## 2 Background  
### Overview of IDMSs  
Digital identities are needed to identify users when they request access to digital resources. To manage these digital identities, in addition to related information and credentials, an efficient IDMS is required. There are many identity management models that have been created and categorized based on the use of identity and the need for a cross-domain, such as an isolated user identity model, a federated identity model, and a user-centric model [1]. 2.1.1. The Isolated User Identity Model (SILO) or Centralized Model  
IDMSs have undergone multiple stages of development. First, there was the Isolated User Identity (SILO) model, which is the cornerstone and the most simple model most widely used [4]. It is based on identity management between only two parties, the IDP and the user. The IDP in this system plays the role of the SP, as it allows the user to create a digital identity to obtain services provided in a specific field, which means that the user needs to create several digital identities to obtain services in multiple domains [5]. This is perhaps a major defect in this model owing to the difficulty of managing multiple identities by the user, in addition to full dependence on the IDP, which may cause a violation of user privacy, such as user movements tracking. 2.1.2. Federated Identity Model  
Another IDM was then created, which is Federated IDMS [1]. It differs from the previous system as it is based on three parties instead of two: the IDP, the SP, and the user [6]. The IDP here is the responsible party for user identity creation, authentication, and necessary credentials. In this model, the user depends on the IDP to issue credentials related to their identity and authenticate them to the SP. Therefore, there must be an element of trust between the IDP and the SP (Circle of Trust principle), which means that for every IDP in the system, there is a group of trusted SPs that the user can obtain services from [2,4]. Full dependence on the IDP, in addition to being fully informed of all user behaviors and relationships, is a threat to user privacy and may lead to the SPOF. These are serious problems in the centralized identity management approach that depends on a central party to provide the required identity creation and authentication services; the IDP. 2.1.3. User-Centric Model  
This model is also referred to as the Open Trust Model, as all parties in the system are required to trust each other [1]. In this model, the user can select the attributes and credentials to be sent, in addition to the ability of choosing the IDP. It is very similar to the federated model, and it also has the same privacy concerns. The second law of identity (justifiable parties) is not satisfied in this model and the sharing policy with SP can be defined by the user, but it is still under the control of the IDP [5]. User privacy is violated in this model because of the IDP control. 2.1.4. Self-Sovereign Identity Model (SSI)  
The abovementioned IDM model requires full dependence on a third party, the IDP, to manage and control the identity, in addition to providing the credentials necessary for authentication. This represents a clear threat to the user's privacy, as all user behavior and movements are exposed to the IDP. To raise the level of user privacy in the field of digital identities, and to find a solution to the problems associated with the user's dependence on the IDP (problems related to the centralized approaches), a model based on the principle of decentralization has appeared in the field of IDM. The adoption of a decentralized IDM approach has been instigated by many researchers to find solutions regarding the privacy and SPOF problems in the previous centralized models. The Self-sovereign Identity model (SSI) is an emerging decentralized IDMS that provides the user with the ability to control their identity, as well as its related data and transactions [7].