I've been building a custom web service to handle updates to a
specific event list on a WSS site.

Building the framework of the service is easy enough, as is accessing
the sharepoint object model so that I can add and remove items from
the list in principle. The system fallsdown though because I haven't
presented any security credentials.

The form digest security is not applicable since this is a web
service, not a web page. I had assumed I could mimick the provided
web services by populating a credentials "property" but I cannot find
how this is referenced.

Can someone give me some pointers on the matter?