VaxPass: Decentralized Two-Tier Verifiable Blockchain Platform for COVID-19 Certificate Verification

Abstract

[evdate 2022-04-29] 

HKUST President’s Cup Final Report VaxPass: Decentralized Two-Tier Verifiable Blockchain Platform for COVID-19 Certificate Verification

Submitted by: TIAN Xiangan WU Lijia WU Yijian

Supervised by: Prof. Dimitris PAPADOPOULOS

Department of Computer Science and Engineering School of Engineering Hong Kong University of Science and Technology

Details of Authors Author 1 Full Name TIAN Xiangan Student ID

School SENG Department CSE Major COMP & MATH(GM) Year of Study 4 Email

Mobile No.

Author 2 Full Name WU Lijia Student ID

School SENG Department CSE Major COMP Year of Study 4 Email

Mobile No.

Author 3 Full Name WU Yijian Student ID

School SENG Department CSE Major COMP Year of Study 4 Email

Mobile No.

VaxPass: Decentralized Two-Tier Verifiable Blockchain Platform for COVID-19 Certificate Verification

Figure 1. The vaccine passport is a way of proving one has been vaccinated against particular diseases. Many countries and regions have adopted their own vaccine passport systems for fighting against COVID-19 pandemic.

Abstract Many countries around the globe have adopted the COVID- 19 vaccine passport system to fight against Corona Virus and protect public health. In our report, we introduce the design and implementation of VaxPass, an end-to-end decentralized blockchain-based vaccine passport verification system that provides a secure and reliable way for certificate storage, is- suing, and verification. VaxPass utilizes a two-tier verifiable blockchain architecture for better scalability and maintain- ability. The first tier is the Global Chain, a Hyperledger Fabric permissioned blockchain whose nodes are maintained by the authorities by different countries. It only stores the mem- bership proof of certificates to keep lightweight so that it is convenient to share data across the globe. The second tier is the Local Chains, which contains many blockchains held by different groups of institutions such as vaccine producers, hospitals, etc. Local Chains store detailed information on the certificates. Periodically, a certificate issuer generates a membership proof of all the certificates issued within the period and records it on the Global Chain. Then the certifi- cate verifier can verify a certificate according to the proof on the Global Chain and the detailed information on the corresponding Local Chain. Keywords: blockchain, distributed ledger, cryptography, vac- cine passport, COVID-19

1 Introduction Since the first infected case was identified in December 2019 [22], there have been more than 445 million people infected with COVID-19 and 6 million deaths by March 2022 world- wide [15]. The World Health Organization (WHO) declared this Coronavirus Disease (COVID-19) outbreak caused by the virus ‘Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2)’ a Public health Emergency of International

Concern on 30th January 2020 [13]. The pandemic has re- sulted in a massive impact and unprecedented challenges to the world. One of the most immediate solutions to such in- fectious diseases is vaccination. The vaccines provide human beings with enhanced immunity and reduce the transmis- sion of the virus among the population. Many COVID-19 vaccines such as Corona Vac, Comirnaty, Covaxin, etc., have been approved by many countries and are available for vac- cination. As vaccination is one of the most effective ways we can do to prevent COVID-19 resurgence, governments usually require the citizens to get vaccinated and present the proof of their vaccination when entering public places. A vaccine passport can prove that you have tested negative for the disease or been vaccinated and protected against the virus. Before the COVID-19 outbreak, traveling from places where endemic diseases such as yellow fever, measles, or tubercu- losis remain prevalent have been required to demonstrate the proof of vaccination while traveling abroad. To fight against Corona Virus, many countries have implemented their own COVID-19 vaccination passport systems. And a great number of providers such as AOKpass, Common- Pass, and Docket have developed different vaccine passport platforms for public use. Nowadays, people usually have to present their COVID-19 test results and vaccination cer- tificate on the vaccine passport while traveling from one country to another. As in different countries, vaccine pass- port systems may vary. Sometimes it is difficult and incon- venient to synchronize the vaccination records between two countries. Some vaccine passport systems have certain defects and are not secure enough. Thus, the certificates can be easily forged. People who have not been vaccinated can purchase fraudulent certificates for their own purposes. Such fraud cases have already been reported in many countries includ- ing UK [2], Saudi Arabia [1], Germany [20], etc. Fake vaccine

