibility rates across vast, diverse package collections. Second, and the specific focus of this work, is the lack of effective infrastructure for monitoring reproducibility at scale. Existing monitoring solutions are typically centralized, computationally expensive, and often restricted to subsets of packages or recent versions. These limitations hinder both the practical maintenance of reproducible builds and the ability to conduct large-scale empirical studies on build reproducibility.

Methodology and System Design
The authors introduce Lila, a decentralized system designed to assess and aggregate reproducibility results specifically for functional package management ecosystems, with an initial implementation targeting Nix.

• Decentralized Attestation: Lila leverages the post-build-hook mechanism within the Nix build system. Upon the completion of a local build, the Lila client inspects the generated output paths. It constructs a cryptographic attestation containing:
  • The hash of the build recipe (derivation).
  • The cryptographic hashes of the contents of all produced outputs.
  • A signature generated by the builder's private key.
• Aggregation Architecture: These signed attestations are submitted via an authenticated REST API to a central aggregation server. The server validates the reports and stores them in a queryable database.
• Data Ingestion: The system is designed to ingest data from independent builders across varied hardware and environments, as well as from the NixOS continuous integration platform, thereby creating a comprehensive dataset.
• Interface and Utility: The server exposes a web interface and API (detailed in Table 2 of the paper) that allows users to generate reproducibility dashboards, monitor specific package sets, and detect reproducibility regressions over time. The system also supports historical tracking, allowing for the analysis of older package versions.

Key Contributions

1. Decentralized Monitoring Infrastructure: Lila addresses the scalability challenge of reproducibility monitoring by distributing the build execution and attestation generation to independent participants, rather than relying on a single centralized builder farm.
2. Empirical Data Foundation: By aggregating reports from diverse environments, Lila creates a persistent database suitable for future empirical research. This allows researchers to study the impact of specific build environment factors on bitwise reproducibility at a scale previously unattainable.
3. Practical Tooling for Maintainers: The system provides immediate utility to developers and package maintainers by offering dashboards that facilitate the timely identification of packages that have regressed in reproducibility, enabling faster remediation.
4. Integration with Functional Package Management: The design is tailored to the functional package management model (specifically Nix and Guix), utilizing cryptographic derivation hashes to uniquely identify build recipes and outputs.

Results and Deployment Status
The paper reports on the gradual deployment of Lila within the Nix community. An experimental instance has successfully collected over 150,000 reproducibility reports. This deployment has already proven useful by generating dashboards that helped experts identify specific reproducibility issues. The authors note that their initial deployment with a small group of users revealed no inherent scalability issues in the design. Furthermore, the system has demonstrated the ability to track reproducibility over time, including historical versions of packages.

Significance and Claims
The authors position Lila as a foundational component for advancing software supply chain security and academic research.

• For Practitioners: Lila serves as a practical tool to minimize trust relationships in the binary distribution supply chain by providing verifiable, aggregated evidence of build reproducibility.
• For Researchers: It offers a new infrastructure to answer research questions regarding the variability of reproducibility rates across ecosystems and the specific impact of build environment components on artifact reproducibility.
• Modest Scope: The authors acknowledge that while Lila decentralizes the generation of attestations, it currently relies on a centralized server for aggregation. They view this as a pragmatic first step, noting that a fully peer-to-peer aggregation architecture would require significantly more engineering effort regarding coordination and consensus. The system is designed to be extensible, with compatibility for the Guix ecosystem, and is released as open-source software under the EUPL-1.2 license.