Running a SharePoint 2010 Client Object Model application from outside the domain

I've been asked to prepare an application that can be hosted elsewhere that should interact with a SharePoint 2010 farm from a system outside the AD domain. I know to use the SP Client Object Model but how can I can I pass the credentials of the target domain user that I want to use onto the ClientContext? 

Assuming that I need to supply these in one way or other, is there a way to store the domain name, user name, password securely other than encrypting these myself?

Thanks,

Hans