credentials are even for sale on dark webs. These fake cer- tificates increase the risk of virus transmission and make it more difficult to trace the origin of virus outbreaks, thus lead- ing to unpredictable consequences and threatening public health among nations. 2 Background 2.1 Blockchains Technologies Blockchain is defined as a particular distributed ledger for decentralized data storage and consensus using peer-to-peer networks. It is firstly introduced in Bitcoin whitepaper [12] by Satoshi Nakamoto. By access level, there are three types of blockchains: 1. Public blockchains: the most commonly seen blockchain network open free access to all participants such as Bitcoin and Ethereum. Everyone has access to read and write records on the blockchain. 2. Private blockchains: it is usually privately owned by a single entity. It is not decentralized but a type of dis- tributed ledger. Access rights are only given to people who own an invitation. 3. Permissioned blockchains: a mix of public and private blockchains with more customizable access configu- rations. The one with proper identification verifica- tion can join the permissioned network and execute specific network operations. Hyperledger Fabric is a permissioned blockchain that is widely used in the industry. The project is going to build a system based on blockchain technology. Vaccine certificates will be stored in the data block in a decentralized storage manner. It is more secure and credible for all users and issuers since no third party can modify or delete the data. Unlike the current established blockchain-based vaccination verification system, this new system will build a two-tier blockchain architecture for better performance. It will try to provide an efficient and stable platform for travelers and any related organization. 2.2 Cryptography Tools Hash Digest & Summary. Hashing algorithms take input data with arbitrary size and output a fixed-size hash value. In our system, we use a hash algorithm to generate digested data from personal information, rather than directly expos- ing it, and simplify the vaccine and certificate information in a verifiable manner. A cryptographic hash function must satisfy three criteria: 1. Pre-image Resistance. The reverse of a hash value to get the original data is computationally infeasible. 2. Second Pre-image Resistance. It should be very chal- lenging to find a second input data that produces the same hash value. 3. Collision Resistance. The hash value of two different data should be different.

In our system, we use SHA-256 as our hash algorithm. Digital Signature. Digital signatures [11] are widely used for verifying the authenticity of digital messages or docu- ments. When issuing a certificate, the Issuer needs to sign a transaction and spread it to the network. In our system, we will use the RSA algorithm for implementing digital signa- tures. Proof of Membership. Proof of membership refers to a type of interactive proofs for the statements like 𝑥 𝐿 where 𝐿 is some formal language [16]. In our design, a proof of membership of a batch of certificates during a certain period is generated by the certificate issuer and recorded on a blockchain. Then the certificates in the batch can be verified by checking the proof. Merkle tree is a commonly used proof of membership algorithm and will be used in our system. 3 Objective The goal of this project is to build a decentralized two-tier verifiable blockchain platform for COVID-19 certificate veri- fication that addresses two major issues in the current vac- cine passport system: 1. The difficulty of synchronizing vaccination data be- tween different vaccine passport platforms implemented by other countries. 2. The fraud of fake certificates caused by the security defects of the current vaccine passport system. During the project development, the following objectives were focused on: 1. Develop a secure and end-to-end platform to assist travelers with their vaccine record verification. To this end, we specifically adopt a two-tier blockchain ap- proach. At a high level, we will store in the first (local) tier a considerable amount of information related to vaccination certificates. Periodically, we will generate and upload a concise membership proof of the 1st-tier stored data on the global 2nd-tier blockchain. 2. Implement a user-friendly mobile application. This will be used for presenting digested personal informa- tion during the certificate verification process so that no exact information is revealed. The mobile applica- tion will be used for self-keeping (but not for verify- ing) vaccination records as well. We will develop our application for both Android and iOS platforms. 3. Implement a web-based service. This will be used for uploading vaccine records in an encrypted manner and issuing vaccine certificates for verification pur- poses. 4. Ensure desirable properties of our design and safe- guard against various attacks. For this, we will employ cryptographic components and techniques, develop

VaxPass: Decentralized Two-Tier Verifiable Blockchain Platform for COVID-19 Certificate Verification

asymptotically efficient algorithms, and implement them.

