Question

I have sensitive files to download to users, and each user is permitted to download a given file exactly once. If the download fails, I want to permit the re-download, but not otherwise.

It’s not sufficient to rely on logging/processing the file download request at the server – I need to know deterministically when the file is complete and in place at the client, since many of my users work in an environment with frequent connectivity drops.

The most straightforward way for this to work would be if the browser exposed a “file saved” event from the Save As… dialog that could be wired to a JavaScript function on the download page (which could post back to the server). But, intuition suggests there might be security holes if browsers exposed this functionality, as it sneaks somewhat outside the sandbox. I’m not sure this is even possible.

I found several other questions in this area, but nothing about this problem specifically.

Any ideas?

Edit: I should not have used the word “security” in the original question, sorry for triggering the red herrings.

Edit 2: My “security” phrasing misled folks into offtopic technical security issues, but both of you confirmed my suspicion that “no, there’s no browser support for that.” I’m marking the first commenter with the answer since his first sentence had what I was looking for. Thanks all.

Answer 1

There’s no such browser event in JavaScript and even if there was you can not trust the user’s browser to provide security for you.

You’re better off using a GUID to generate a unique URL for each download. You can then for example:

• let the URL be valid only for a specific time period
• allow transfers only from a specific IP address associated with the unique URL
• let your server-side code detect when the content for a unique URL has been fully transferred and then invalidate the URL.

Let me clarify the last bullet. Say you’re using Java – you will in.read(buffer) and out.write(buffer) in a loop until EOF. If the client disconnects you will receive an IOException during out.write() and will be able to tell a successful download from an interrupted one. On other platforms, I’m sure there are ways to tell whether the connection was lost or not.

EDIT: You could actually fire a browser event using the trick outlined in the accepted answer of one of the questions you linked to. That would however not be a reliable solution to limit the number of downloads.