  Digital identity has always been considered the keystone for implementing
secure and trustworthy communications among parties. The ever-evolving digital
landscape has gone through many technological transformations that have also
affected the way entities are digitally identified. During this digital
evolution, identity management has shifted from centralized to decentralized
approaches. The last era of this journey is represented by the emerging
Self-Sovereign Identity (SSI), which gives users full control over their data.
SSI leverages decentralized identifiers (DIDs) and verifiable credentials
(VCs), which have been recently standardized by the World Wide Web Community
(W3C). These technologies have the potential to build more secure and
decentralized digital identity systems, remarkably contributing to
strengthening the security of communications that typically involve many
distributed participants. It is worth noting that the scope of DIDs and VCs
extends beyond individuals, encompassing a broad range of entities including
cloud, edge, and Internet of Things (IoT) resources. However, due to their
novelty, existing literature lacks a comprehensive survey on how DIDs and VCs
have been employed in different application domains, which go beyond SSI
systems. This paper provides readers with a comprehensive overview of such
technologies from different perspectives. Specifically, we first provide the
background on DIDs and VCs. Then, we analyze available implementations and
offer an in-depth review of how these technologies have been employed across
different use-case scenarios. Furthermore, we examine recent regulations and
initiatives that have been emerging worldwide. Finally, we present some
challenges that hinder their adoption in real-world scenarios and future
research directions.
