# Towards a GDPR-Compliant Blockchain-Based COVID Vaccination Passport
## 4 Architectural Framework for COVID Vaccination Passport
### Compliance with GDPR  
The proposed architecture uses smart contracts for accessing vaccine information. The immigration authority and other concerned authorities will be able to retrieve the data  
Figure 2: Description of the verification procedure.  
by invoking smart contracts. From the GDPR compliance perspective, both the user and the hospital authorities will be informed about this event, which shall give compliance to Article 5 of GDPR. It is to note that the same process has also been recommended in prior literature [33].  
GDPR specifically emphasizes the data subject's consent collection and its relationship with data analysis. This is crucial for the data controllers and processors. For compliance with GDPR, the use of smart contracts can be useful. The proposed architecture includes the consent conditions that will be designed and encoded into smart contracts. When the vaccine receiver registers with the vaccination authority the consent will be collected. This technique shall make our proposed system compliant with article 7 of GDPR [50; 51].  
Furthermore, the data deletion and modification procedure that is \"Right to be forgotten\" and \"Right to rectification\" of GDPR Articles 17 and 16, respectively have to be considered. Whenever the vaccine receiver wants, he or she should be able to request the controller for data deletion as well as data correction in case any wrong information is entered. This problem can be solved by using off-chain storage for original data and on-chain storage for proof of existence [35; 36; 37; 41]. A link will be established between the off-chain and on-chain storage. Whenever user requests for data deletion, this connection link will be destroyed causing logical deletion. It is worthwhile to mention that in the case of the proposed architecture, the off-chain data will also be deleted after logical deletion. So, there will be no personally identifiable information available on the off-chain. Besides, blockchain stores the hash only which is irreversible and is not understandable at all. So, it is impossible to retrieve or understand personal information only by looking at the hash.  
Data controller and processor identification (Articles 24, 26, and 28) is another vital requirement for blockchain-based architectures. There are several recommendations for this purpose, for example, each node can act as a controller [37; 38]. However, in some cases, miners with maximum financial stake can also act as controllers. If a smart contract is used in a system, smart contract developers can be also used as processors [39]. Since in the proposed architecture the permissioned blockchain is used, joint controllers (Article 26) could be used here. The miners who will be validating the blocks can be the processors (Article 28) [39; 52; 53]. Therefore, the proposed architecture is compliant with articles 26 and 28 of GDPR.  
Some characteristics of blockchain are inherently compliant with the GDPR. For example, blockchain complies with the GDPRs privacy by design since it uses a cryptographic approach by default [54; 55]. In addition, private blockchain helps implement security measures for cross-border data transmission.