I got an high trusted provider hosted application that use the App Only Policy. If I try to use server object model, sharepoint returns a context error. Is it possible to use the client object model to read secure store credentials?
This is something that is not possible, however you may create a custom web services instead as a work around.
This C# solution will be able to help you to build your web-services to get credentials,
I guess, you need the credentials so as to connect to an external system from under the provider-hosted add-in. In this case you may not need the credentials as such, but you can create an external content type and access a list based on this ECT in order to read the data from the external system. So the solution may be: