# Secure Delivery Scheme of Common Data Model for Decentralized Cloud Platforms
## Materials and Methods
### Materials
#### 2.1.2 Decentralized Identifier (DID)  
Identifiers uniquely identify an item of datum and give the datum a unique name to express its characteristics. DID, which is unique worldwide, is a new type of identifier that can be operated with high availability and can be cryptographically verified [4]. DIDs also support the principles of self-sovereignty [10], that is, they allow the identity owner to create, manage, and discard identifiers that appears to be suitable. It supports the use of multiple simultaneous identifiers, thus meeting the privacy enhancing properties. CDM can increase the interoperability of medical data between hospitals, but it is required to provide a secure medical data access environment for medical data analysis by various researchers, including disease prevention.  
Identity management for the CDM operation process of hospital medical and prescription data is classified into four types: isolated, centralized, federated, and distributed ID. The isolated type is a form in which the user's identity is managed for each service, and the user needs the identity registration (member registration) and verification (authentication) procedures for each service, and the identity management system is securely built and operated for user authentication and access control of the cloud CDM service. In order to do that, it is expensive. In terms of security system, cloud CDM users need to register for identity verification and store or manage identity verification information (ID/PW, etc.), separate from hospital ID.  
The centralized type is a method of centrally managing the identities of trusted users, and the construction and operation efficiency of the identity management system is higher than that of the isolated type. Users can register their identities only in the centralized management system and use the SSO (Single Sign On) function to connect multiple services through one authentication. With this technical feature, the centralized type is suitable for providing multiple services through a single cloud CDM. However, when a failure occurs in the central management system, the entire service cannot be used, and the interoperability and scalability are limited due to the closed structure.  
The federated type is a method in which different service providers form a trust relationship for the convenience of users, and jointly manage the user's identity [11]. The federated type is suitable for SSO support between the hospital and the cloud CDM network. However, federated ID management should be preceded by establishing a trust relationship between the hospital and the cloud CDM, and if identity management is concentrated on a specific service provider, it has the same limitations as the centralized type.  
The distributed ID type is operated in a way that users create their own identification information and share it through DLT (Distributed Ledger Technology) [12], and use DLT to share information to identify each service provider. The convenience of identity management can be improved by eliminating the need to register verification information. In terms of security system, since the user's identity is not stored in the central server, DB, etc., there is little risk of ID leakage, and theft and has the advantage of ensuring the integrity of the identification information through DLT technology.