Reading Chrome, Firefox and Opera extensions in Emacs

If you're a fan of emacs' archive-mode that lets you load zip files and the like as it lets you load regular directories, and know that Firefox xpi, Chrome crx and Opera oex extensions are really just thinly wrapped zip files under the hood, you might expect Emacs to grok them, just the same. I did, and was a little surprised when it didn't plain work. (So maybe I'm running an old Emacs build?)

Anyway, I looked into the matter and did a little poking about to see what setup would do the trick. In the case of Firefox and Opera, it's really easy, as the xpi and oex "formats" only amount to changing the filename extension and making sure you have the right files in there -- so all you have to do is make sure that ("\\.xpi\\'" . archive-mode) exists in your auto-mode-alist, and ("\\.xpi\\'" . no-conversion) in your auto-coding-alist (there is a little snippet for your .emacs at the end of this post), and the corresponding oex variant.

In the Chrome case (which prepends a Chrome header with a format version, an RSA public key and signature from the extension author), besides the corresponding crx change for same variables, I also had to patch the lisp/arc-mode.el file lightly:



If you try loading a crx and find it already works, someone upstream already applied the same or a better patch than mine to your installation, but otherwise, read on. To find where it is on your system, type C-h archive-mode <return> and follow the archive-mode is a compiled Lisp function in `arc-mode.el'. link, apply the patch and M-x byte-compile-file the result again.

The .emacs snippet that makes it all kick in automagically:



Note that for the Chrome extension case, you won't be able to edit and save any of the contents in place, as you can with plain zip (xpi, oex) files; the magic to remove/re-add public keys and signatures will be somebody else's late Saturday hack to complete. :) Enjoy!