Let's start to explore Cryptography. It's a very good project for the coders who would like to take the cryptography security concept away and bring it to their projects. The HelloCrypto lucky draw system with practice of the java keytool can be used by any organisation for their events.
As the data could be increasing dramatically, housekeeping batch is required to clean up those data which is no longer used.
Ideally, propose to implement housekeeping batch as the independent application with Spring Batch framework, 1. decoupling with the existing Rest Services, 2. easy to extend the housekeeping functionality with Admin process to clean up the set of data.
For the org lucky draw, there is a chance one user join one group multiple times, then it will get higher chance to win the lucky draw. It's unfair to those folks who join the group only once.
Proposal:
define whether the group would like to add the feature that the participants need to have their unique identifier (such as corporation ID) or not.
once the group asks for the unique identifier, during joining the group, the system should check the unique key to avoid any duplication for the group.