## 2 Preliminaries
### Decentralized Identifiers (DIDs)  
In the Bitcoin system, transaction source and destination are simply recorded, ensuring that identity information remains non-sensitive for any participant in the blockchain. However, in a decentralized system such as blockchain it is necessary to establish a trusted identity mechanism. Especially when applied to specific scenarios, the need arises to authenticate transaction data or representatives based on identity. In such cases, user identity information becomes essential.  
Decentralized identity [15] is an identity mechanism adapted to a decentralized environment. It constitutes a standard for identifying entities, including individuals, organizations, or devices. DIDs, or digital identity identifiers, uniquely distinguish entities without reliance on centralized identity providers. The primary objective is to overcome the limitations of traditional identity verification models, empowering individuals to manage and control their digital identities effectively.  
As depicted in Figure 2, DIDs adopt the Uniform Resource Identifier (URI) format, commencing with \"did:\" and followed by a unique identifier, such as \"did:example:123456\". This identifier is linked to a DID Document presented in JSON format, containing DID-related details such as public keys, authentication services, and endpoints. This structure enables others to verify DID ownership and employ the DID's public key for authentication or data encryption. Due to the diversity of DID information, different users may choose different verification methods during DID verification.  
The DID Document encodes public key information in the form of a JSON Web Key (JWK). Different DIDs may utilize distinct encryption algorithms and key types, resulting in potential variations in verification steps depending on the specific DID implementation [16].  
DIDs feature decentralized management, realized through the adoption of DID methods. DID methods constitute a set of specifications defining how DID identifiers are created, parsed, and updated. These methods use different technologies and protocols to cater to diverse application scenarios. For instance, Ethereum's smart contracts can implement the related DID verification process [17]. [PERSON] et al. [18] utilize DIDs to construct a decentralized identity identification system, addressing identity verification and authorization challenges in content-centralized networks. [PERSON] et al. [19] enhance the security and credibility of IoT networks by introducing decentralized identity and access management mechanisms. [PERSON] et al. [20] propose an identity management system based on range proof that provides an efficient, privacy-secure authentication solution to the social networking field, providing valuable ideas for the research and practical application of decentralized identity management.  
Figure 1: Bitcoin structure diagram. The block header stores some verification information of this block, and the block body stores the specific information of this block, of which transaction information is the main storage part. The pink box in the figure represents transaction information.  
The integration of DIDs introduces enhanced flexibility, privacy protection, and user control to the digital identity domain. Users gain the capability to share and verify identities across various networks and applications without relying on centralized authentication services. This advancement contributes to the creation of a more secure, transparent, and user-friendly digital identity ecosystem, thereby promoting widespread adoption and application [21].  
In the BDIDA-IoT solution, DID is used to uniquely identify a certain piece of data or a user, which means that the controller that identifies the DID of the data can be another DID that identifies the user. DID is not static data, it is updated dynamically, and the update status of DID can be recorded in the blockchain immutably.