I'm looking for a good way to encrypt ePub files. DRM is a too expensive option and not really needed: so good encryption will do the job but I'm not sure what are the standards/the way to go.
I get 'clean' unprotected ePub files which I can encrypt serverside. Then they can be downloaded with an iOS client. If I put the decrypt logic in the app, it will be unsafe, right? The user may not have access to the clean ePub file.
When working with an encryption key, it also have to be downloaded by the client, so unsafe too, right?
What's a right way to do this? For example: how is Spotify doing this?