4 Related Work 4.1 Vaccine Verification Platforms A vaccine passport is currently one of the most popular tools for vaccination verification. Before the outbreak of COVID- 19, patients with diseases such as measles, yellow fever, or hepatitis A are required to show their vaccine passports [21] for verification before they can travel around. The vaccine passport will specify whether a person has got a recognized vaccine and the vaccination details. There are also additional health records to store the physical conditions of individuals and trace potential virus transmission. As one of the exam- ples, the European Union launched the EU Digital Covid-19 Certificate for travelers. The certificates are based on QR codes, and valid in both digital and soft copy [18]. This is the most efficient and straightforward way of making and issuing certificates and usually is free of charge. However, people raise concerns about privacy, especially for sensi- tive information such as health records. Security is another limitation while forged digital and paper certificates can be produced. Verifying the authenticity of every certificate is a huge problem.

4.2 Application of Blockchains in the Industry Blockchain technology has become more popular these years, and blockchain-based platforms have several applications across diverse industries. For instance, IBM provides finan- cial services solutions based on blockchain technology. It makes cross-border transactions much more secure and faster by a new digital currency backed against USD [6]. People who live in the poor region could have difficulty reaching a physical, financial, or banking service. By using the dig- ital token, the platform utilizes the safety and speediness of blockchain technology. However, at present, the applica- tion is not promoted to the worldwide population due to a relatively low acceptance of blockchain-based services. Ap- plying a single-tier blockchain structure to record all citizens’ vaccination data will encounter problems of low throughput because of vast amounts of data. Generally, the efficiency of such systems can be lower than expected.

4.3 Blockchain Platform for Vaccination Verification There are some ideas about blockchain platforms for vacci- nation verification. Ricci et al. [14] proposed that blockchain technology can support the fight against COVID-19 with several proper ad- vanced cryptographic techniques. These techniques include a zero-knowledge proof, Diffie Hellman key exchange pro- tocol, blind signatures, and proxy re-encryption. Although this paper has proven how security can be ensured with the

above technologies, it still has limitations. It did not discuss the efficiency and efficacy in the real-world scenario. Androulaki et al. [5] also proposed a privacy-respectful platform with a decentralized Trusted Registry that can pro- vide the health status of the user. In their idea, the traditional Public Key Infrastructure with digital signature is integrated with Distributed Ledger Technology (DLT). A registry needs to be administered by more administrative authorities like the government’s health department. Different authorities have their own DLT node in the trusted registry, based on Hyperledger Fabric. This idea can guarantee that the registry can always take correct action, though the efficiency of a one-level ledger may not be high enough for daily usage. Hernández-Ramos et al. [10] provide a case study and per- formance evaluation in the use of blockchain for sharing pandemic vaccination certificates in European countries. A permissive blockchain stores vaccination information for EU countries, and each country has a node. This blockchain is deployed by Hyperledger Fabric. Different ordering services have been tested, including ZooKeeper, Kafka, Orderer, Peer, Smart Contract, etc. Their results indicate that their use of the EPIC platform is reliable under stress. Their study also shows that the Orderer node has fewer CPU requirements than the MS node, and peer nodes require more memory. However, their case study did not provide more discussion about security and privacy and failed to consider the adver- sarial scenario that a one-tier blockchain would take more time to issue and verify certificates when the blockchain is packed with information of a large population. There also exists a public blockchain approach for vaccine certificates [19]. In a public blockchain, more than 50 % of malicious nodes would be needed to attack the blockchain successfully. Vaccine data would be stored in the distributed ledger. Actors of this system are beneficiaries that register for vaccination, manufacturer, IoT sensor monitoring devices, medical centers, and doctors. The certificate issuing proce- dure is based on the Asymmetric Key Cipher. But the system mainly focuses on transparency of delivery and integrity of the vaccines and has its limitations in vaccine verification for cross-border traveling. The high cost of implementing multi- ple IoT devices also makes it impractical. Another approach is to use a private permissioned Blockchain and off-chain IPFS storage to construct a system [3]. Issuer, holder, and verifier will be authorized by CA to access the blockchain by their private key. The off-chain IPFS storage will be used to store personal data like vaccination details. And the hashes of the IPFS storage will be held on the chain to provide verifi- cation service. Although this architecture ensures high-level privacy requirements, individuals must set up nodes that make IPFS not user-friendly. High bandwidth consumption also increases the cost of implementing this architecture. Eisenstadt’s solution uses a hybrid system combining a Con- sortium Blockchain and decentralized personal data plat- form' Solid' [9]. A W3C standard' Verifiable Credentials'

