For a simple 6 digit number such as this, perhaps you should consider doing a simple XOR encryption. It's quick, easy to implement, and won't stretch out the string.

I can see two approaches to implementing this. One, create a universal 6 digit key, that all users would be encrypted against. Two, create a key for each user.

Obviously the first method is easier to manage, but is also 'less secure'. If a user knows what their ID is, they can easily figure out the key, and from that can get anyone else's user ID. Of course this requires the user knowing that the encryption is XOR and that all IDs are encrpyted with the same key. So the big question becomes, how secure do you need this? Is it likely that a valid user will try to figure out someone's ID? How valuable is the information that is encrypted?

Happy Hacking!

Rob.