Ideas about Generating Untraceable Invoice IDs

Ideas about Generating Untraceable Invoice IDs

I want to print invoices for customers in my app. Each invoice has an
Invoice ID. I want IDs to be:
Sequential (ids entered lately come late)
32 bit integers
Not easily traceable like 1 2 3 so that people can't tell how many items
we sell.
An idea of my own: Number of seconds since a specific date & time (e.g.
1/1/2010 00 AM).
Any other ideas how to generate these numbers ?