is adopted for digital certification. This approach, however, takes around 20 seconds to issue or verify a Server Hash and local Hash when parallel requests increase to 100, which makes it impractical for vaccine passports. Here comes an opportunity for a decentralized blockchain that can be deployed to construct a vaccine passport system based on a two-tier blockchain architecture, which would not only potentially increase the efficiency of certificate issuing and verification but also ensure the integrity of the certificate and the confidentiality of personal information. In this report, we will propose the first implementation of such a system.

Figure 2. Size of the Bitcoin blockchain from Jan. 2009 to Feb. 2022 by Statista

5 Design Our system is designed to provide a secure, immutable, and efficient COVID-19 certificate platform that helps the preven- tion of COVID-19 certificate frauds, international verifica- tion of the certificates, and the protection of personal private information. This section describes how the system uses a two-tier blockchain structure for recording and verifying vaccination and certificate information and improves effi- ciency compared with the traditional one-tier blockchains. 5.1 Challenges Blockchain-based platforms usually have scalability issues. Data Storage Limitation A blockchain is an endless ledger. Its integrity is en- sured by the availability of tracing records on the chain nodes from the origin of the blockchain, the Genesis Block, to the latest validated blocks. As of 2022, the complete copy of the Bitcoin blockchain oc- cupies up to 380 GB of storage space [8], which has already become a fierce issue for maintenance and transfer. See Fig 2 for detailed information. As a result, it is not ideal for storing all the detailed information on a single blockchain.

Low Transaction Throughput For the operation of a typical blockchain, a certain amount of time is needed for all the nodes in the sys- tem to reach a consensus on the validity of a newly generated block. Thus, a limited number of trans- actions on the block can be confirmed during the time interval. Currently, many well-known public blockchains have low TPS (transaction per second) issues. For instance, Bitcoin has a TPS of less than 10, and Ethereum has a TPS of around 30. 5.2 Actors in System In our system, there are four types of actors: Certificate Association, Issuer, Verifier, and Holder. 1. Certificate Association (CA) can be healthcare in- stitutions, and governments from different countries. They are a group of authorities that manage the oper- ations of the whole system. They are responsible for authorizing new Issuers to the system, removing exist- ing Issuers, and adding new members to the Certificate Association. Furthermore, they also decide what type of certificates can be issued. 2. Issuer can be healthcare providers like hospitals and test/vaccine centers which are authorized by CA. They are responsible for issuing certificates such as vaccina- tion certificates, test certificates, and recovery-from- Covid19 certificates. Ideally, they will not spread fraud information to the system. 3. Verifier can be individuals or organizations that need to check the health status (e.g., vaccination records, nu- cleic acid amplification testing results) of the Holders. They perform verification of the certificates owned by the Holders. 4. Holder is the individual that possesses the certifica- tions. They must show their identity in the system when verifying their health status. 5.3 Architecture The system makes use of a two-tier verifiable blockchain architecture for certificate issuing and verification. The first tier is a global permissioned blockchain that only records the issuing actions. The second tier is local blockchains main- tained by different health care providers for storing detailed information about the vaccines and certificates. When re- quiring a certificate, Holders do not need to expose their personal information to the remaining of the system. The architecture overview is shown in Fig 2. 1. The First Tier: Global Chain The Global Chain is a permissioned blockchain man- aged by Certificate Association across different coun- tries. It is only used for recording the issuing actions. Detailed contents of vaccines or certificates are not directly stored on them. Only hash digested values

VaxPass: Decentralized Two-Tier Verifiable Blockchain Platform for COVID-19 Certificate Verification

Figure 3. Architecture overview

of them will be written on the chain. As it is a per- missioned blockchain, different actors have different permissions to operate with the chain.

