# Privacy-Preserving Solution for European Union Digital Vaccine Certificates
## 5 Privacy-Preserving Solution for EU Digital Vaccine Certificates
### System Architecture  
* **Issuer/Revocation Authority (I/RA)**. This is the entity responsible for issuing individual attributes to the end-users. To do so, it runs the Issue protocol. The I/RA signs all issued attributes with its private key. We suppose that the I/RA is entrusted with the authority to process the users' personal data. * **Verifier (VER)**. This is the entity responsible for verifying the ownership of required attributes by users who are interested in using the service. If revocation mechanisms are implemented in the system, the VER also verifies the revocation status of the presented certificates. Certificates are verified using the Verify protocol. In order to verify the attributes, the VER must possess the I/RA public key. * **User (USR)**. This is the entity that holds the attributes issued by the I/RA and anonymously provides proof of their ownership to the VER in whose services it is interested. For this purpose, the USR runs the Show protocol. Our cryptographic core is based on using the Fast Keyed-Verification Anonymous Credentials (FKVAC) [21]. Similarly to the FKVAC scheme, our scheme employs the weak Boneh-Boyen (wBB) [23] digital signatures to create the cryptographic credentials over user attributes. In contrast to the FKVAC scheme, our scheme offers to separate the cryptographic keys of the VER and the I/RA, resulting in an asymmetric cryptographic scheme. This way, it can be used in the application scenario of EU Digital Vaccine Certificates, where the VER and the I/RA are distinct entities which have different rights to the users' data. All this is performed, while ensuring the same (i.e., low) computing, memory, and cryptographic requirements on the user side (where users have an authentication device in the form of a smartphone or smart card). The system architecture of our solution is shown in Figure 1.  
The communication model for presenting the vaccination certificate is shown in Figure 2. The USR is equipped with a smartphone capable of Bluetooth Low-Energy (BLE) communication. The service provider (acting as the VER) broadcasts the BLE advertisement (named, for example, PPCOVIDPASS). If the USR is interested in using the services of the service provider, it initiates a search and connection process through the BLE advertisement. The USR's mobile app will then connect to the VER app, and both apps will exchange  
Figure 1: System architecture of the privacy-preserving solution for EU Digital Vaccine Certificates. verification data. Namely, the VER sends the verifier certificate with the verifier public key and required attributes for disclosing and cryptographic nonce. Note that the verifier certificate is signed by the I/RA, which is trusted by both communication parties. The USR's app verifies the validity of the certificate, i.e., the validity of the generated I/RA signature. If the signature is valid, the USR decides whether to accept or decline the attributes required by the VER. If the user approves the attributes, the app sends an encrypted USR's photo (one of the user attributes), other required attributes (such as, for example: Dose number, Date of Vaccination), and unencrypted proof of knowledge of hidden attributes. In case of successful verification of the presented attributes and identification of the person presenting these attributes by the VER, the USR is granted access to the service; otherwise, it is denied. The detailed structure of the digital vaccine certificate, including the representation of individual attributes, is shown in Figure 3. The cryptographic credential contains a total of 20 attributes. However, it can be expanded if it is needed. The first 16 attributes correspond to the attributes from EU Digital Vaccine Certificate. The following attributes extend the certificate by (1) a photo of the certificate holder enabling the connection of the holder with the certificate, (2) time stamp, (3) information about the completed vaccination (i.e., yes, no), and (4) one optional field according to the needs of the service provider.