  During the normal operation of a Cloud solution, no one usually pays
attention to the logs except technical department, which may periodically check
them to ensure that the performance of the platform conforms to the Service
Level Agreements. However, the moment the status of a component changes from
acceptable to unacceptable, or a customer complains about accessibility or
performance of a platform, the importance of logs increases significantly.
Depending on the scope of the issue, all departments, including management,
customer support, and even the actual customer, may turn to logs to find out
what has happened, how it has happened, and who is responsible for the issue.
The party at fault may be motivated to tamper the logs to hide their fault.
Given the number of logs that are generated by the Cloud solutions, there are
many tampering possibilities. While tamper detection solution can be used to
detect any changes in the logs, we argue that critical nature of logs calls for
immutability. In this work, we propose a blockchain-based log system, called
Logchain, that collects the logs from different providers and avoids log
tampering by sealing the logs cryptographically and adding them to a
hierarchical ledger, hence, providing an immutable platform for log storage.
