Publish/subscribe is a scalable communication method for one-to-many communications. Centralized Pub/Sub protocols rely on a centralized broker that is a trusted third party managing the subscriptions. To prevent data alteration by the broker, or by an entity outside of the system, the literature proposes E2E security solutions. The issue with this solution is that they all introduce a third party who is hardly trustable in practice. Recently, Pub/Sub protocols have been proposed using the blockchain to manage the subscriptions and provable data deliveries. In this paper, we focus our work on one publish/subscribe protocol, called SUPRA, that uses the blockchain only when necessary. We explain why the current version of the protocol does not respect the publish/subscribe paradigm because a subscriber has to encrypt and sign each data as many times as there are subscribers. Then, present how to update the protocol in order for each data to be encrypted and signed only once, regardless of the number of subscribers, while keeping the security properties of the protocol. The result is a publish/subscribe protocol with strong delivery guarantees and traceability that can be used to share sensitive data.