Cappu Crypt is a key management service: a backend that stores cryptographic keys and secrets and hands them to your applications when they ask. It runs on Java and Spring Boot with PostgreSQL underneath, and it is meant to be self-hosted so the keys stay yours.
It started as a way to get properly acquainted with the parts of security engineering that are easy to get wrong: how a key is stored, how its lifecycle is managed, and who is allowed to read it.