  Self-sovereign identity (SSI) has gained a large amount of interest. It
enables physical entities to retain ownership and control of their digital
identities, which naturally forms a conceptual decentralized architecture. With
the support of the distributed ledger technology (DLT), it is possible to
implement this conceptual decentralized architecture in practice and further
bring technical advantages such as privacy protection, security enhancement,
high availability. However, developing such a relatively new identity model has
high costs and risks with uncertainty. To facilitate the use of the DLT-based
SSI in practice, we formulate Self-Sovereign Identity as a Service (SSIaaS), a
concept that enables a system, especially a system cluster, to readily adopt
SSI as its identity model for identification, authentication, and
authorization. We propose a practical architecture by elaborating the service
concept, SSI, and DLT to implement SSIaaS platforms and SSI services. Besides,
we present an architecture for constructing and customizing SSI services with a
set of architectural patterns and provide corresponding evaluations.
Furthermore, we demonstrate the feasibility of our proposed architecture in
practice with Selfid, an SSIaaS platform based on our proposed architecture.
