Note: this is a pretty long article which does a deep dive into breaking some amateur crypto. I go on for quite a bit. Make a cup of tea before reading, and get ready to read some code!
introduction
Everyone knows it. Rolling your own cryptography is a terrible idea. Here’s Bruce Schneier writing about it in 1999. Here’s an excellent answer on the Infosec Stack Exchange about why you shouldn’t do it. Here’s another Scheiner post with an excellent opening sentence.
This, then, is a post about a broken homegrown cryptosystem; namely, that used in CodeIgniter, pre-2.2. This version was current until the release of CodeIgniter 2.2, on the 5th of June, 2014, and you can still find sites on it today.
The attack described in the post depends on a lot of things to go right (or wrong, if you will); it’s not just that they used a bad cipher, but also the fact that they rolled their own session storage, and implemented a fallback, and a dozen other things. This is probably typical for most bugs of this class; a bunch of bad decisions which aren’t thought through find their logical conclusion in complete insecurity.
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