# Self-Sovereign Identity-Based E-Portfolio Ecosystem
## 2 Background and Related Work  
### Blockchain  
Blockchain serves as an unalterable distributed ledger, enabling the exchange of information within a network of participating nodes. Its applications extend across diverse domains, notably in the realm of cryptocurrencies like Bitcoin [2]. The architecture of blockchain involves organizing transaction records into blocks, linked by a cryptographic mechanism. This not only ensures data confidentiality but also establishes a decentralized trust foundation, eliminating the reliance on centralized authorities. Blockchain's decentralized structure ensures that all involved nodes uphold a consistent ledger, thereby providing a dependable method for sharing information.  
Blockchain systems can be classified according to node type: public, consortium, or private. In a private blockchain, one entity retains control, and transaction visibility is limited to its members. Public blockchains, in contrast, are accessible to everyone, allowing users to read the ledger, conduct transactions, and participate as nodes in maintaining the network. Consortium blockchains involve multiple entities collaborating within a semi-centralized ecosystem. These frameworks facilitate cooperative transactions through smart contracts, fostering trust among the participating parties.  
Ethereum [3] was proposed in 2014 by [PERSON]. Programmable code on the blockchain, known as a smart contract, initiates actions automatically when specific conditions are satisfied. By combining with blockchain, smart contracts establish a trusted program execution environment that does not rely on any centralized server. Programmers can develop their own decentralized applications (dapps) by creating smart contracts and implementing them on the blockchain, which has been applied to several field, such as healthcare [4; 5; 6; 7], the Internet of Things (IoT) [5; 8; 9], contract production [10], product traceability [11], supply chains [12; 13], and open banking [14].  
Consensus mechanisms [15] in blockchain are methods to achieve agreement among participants, ensuring that all have a consistent view of the data's state. These mechanisms resolve how nodes in a decentralized network can trust each other and maintain the integrity and consistency of the blockchain data. Here are some commonly used consensus mechanisms:  
Proof of Work (PoW): PoW is the earliest and one of the most widely used consensus mechanisms, central to the Bitcoin blockchain. In PoW, miners must solve a complex mathematical puzzle to add a new block to the blockchain. This process requires substantial computational power and energy but is effective in preventing tampering, ensuring security.  
Proof of Authority (PoA): PoA is a reputation-based consensus mechanism, primarily used in private or consortium blockchains. Unlike PoW, PoA does not rely on high levels of computational resources or token holdings. Instead, it depends on verified authority nodes to generate and validate blocks. Only authorized nodes are able to add new blocks, making PoA highly efficient and well suited for scenarios requiring fast validation and stability, such as supply chain management or enterprise consortium blockchains.  
Practical Byzantine Fault Tolerance (PBFT): PBFT is a consensus mechanism specifically designed for fault tolerance, often used in private or consortium blockchains. PBFT can achieve a consensus even when some nodes display inconsistent behavior, ensuring data consistency. It is commonly applied in blockchain applications requiring rapid validation.  
These consensus mechanisms aim to enhance blockchain security, efficiency, and decentralization. Blockchains choose mechanisms based on specific application requirements to achieve optimal performance for their use cases.