It appears, from Nintex documentation, that the K2 Workflow REST API only supports Basic authentication and OAuth authentication.
We have the requirement to be able to start a workflow via our own REST API, calling the K2 Workflow REST API by using the credentials of the currently logged in user as the originator of the workflow.
We would need to be able to do this by calling our API via a url, and also, by using our controller method in a Smart Object.