Trillian
A transparent, highly scalable and cryptographically verifiable data
...Common use cases include certificate transparency, package registries, and audit logs where public verifiability or tamper evidence is required. Trillian exposes both “log” and “map” primitives so developers can choose between append-only timelines or verifiable dictionaries depending on their data model. By making verification independent of trust in the operator, trillian helps build systems that are auditable by external parties.