I’ve been reading through the source of SC4, a self-contained crypto web application. It’s like PGP in a .html file.

You use one HTML page (sc4z.html) to generate another (randomly named, like SC4_9EzCzXFM2UARR9.html) and you’re pretty much all set.

As part of the standalone page, SC4 uses localStorage to store keys. This is fine when you’re serving the content from a normal domain (https://origin.tld/path/), but what I didn’t know is that different browsers handle this differently when it’s served from a file:// URI.

This is mentioned in the SC4 security audit, but it’s news to me. Firefox will store a separate localStorage database for each directory; in other words, only files located within the same directory can access the same localStorage database (including subdirectories, apparently). There is a good page on the Mozilla Developer Wiki detailing the Same-Origin Policy for file: URIs. Chrome works differently, though: it stores all data for localStorage for file: URIs in a single database. What this means is that any file served from a local filesystem can read the localStorage database.

There is a lengthy discussion on Mozilla’s Bugzilla about how localStorage should work with file: URIs. It’s not a recent bug; the original, more-Chrome-like implementation was discussed back in 2009. If you try accessing localStorage from a separate directory, you won’t get the SC4 keys; you won’t get anything. In Chrome, though, you will. This isn’t a huge security concern, but it’s a little bit surprising. HTTP-served resources are restricted by domain (according to the same origin policy), but file: URIs are the wild west, so to speak. Go ahead and console.log the localStorage from any local file in Chrome: you’ll get anything & everything other files store.

I did some further digging and found that data: URIs are handled even differently from the file: URIs. I was wondering if the SC4 pages could be served entirely from data: URIs (humongous links, more or less), but it appears to be not so. Long story short: they don’t support localStorage. I don’t have a good narrative why this is so (there are discussions on bug trackers), but it’s not outrageous.

This is pretty uniform across all browsers. There’s a Chromium bug marking this working as intended, so I don’t expect any changes soon. Here are some error messages that come up when you try to reference localStorage from a data:-served resource:


SecurityError: Failed to read the 'localStorage' property from 'Window': Storage is disabled inside 'data:' URLs.


[Exception... "Component is not available" nsresult: "0x80040111 (NS_ERROR_NOT_AVAILABLE)" location: "JS frame :: data:text/html;base64,PGh0bWw+CjxoZWFkPgo8bWV0YSBjaGFyc2V0PSJ1dGYtOCI+CjwvaGVhZD4KPGJvZHk+CjxzY3JpcHQgdHlwZT0idGV4dC9qYXZhc2NyaXB0Ij4KdHJ5IHsgZG9jdW1lbnQud3JpdGVsbihKU09OLnN0cmluZ2lmeShsb2NhbFN0b3JhZ2UpKSB9CmNhdGNoIChlKSB7IGRvY3VtZW50LndyaXRlbG4oZSkgfQo8L3NjcmlwdD4KPC9ib2R5Pgo8L2h0bWw+Cg== :: :: line 7" data: no]


Error: SecurityError: DOM Exception 18

Here’s the HTML I used; it’s visible in the Firefox error message.

<meta charset="utf-8">
<script type="text/javascript">
try { document.writeln(JSON.stringify(localStorage)) }
catch (e) { document.writeln(e) }

Encoding this in base64 yields:


So the data: URI is data:text/html;base64,PGh0...snipped...Cg==.

I’m really not sure how Internet Explorer handles all this, but I’m not exactly inclined to find out. We’ll see. I like SC4, localStorage or not.