# Blockchain and Self Sovereign Identity to Support Quality in the Food Supply Chain
## 2 Review
### Food Certifications as Verifiable Attestation  
Our SSI model aims to provide more transparency and traceability in the supply chains giving to users/participants free access to food/process certifications issued by authorized entities, which regularly monitor and inspect supply chain nodes, issuing, revoking, and updating the certifications to ensure regulatory compliance.  
Certifications are hence, within this system, verifiable attestations, and as such, they can be modeled following the indications of the EBSI that proposes the format for Verifiable ID and Attestation signed with a DID Key shown in Figure 2. This format is in according to the DIDs standard developed by the W3C.  
So a Verifiable Credential is a file (The recommended formats for credentials are JWT, JWS, and JSON-LD.) containing the following information (ref. work by [PERSON] [25]):  
* The context defines URI to identify terminology and protocols that allow parties to read the credential. It enables different systems to exchange data with each other.
* URI to identify the credential type.
* URI to identify the issuer.
* Issuance date of the claim.
* URI (URI stands for Uniform Resource Identifier.) to uniquely identify the credential and/or the subject of the credential.
* Cryptographic proof of the issuer.  
The W3C specification provides that the data model can be extended. The following features can be added as indicated in the work by [PERSON] [25]:  
* Claims data or metadata.
* Expiration conditions for defining when the claim is no longer valid.
* Location of the credential status.  
In general, in an SSI model, it is recommended that the individuals/holders use private repositories/digital wallet to store and manage all their credentials to guarantee data protection requirements. Since our system aims to be as transparent as possible the off-chain model was chosen to meet only transactionability and storage requirements. The database chosen for storing the food/process certificates is the IPFS, that is publicly accessible. The blockchain is queried for verification of the certifications. The subject and issuer DIDs can be found and resolved in the blockchain registries, as well as the credential status (certification status) and the identifier of the certificaton. The certification identifier, that allows to access its location, as we are going to see in the next sections, is the IPFS hash. In fact, when uploading a document on IPFS, the system directly computes and returns the hash of the document and use it as a unique identifier to retrieve the document. We implemented the proposed system using the Ganache tool to simulate a local Ethereum blockchain so the queries for the verification of certifications do not generate transactions, hence costs. In the future, when there will be complete and functioning SSI models, less data may be stored in blockchain, as suggested by EBSI and [PERSON] [25].  
Figure 2: Format for Verifiable ID and Attestation signed with a DID Key.  
All claims data, including the food/process certifications data, could be stored in the same registry.