RELATED IDEAS
Add support for Client Certificates for authentication in the 'Call Web Service' action.
When a web service end point requires a client certificate for authentication, a client certificate needs to be specified for use.
Note for the Product Team: Although SharePoint's Secure Store is capable of holding a client certificate and client password, the Call Web Service action will fail to use the secure store target application as Nintex Workflow appears to be expecting a username and password from the secure store target application.