  The COVID-19 pandemic has recently emerged as a worldwide health emergency
that necessitates coordinated international measures. To contain the virus's
spread, governments and health organisations raced to develop vaccines that
would lower Covid-19 morbidity, relieve pressure on healthcare systems, and
allow economies to open. As a way forward after the COVID-19 vaccination, the
Vaccination certificate has been adopted to help the authorities formulate
policies by controlling cross-border travelling. To resolve significant privacy
concerns and remove the need for relying on third parties to maintain trust and
control the user's data, in this paper, we leverage blockchain technologies in
developing a secure and verifiable vaccination certificate. Our approach has
the advantage of utilising a hybrid architecture that implements different
advanced technologies, such as smart contracts, interPlanetary File System
(IPFS), and Self-sovereign Identity (SSI). We will rely on verifiable
credentials paired with smart contracts to implement on-chain access control
decisions and provide on-chain verification and validation of the user and
issuer DIDs. The usability of this approach was further analysed, particularly
concerning performance and security. Our analysis proved that our approach
satisfies vaccination certificate security requirements.

Cryptography and Security