Prioritization of Quality Requirements in Decentralized Identity Applications

In the evolving landscape of digital identity management, Decentralized and Self-Sovereign Identity (DI/SSI) systems represent a paradigm shift toward user-centered control, prompting a re-evaluation of how quality requirements are defined and prioritized across system stakeholders. This thesis advances Requirements Engineering (RE) for DI/SSI systems by prioritizing Non-Functional Requirements (NFRs) through the distinct perspectives of identity holders, issuers, and verifiers, moving beyond the typical organization-centric approach and addressing user-centric goals such as control, privacy, and trust. The study establishes stakeholder-specific NFR rankings and investigates how concrete system functionalities are perceived as supporting these qualities. Employing a structured responsibility scheme and role-based mapping, this thesis introduces the Software Quality Requirements Importance (SQRI) scale and applies Best-Worst Scaling (BWS), a novel prioritization technique in this context, within clear, scenario-based surveys designed for each stakeholder group. The instruments, tailored to each stakeholder as surveys and available in both English and German, enable robust, stakeholder-specific assessment of NFRs for DI/SSI systems, advancing current requirements engineering methods and supporting nuanced analysis of priority patterns across identity holders, issuers, and verifiers. The research delivers ranked sets of NFRs for each role, providing new insights into functionality-quality mappings via prioritization matrices that compare Protection, Authenticity, Security, and other NFRs to prior classifications and highlight quadrant-based role differences. The findings offer actionable guidance for DI/SSI system design and contribute to the broader Requirements Engineering literature by clarifying stakeholder priorities and the alignment between user expectations and system features.