Logging to API (for folder/file listing/downloading) with least number of credentials?

Thanks for elaborating. The end users of your app don't need to enter any app name or anything to identify your app. When a user uses your app, e.g., by browsing to your app's web site, your app has its own app key already, and directs the user through an OAuth flow. This sends them to Dropbox to authorize your app. At that point, they will log in to Dropbox, if they aren't already logged in, however they normally login, e.g., using their username and password. The user is then sent back to your app. This way, your app never needs to handle user credentials. You can find more information on how this works in the OAuth guide here:

https://www.dropbox.com/developers/reference/oauthguide

We recommend using one of the official SDKs, if possible, as they'll do most of the work for you:

https://www.dropbox.com/developers/core

Once a user is linked to your app, you can use the /delta endpoint to get the full list of files and folders in the user's Dropbox account:

https://www.dropbox.com/developers/core/docs#delta
https://blogs.dropbox.com/developers/2013/12/efficiently-enumerating-dropbox-with-delta/