CA Issuer Verifier Holder Read Y Y Y N Write Y Y N N Table 1. Global Chain Permissions

Members in the certificate association run the full nodes of the Global Chain network. They keep the whole copy of the blockchain data. Issuers, authorized by CA, can join the network with light nodes and mainly perform new transaction generation. We will finalize our consensus protocol after the per- formance evaluation and benchmark. As it is a permis- sioned blockchain, protocols with better scalability and higher throughput such as Proof-of-Authority (PoA) can be considered. 2. The Second Tier: Local Chains The second tier is blockchains that are maintained by different health care providers. They are used for stor- ing the detailed information of the vaccines provided by vaccine producers and the certificates issued by the Issuers. 3. Personal Information Personal information is kept privately by the Hold- ers. It will not be recorded on Global Chain or Local Chains. 5.3.1 Operations. 1. Issuer Registration Issuer registration is the process that Certificate As- sociate authorizes a healthcare provider to join the Global Chain network and write new transactions to the chain and become an Issuer (shown in Fig 4). After an Issuer candidate requests registration, the Certificate Associate will investigate the candidate’s background and evaluate their qualification for the role. More than half of the members should approve the request before issuing the registration certificate to the candidates. When the candidate obtains the certificate, it becomes an Issuer. It needs to generate an asymmetric key pair for a digital signature on the Global Chain. Now it can join the network with self- maintained nodes and perform the role of an Issuer. 2. Certificate Issuing When being issued a certificate, a Holder must sub- mit their unique ID in the system and hash digested personal information as the unique identity to the Issuer. Then the Issuer retrieves vaccine information and records the certificate to be issued to Local Chains. Finally, the Issuer composes and signs a transaction containing a hash summary of the certificate infor- mation and Holder’s unique identity and records it to Global Chain. After recording the transaction, the Issuer sends the certificate information to the Holder to keep in the client software. 3. Certificate Verification To verify the certificate, the Holders need to provide the same unique identity and the transaction hash that records the certificate issuing. The Verifier checks the transaction on Global Chain and verifies the certifi- cate information via the hash summary from on Local Chains.

Figure 4. Issuer Registration

Figure 5. Certificate Issuing

6 Implementation 6.1 Two-tier Blockchain Architecture

Figure 6. Certificate Verification

very convenient for customization and adding new features. At this stage, we have finished the Local Chains’ smart con-

Global Chain. The Global Chain is designed for certifi- cate verification aboard. In this stage, we have designed a smart contract for the global chain to store the Merkle Tree root that proves the membership of the certificates from the Local Chain whenever they have been issued.

Local Chain. We choose Hyperledger Fabric [4] as our blockchain framework. It is a commonly used enterprise- level permissioned blockchain with outstanding performance, scalability, and security. Its modular architecture makes it

tracts, allowing issuers to create, read, and verify certificates on the local chain. We also connect these functions to the Golang backend server by using packages including Hyper- ledger Fabric SDK, gin, etc. Frontend and the client applica- tion can access these APIs to issue and verifier certificates on the local chain. Interoperability between Global and Local Chains. Local Chains are permissioned blockchains that regulate different parties with different access levels. When an Issuer creates a certificate on the Local Chains, the Local Chains

VaxPass: Decentralized Two-Tier Verifiable Blockchain Platform for COVID-19 Certificate Verification

