[andrenotgiant]

Here's the documentation: http://www.rubydoc.info/gems/aes/0.5.0

Reading through, it's NOT clear that `key` must be hexidecimal, but you can see why the gem's author didn't think to call it out:

The gem also provides a helper method for generating keys: AES.key (which outputs a random hexidecimal string). The developer probably never expected users to submit their own keys, despite the design of the gem making it easy to do.