## 4 The System Proposal: An Introduction
### ABCDE Method: On-Chain Subsystem
#### 4.3.2 Format for Verifiable Files  
The system proposal assumes that both the typologies of files that stem from the mechanisms of notarization and certification, mentioned above, can be exchanged using secure and protected digital channel and using the suggested formats by EBSI, such as JSON-LD. This format is also in accordance with the DIDs standard developed by the W3C. In Figure 5, an example of Verifiable ID in Json-ld format signed with a DID Key is shown, the definition of its properties, such as Contexts, Identifiers, Types, Issuers, Credential Subject, Issuer, Issuance Date, Expiration, Status and Proofs, follows. It is extracted from the EBSI's technical specification in web site [[https://ecas.ec.europa.eu/](https://ecas.ec.europa.eu/)]([https://ecas.ec.europa.eu/](https://ecas.ec.europa.eu/)) (accessed on 8 April 2022). * _Contexts: JSON-LD contexts define the terminology used to describe data, in order to ensure that both producers and consumers of data have a shared understanding of the semantics;..._
* _Identifiers: The W3C specification requires the use of URIs to unambiguously refer to a person or other subject of a Verifiable ID or Attestation. ESSIF further constrains this by requiring the use of Decentralized Identifiers (DIDs) for both the issuer and Credential Subject;_
* _Types: The W3C specification requires at a minimum a single type to be used (VerifiableCredential) and allows additional types. ESSIF defines the following additional types. EssifVerifiableID: To be used in the case of an ESSIF Verifiable ID. EssifVerifiableAttestation: To be used in the case of an ESSIF Verifiable Attestation;..._
* _Issuer: The Issuer of an ESSIF Verifiable ID or Verifiable Attestation is a public institution, company, or other organization that is a trusted source of claims of a certain type. For example, a university may be an Issuer for Verifiable Attestations of type DiplomaVerifiableAttestation. A bank may be an Issuer for a credit score. An employer may issue confirmations of employment. A government may be an Issuer for Verifiable IDs. In ESSIF, Issuers of Verifiable IDs and Verifiable Attestations are identified by DIDs;_* _Credential Subject: The Credential Subject is the individual (or organization, thing, animal, etc.) described by a Verifiable ID or Verifiable Attestation. In ESSIF, Credential Subjects of Verifiable IDs and Verifiable Attestations are identified by DIDs;_
* they are semantic statements that express a certain value for an attribute of the DID Subject. For example, if an attribute is placeOfBirth, then a claim could be \"The Credential Subject identified by did:cbsi-eth:00000001 has an attribute placeOfBirth with the value Vienna\";_
* _Issuance Date:...Even though this property is called issuanceDate, it actually indicates the date and time when the credential becomes valid. This may or may not be the same as the date and time when the credential is issued;_  
_Expiration Date: The Expiration Date property (expirationDate) has the purpose of expressing expiration information of a Verifiable ID or Attestation, i.e., if and when it expires. If present, the value of this property must be a string value of an combined date and time string representing the date and time the credential ceases to be valid;_  
_Status: The Status property (credentialStatus) has the purpose of allowing the discovery of information about the current status of a Verifiable ID or Attestation, such as whether it is suspended or revoked. The value of this property must include a) the id property (which must be a URL) and the b) type property, which expresses the credential status type (also referred to as the credential status method);_
* _Proofs: The W3C specification requires at least one proof mechanism to be used for Verifiable IDs or Attestations, and allows multiple proof mechanisms.