should automatically generate a certificate on the global chain. When Verifiers want to access the Global Chain, they can use their existing credentials to access them on Global Chain. There are two possible solutions for this property: 1. Weaver DLT with Multi-party Trust Weaver DLT [14] is a tool developed by IBM to intro- duce Interoperability between different permissioned networks. It enables multi-party trust to the blockchains, which can secure the two-tier blockchain and avoid putting too much trust on a single host. To implement Weaver DLT, we need to implement the weaver SDK helper, Identity services, and drivers in the backend application. We need to develop fabric interpretation smart contracts. We also need to implement relays in infrastructure. 2. Modify existing backend API with Single-party Trust A simpler but more risky solution is to enable single- party trust. That is to allow one node in the Local Chain to be a node in the global. When this node receives a request for creating certificates on the Local Chain, it automatically updates the Merkle Tree roots on the Global Chain. 6.2 Backend Services We choose Golang as our backend development language because of its simplicity, cross-platform support, and out- standing performance. More importantly, many decentral- ized applications are now developed in Golang, including Ethereum, IPFS, and Hyperledger Fabric. There are plenty of valuable and convenient frameworks and libraries such as libp2p, ZeroMQ, and gRPC written in Go to accelerate our de- velopment. We have implemented a Golang backend server using Gin for blockchain-related APIs and a PHP Backend ap- plication for login/register process for holder/issuer/verifier. As shown in Fig 7a, this time sequence diagram represents the progress accomplished by now. Issuers interact with the web frontend to create certificates on the Local Chains. The next step is to allow the Local Chain to automatically create a digest of certificates on global chains. As shown in Fig 7b, the verifier can access backend API to verify certificates on-chain. Database. Blockchains are performed as the central databases in the system. In addition, we need other kinds of databases for storing data used by client software. MySQL is chosen for storing strongly relational data. At this stage, we have already established a MySQL database for storing credentials for holders, issuers, and verifiers of the certificates. Digital Certificate. During the Issuer registration pro- cess, a candidate needs to obtain the certificate generated by CA to join the Global Chain network and become an Issuer. One commonly used approach for this digital certificate is X.504 certificate. It can bind the Issuer’s identity with its

public key. So, for the Issuer registration, Certificate Associ- ation (CA) signs an X.504 certificate to bind the candidate’s identity with its public key. Then the Issuer signs the trans- action (certificate issuing record) with its private key, and the transaction can be verified by the public key and the certificate. Containerization. All the node software will be released as Docker images and executed as Docker containers orches- trated by Kubernetes. Containerization applications are easy to deploy, upgrade, and maintain. It is also very convenient to run replicas of containers or different containers on a single machine. Thus, a machine can run different node containers for performing various roles in the network. 6.3 Web Client for Issuer, Verifier, and CA A prototype of web client side for issuers and verifiers have been implemented. The website is implemented with PHP and JavaScript. Currently, both functionality of issuing and verifying are integrated in the same website for easier testing purposes. Before accessing the web client-side, the users have to log in first. That means only recognized individuals or or- ganizations can access it. At the current stage (Fig 8, Fig 9), there is a simple register and login system. According to our previous design, the Certificate Association (CA) will create and initialize accounts for issuers and verifiers. The register system will pass the account data via PHP. The username and password of the accounts and relevant information are stored in the MySQL database we created. The issuers and verifiers can use the created account to log in to the web client. The users will see the home page after logging in Fig ??. This is the home page of the web client. The username will be shown in the top-right corner. Issuers and verifiers can click on the buttons on the top bar. Under the 'ISSUE' tab, the issuers can see the above page. They can input the following information of the vaccina- tor after the vaccination: certificate id (vaccine or testing id number), their name, personal ID number, the brand of vaccine, the number of current doses, vaccination time, and any remarks. The certificate is packed into a JSON object. By calling the backend API, the certificate will be sent to the 'local' blockchain, the second tier of blockchain. A QR code is also generated, which contains the detailed information of the uploaded certificate for now. This is for testing the reading function of the verifier. Currently, the verifying method is uploading the QR code image in PNG format. The detailed certificate information will be shown on the verifiers’ web page. The basic logic flow of issuing and veri- fying certificates of web clients has been completed. In the future, the web client side will work more closely with the backend and mobile client to enhance security levels.

(a) Create Local Certificate(b) Verify Global Certificate Figure 7. Time Sequence Diagram

6.4 Mobile App for Holder As smartphones are getting very popular these days, we assume that most people have a smartphone with a camera. Users will not be required to input any information. Holders only need to scan the QR code once they receive the paper of vaccination certificate. This allows our system to have a convenient user experience. User Interface. The user interfaces of the mobile client are shown in Fig 14. There will be a tab for the informa- tion page and scanning the QR code. When the QR code is released by the Issuer of the health authority officer, the trav- eler needs to check the code using the 'scan' function. Access to the camera needs to be granted by the traveler. After scan- ning the QR code, the client will analyze the information and store it on the mobile phone. Cross Platform. We use Flutter and Dart language to build our mobile App compatible with Android and iOS systems. Local Data Storage. To prevent data leaks, our Mobile APP client will only send the UID of people to the backend server or display in the QR code. The user’s personal in- formation will only be saved on the mobile phone using a symmetric key cipher. The key will be the password that the user input during registration. The two-tier blockchain will only need to know the UID of the holder. All personal information will be lost if the APP is uninstalled, including the UID. To regain the vaccine passport, our user will need to bring their certificate document issued by local authorities to

the issuers of their local community and go through the reg- istration process again. Old vaccine passports on the chain will not be erased because the vaccine passport depends on the UID.

