Read the "Design Principles" section more carefully. Simple concatenation suffers from several different attack vectors.
I am concerned about changing the algorithm at this point in case anyone has already used it. I don't think there are any serious concerns but will report back if I find any (after taking some more time). Of course if anyone knows this stuff very well I'd be very eager for some feedback!
Exactly why you should be considering changing the algorithm.
http://happybearsoftware.com/you-are-dangerously-bad-at-cryp...