If you want to run an application with Handysignatur support locally, you will soon run into the following problem: A-Trust will only send you the reply for a Handysignatur request via https, and only if you are providing a certificate issued by one of the authorities accepted by a-trust. This condition will probably not hold for your local development environment. An alternative that is viable for debugging purposes is to host a kind of proxy that will receive the responses to Handysignatur requests and forwards them to a given URL.
The application handysig_proxy provides exactly this. It implemens a Handysignatur proxy that can be hosted in a Google App Engine Standard Environment. All you need to do is set the two values for localUrl and remoteUrl in the config.json properly and deploy the application to Google App Engine.
localUrl is supposed to contain the relative local app engine URL where your proxy should be reachable. The Handysignatur proxy will be available at https://app-engine-project-id.appspot.com/localUrl
remoteUrl is supposed to contain the URL that all responses received through the local URL mentioned above will be routed to. So if you are running your application locally on a machine with IP address 123.45.67.89, your remoteUrl will be something like http://123.45.67.89:8080/App .