  Self Sovereign Identity (SSI) is an emerging identity system that facilitates
secure credential issuance and verification without placing trust in any
centralised authority. To bypass central trust, most SSI implementations place
blockchain as a trusted mediator by placing credential transactions on-chain.
Yet, existing SSI platforms face trust issues as all credential issuers in SSI
are not supported with adequate trust. Current SSI solutions provide trust
support to the officiated issuers (e.g., government agencies), who must follow
a precise process to assess their credentials. However, there is no structured
trust support for individuals of SSI who may attempt to issue a credential
(e.g., letter of consent) in the context of business processes. Therefore, some
risk-averse verifiers in the system may not accept the credentials from
individual issuers to avoid carrying the cost of mishaps from potentially
inadmissible credentials without reliance on a trusted agency. This paper
proposes a trust propagation protocol that supports individual users to be
trusted as verifiable issuers in the SSI platform by establishing a trust
propagation credential template in the blockchain. Our approach utilises (i)
the sanitizable signature scheme to propagate the required trust to an
individual issuer, (ii) a voting mechanism to minimises the possibility of
collusion. Our implementation demonstrates that the solution is both practical
and performs well under varying system loads.
