Getting client secret for an application that is already written

I’m a noob so please don’t shoot me down. I have inherited an application which uses the API. This was written a short while ago by a 3rd party. They have not released the API keys to my new client. I have access to the Infusion suite and can see the API key and have been told what the API Passkey is. These don’t work when trying to access the API. I have tested my new code against a sandbox application I have constructed and everything works as it should (Via OAuth) using those keys.

How do I get to the original client id and client secret for the application, or do I have to start again?

I have it now, for noobs it’s a bit confusing but if you have the clients Infusion credentials it allows your application to attach to their infusion suite via OAuth.