#[05AB1E](https://github.com/Adriandmen/05AB1E), 5 bytes, [cracked](http://codegolf.stackexchange.com/a/77462)

I hope that this submission isn't as easy as my other ones :p. Outputs:

 a(0) = 0
 a(1) = 1
 a(2) = 6
 a(3) = 24
 a(4) = 80
 a(5) = 240

Obfuscated code:

 ####O

Contains some non-ASCII characters though, uses **CP-1252** encoding.

[Try it online!](http://05ab1e.tryitonline.net/) might come in handy :p.