I have developed a Client/Server application for IM with Qt. So far messages are sent and displayed at the client side, but when the program is closed the messages are no longer available since a proper storage is missing.
I would like to keep the messages on the client devices and avoid to store everything on the server. I don't want to use a DB either since it needs to be installed and I would like to keep everything quite easy.
Therefore I was thinking of simply storing everything in an encrypted file, but I couldn't think of a proper format to do that. Has anyone experience with that or any suggestions how to save the messages from different clients?
You do have a concern with data integrity in face of unplanned termination of your software, due to bugs in your code, transient hardware errors, power outages, etc. That's the problem that everyone using "plain files" usually ignores, as it's a hard problem to solve and requires extensive testing and know-how.
That's why you should use an embedded database. It will solve that, and many other problems as well. SQLite is a de-facto standard for applications such as yours. You can add any encryption you wish, as SQLite provides hooks that let you implement writing and reading of the pages. You'd do the encryption there.
One little-appreciated aspect of SQLite specifically is the amount of testing it gets during development. The test harness, most of it non-public, is probably worth way more than the published SQLite code (>1M USD). SQLite is used in aerospace applications, e.g. IIRC in code classified as DAL-B under DO-178B.