I can't comment on "true" software serial creation/validation, but from a web point of view (where hopefully no one can see your validation code)then this is really quite easy and a fun exercise..
I have often created codes similar to the typical "enter your code to receive your free whatever" in carts and usually it is preferable to use lookup tables, but sometimes where these codes need to be unlimited and unique you have to resort to a bit of trickery - you know you're not going to fool someone with clue and a lot of spare time but you can make it hard for them.
As mentioned above, use a lot of false trails (data which you can ignore) and leave youself with a chunk of data you can apply a bit of basic maths to.

regrds

c

1 and sleep;