  Logs contain critical information about the quality of the rendered services
on the Cloud and can be used as digital evidence. Hence, we argue that the
critical nature of logs calls for immutability and verification mechanism
without the presence of a single trusted party. In this paper, we propose a
blockchain-based log system, called Logchain, which can be integrated with
existing private and public blockchains. To validate the mechanism, we create
Logchain as a Service (LCaaS) by integrating it with Ethereum public blockchain
network. We show that the solution is scalable (being able to process 100 log
files per second) and fast (being able to "seal" a log file in 23 seconds, on
average).
