Limiting potential users of a SOAP web service? Possible?

[Cormac M Redmond]

Hi, I have created a SOAP web service in .NET. I have a client application which consumes some methods - it essentially passes a username and password to the web service and gets back a 'signature' (for what is not important), once the username and password is correct. Obviously, this web service is exposed to the world - and could be a target for malicious users, etc. Is there any way in .NET to say "Only allow this application to use this web service"? I cannot limit the IP range, as the client app should be usable from anywhere. I'm sure this is a common problem, but have found no concrete help. Thanks, Cormac