Toward a Common Understanding of Cryptographic Agility: A Systematic Review

Cryptographic agility has become a recurring theme in both academic and industrial discussions, driven by the need to adapt cryptography under evolving threats and disruptive changes such as post-quantum cryptography. Yet despite its rising importance, the concept itself remains inconsistently defined and often conflated with related notions, limiting its value for research and practice. This paper offers the first systematic study of cryptographic agility by providing the following research contributions: First, we review existing definitions and classify their shared characteristics across 84 screened sources, of which 48 are included in our analysis (24 peer-reviewed, 24 gray literature), thereby exposing the lack of consensus within the current literature (RQ1). Second, we synthesize these findings into a literature-based definition that unifies the most significant aspects identified in the literature (RQ2). Third, we refine this foundation into a general, context-independent definition that isolates agility as the property of changeability of cryptographic entities (RQ3). Fourth, we extend the concept by introducing a layer model with four overarching domains—conceptual, software, hardware, and organization—and by clarifying its relation to neighboring concepts such as cryptographic versatility and interoperability (RQ4). We demonstrate the applicability of our results by analyzing the three real-world exemplars OpenSSL, NGINX, and GitLab CI/CD. Finally, we discuss broader implications, including trade-offs with complexity, its assessment across contexts, and the role of agility in cryptographic migration. Our work establishes a unified terminology and conceptual foundation for cryptographic agility, enabling consistent reasoning and guiding both research and practice in designing systems that can adapt securely and sustainably.