Improving the trust model of Self-Sovereign Identity on blockchain

In the digital world, when users want to prove something about their identities (e.g., age, or degree grade), they need to provide pictures of physical documents (e.g., ID cards, or degree certificates). However, these documents may reveal on the identity of the users more than what the users want to (e.g., the ID card reveals the birthdate, but also the home address). Moreover, a malicious user can reuse those pictures, so to pretend to be another user. Self-Sovereign Identity, together with the blockchain technology, gives back to the users the full control over the information they share about their own identities. In this case, when the users want to prove a claim about their identities to another entity (called verifier), they send a Verifiable Credential, which is tightly tied to them and cannot be reused by any other user. The verifier uses publicly-available information stored on the blockchain to establish the validity of that credential, but can accept the credential only if the entity that has issued it (called issuer) is trusted. This trust is often established by means of invitations, and requires the verifiers to obtain the invitation from each of the issuers they want to trust. In this thesis, we propose an extension of the current trust model, allowing a verifier to trust an issuer without obtaining any invitation from them, provided that another entity, trusted by the verifier, trusts (directly or indirectly) the issuer, effectively creating a chain of trust.