This appears to fix the encrypt/decrypt issues with the new phpseclib.

I wrote a quick test to confirm this is fixed.. you can see it at http://pastebin.com/ZmiHujmS

I didn't try to covert it to a proper phpunit test, as I haven't ever looked at getting that working with fuel. Hopefully there'll be enough there for someone with a proper build environment to convert it to a sensible test.

Also, it's interesting that the new phpseclib is -massively- faster at encrypting and decrypting.  I actually thought there was an error with my tests when I got it working against 1.7.2, it ran almost instantly, as opposed to a 1 or 2 second delay.