Logs from a system are used to report what has happened on that system over time. As the importance of a system grows, so does its logs. For example, if a system handles privileged access to some very important data, its access logs could be used to audit who has accessed what and when. Other examples would be the logs of a flying system, a life-support system or a delivery robot, where their logs are critical to their domain-specific behaviors, and a way to keep them safe in a tamper-proof manner and enable future examinations is much needed. Blockchain is a fast-growing distributed technology that enables us to prove the existence of digital assets without a central authority. Public chains, such as Bitcoin and Ethereum, are popular blockchains with nodes widely distributed across the globe, where their transaction histories are highly unlikely to be manipulated by any particular individuals or organizations, so information posted on them can be viewed as an immutable ground truth. …