Obtaining api_key for REST API?

[Mark McLaren]

I'm hoping to use the REST API from a "service account" (i.e. not a logged in user) application.

How do I obtain a valid api_key header token for the application to use? (e.g. can I configure it to use a shared secret for the api_key value?) 

I've locked down the layouts so only admins can change them.  I'm hoping to use the REST API to export fragment layouts and portlet definitions and store them in Git when an admin makes changes.

Many thanks,

Mark