# Towards a GDPR-Compliant Blockchain-Based COVID Vaccination Passport
## 2 Background
### Blockchain  
Blockchain is very similar to a database or ledger. However, it is more advanced and inherits the elements of security due to its cryptographic nature [20]. A decentralized network is a collection of many centralized networks interconnected in a distributed manner. All the nodes of the network share their data and make a copy of the whole ledger. Blockchain is an ever-growing, interlinked chain of blocks that stores the records of all the transactions committed. Each block is linked to the next with the help of a cryptographic hash value of the previous block. Besides hash value, the blocks also contain transaction data such as the timestamp. Blockchain makes it possible to make transactions without any intermediaries, thus making it indeed a decentralized technology. It also uses public-key or asymmetric cryptography and consensus algorithms to ensure user security and immutability [21]. In Blockchain, all the nodes act as peers and they are treated equally. The nodes can share data directly and do not need any centralized server to be able to send or receive. Blockchain is a reliable space to store and share data in a transparent manner. The path of the transaction can be checked or verified by the other nodes in the network. If any of the nodes think a transaction is risky, the node can cancel the transaction instantly [22]. There are mainly three types of blockchain, (1) permissionless blockchain, (2) private or permissioned blockchain, and (3) consortium blockchain. It is relevant to state that the architectural framework presented in this paper utilizes permissioned blockchain. 1. Public Blockchain: A public blockchain is a non-restrictive, permission-less distributed ledger-based system. Anybody having access to the Internet can register and sign in to the blockchain platform. Each member can check the transaction and verify it. Also, anyone can participate in the process of getting consensus. Bitcoin and Ethereum are the relevant examples [6]. A public blockchain also demands a significant amount of computing capacity, because maintaining a distributed ledger on a wide scale takes a significant amount of processing power. Each node in the network needs to solve a complicated cryptographic puzzle to verify that all nodes in the blockchain have coordinated to reach a consensus [7]. 2. Private Blockchain: It is restrictive in nature and requires permission, which limits it to work only for private networks. It has strict authorization management for accessing data. Only the selected participant has access to the system within an enterprise. Security, authorization, and accessibility are controlled by enterprises unlike public blockchain [9]. Private blockchains are easier to scale up, cut down costs, and feature greater transactions. This approach can enable the organization to have more control over the network and establish integrity by removing anonymity. 3. Consortium Blockchain: This is another type of permissioned blockchain, which is also referred to as a semi-decentralized blockchain. In this type of blockchain, predefined approved parties (nodes) participate in the block validation process. If a group of enterprises wants to implement a blockchain network and requires only a few approved parties to participate in the process, this type of blockchain platform can be implemented. Consortium blockchain allows the implementation of customized organizational policies. In addition to these characteristics, depending on the requirements of customers (organizations), services of the blockchain (i.e., inquiries about the information in the blockchain) can be open to the public or other authorities who are not part of the blockchain. The proposed architecture is based on elements of consortium blockchain enabling the concerned authorities to perform the needful tasks and inquire about any information residing on the blockchain. Moreover, the network can be easily expanded and if required, the participating authorities can be added [23; 24; 25].