Why indy/sovrin stores claims/verifiable-credentials/attestations in ledger

Question

Videos and documentation always say claims are stored in the ledger but only the "credential definition" or "Schemas and claim definitions". Is this really needed?

Answer 1

• As Verifiable Credentials typically contain private data, verifiable claims are not stored on ledger.

• Credential Schemas are written on the ledger. They don't contain any private data and only specify what certain type of document contains. For example, it might specify that a driving licence document should contain name, surname, date of birth of the holder, plus classes of vehicles the holder is eligible to drive. Schema is simply a template according to which an issuer may issue verifiable claims.

• Credential Definitions are also written on the ledger. It also doesn't contain any private data. Credential definition is linked to 1 Credential Schema (which must already exist on ledger). Credential Definition contains public key which an issuer uses to issue Verifiable Claim of certain type.

Example for context

• Government decides that every university diploma in the country will have following fields: Name, Surname, University name, Program, Degree. They write Credential Schema Diploma:1.0.0 on the ledger. Anyone can see how diploma looks like. Universities can use it to issue diplomas (as verifiable credentials) for students.
• Faber Uni. is university in the country. It wants to issue university degrees according to government specification of how diploma should look like. So Faber Uni writes on ledger credential Definition Faber:Diploma:1.0.0. This credential definition also contains public key 123 of Faber Uni for issuing credentials of this type (Faber Uni will safely keeps somewhere private part 456). Each time Faber Uni issues a student diploma as verifiable credential, it will sign the data using the private part of the key, 456.
• When Faber Uni issues diploma for its graduate Bob, there's no trace of this event on the ledger. Faber Uni simply signs a document conforming the Diploma:1.0.0 schema which is on the ledger, using private key 456, whose public counterpart 123 is on the ledger.
• Now Bob wants to get a job at Abcd and is asked to prove his education. He submits the credential issued by Faber Uni, which he safely stores on his device. Abcd could read from the ledger Faber Uni's diploma signing key and use it to verify signatures on the verifiable claim presented by Bob.