# Privacy-Preserving Solution for European Union Digital Vaccine Certificates
## 9 Discussion
Even if smartphones and smartwatches are not considered tamper-resistant  
Figure 12: Application performance depending on the number of attributes on smart card platformdevices like smart cards, they provide sufficient security protection for user data thanks to the separation of applications, processes, and data (use of application sandbox) on the operating system level. That is why they are used in a number of security applications, such as bank or e-government applications. It is true that the acquisition costs of a smartphone are higher than the acquisition costs of a smart card, but if we take into account that most citizens already have a smartphone, this shortcoming is negligible. Although our solution provides high security and privacy protection (see the security analysis in Section 6), we see also other possibilities to continue our work. This is primarily a further increase in the protection of citizen privacy and the decentralization of solutions for the support of cross-border Information and Communication Technologies (ICT) systems. Namely, our solution protects user privacy by disclosing only some information from the digital vaccine certificates. Furthermore, the disclosure of this information is also directly confirmed by the user (i.e., the certificate holder). All transmitted data are encrypted with end-to-end encryption (between the user and the verifier). Encrypted communication and randomized cryptographic proofs make it impossible for an eavesdropper to access data and profile users. Most of the disclosed attributes from the certificate have a general character and, therefore, do not directly lead to user profiling by the verifier (i.e., the service provider). However, the attribute of the user photo has a unique character and has to be always disclosed to allow a visual identification of the certificate holder. Thus, it allows the verifier to track the users (albeit anonymous users) within its services. To eliminate this risk, technologies such as Fuzzy extractors [29] deserve to be investigated. On the other hand, the blockchain technology (i.e., a public verifiable open ledger) and smart contracts (i.e., programmable logic programs integrated into the blockchain) have emerged as promising tools for devising decentralized models to securely settle arbitrary resource transactions such as digital vaccine certificates. This technology holds the potential to eliminate centralized entities involved in certificate management throughout their lifecycle, while ensuring the immutability, traceability, and availability of the involved assets. For instance, storing credential revocation schemes on the ledger would ensure high availability and maintain an immutable record of revoked certificates. Finally, our solution assumes the personal identification of certificate holders based on the presented photo (i.e., revealed attribute) from the digital certificate. However, the identification of the certificate holder itself is up to the inspecting person and is therefore subjective in nature. In order to guarantee the defined accuracy of the correct identification of certificate holders, it would be interesting to incorporate some of the biometric authentication methods into our solution.