6.5 Testing Software testing is a very crucial procedure for delivering high-quality and bug-free applications. And it should be cov- ered through the entire SDLC (Software Development Life Cycle). We have finished configuring CI/CD pipelines for all the code repositories to automate the testing phases, includ- ing building tests, unit tests, Linter checking, and code safety scanning. And we are working on conducting larger-scale tests, including API tests, system tests, Chaos Engineering, etc.

7 Discussion 7.1 Consensus Algorithms Public or permissionless blockchains allow anyone to par- ticipate in the network without credentials, so they should adopt consensus algorithms with economic incentives such as Proof-of-Work (PoW) and Proof-of-Stake (PoS). However, permissioned blockchains like Hyperledger Fabric require each entity in the network to be known and identified. Thus they can use traditional Byzantine-fault tolerant (BFT) con- sensus algorithms [4]. And thanks to the modular consensus approach of Hyperledger Fabric, we can quickly implement various consensus algorithms such as RAFT, PBFT, etc. As

VaxPass: Decentralized Two-Tier Verifiable Blockchain Platform for COVID-19 Certificate Verification

Figure 8. Registration system for is- suers and verifiers

Figure 11. Form for issuing vaccine certificate

Figure 9. Login page for issuers and verifiers

Figure 12. Example of certificate in JSON format and generated QR code

Figure 10. Home page for issuers and verifiers

Figure 13. Verifying QR code to show certificate information

a one-size-fits-all consensus algorithm is hard or even im- possible to find [17], we shall try out and benchmark many consensus protocols and choose the most suitable one in our use case.

7.2 Certificate Issuing & Verification The current design utilizes Merkle Tree as the proof of mem- bership for certificate issuing and verification. Many other methods, such as vector commitments [7], that can accom- plish the desired functionality can be further implemented.

7.3 Encryption In the current design, Local Chains store the plaintext of the detailed information of the certificates. However, the plaintext can be easily compromised by an adversary. To mitigate this risk, encryption methods such as symmetric and attribute-based encryption can be applied and tested in the future. 8 Conclusion Vaccine passport systems are adopted by many countries to fight against COVID-19 pandemic and prevent the resur- gence of the virus. As the systems used by different countries may vary, it is difficult to synchronize the vaccine certifi- cate information between multiple countries. In addition, security defects in the traditional implementations of these systems made fraudulent certificates possible, which causes a significant hazard to public health. In this report, we propose the design and implementation of VaxPass, an end-to-end decentralized blockchain-based vaccine passport verification system, helping to make the vaccine certificate issuing and verification process more se- cure and reliable around the globe. The two-tier verifiable blockchain structure of the system ensures the layered data isolation, easiness of information synchronization, and good maintainability of the distributed ledger shared among dif- ferent countries. We hope the design of our project can contribute to the fight against the epidemic and inspire more innovations of applications of blockchain technologies in the real world.

Acknowledgments

Figure 14. Mobile App UI

[5] Elli Androulaki, Ilie Circiumaru, Jesus Diaz Vico, Miguel Prada,

We would like to acknowledge and give our warmest thanks to our supervisor Prof. Dimitris PAPADOPOULOS, who gave us the wonderful opportunity to work on this project. His willingness to give his time so generously has been very much appreciated. We wish to acknowledge the help of Mr. Vlasis KOUTSOS, who has provided us with continuous support and guidance on the project. Finally, we would like to express our great appreciation to all the frontline healthcare workers for making sacrifices to commit to the fight against the COVID-19 pandemic.

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VaxPass: Decentralized Two-Tier Verifiable Blockchain Platform for COVID-19 Certificate Verification

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