This is the "{project name}-xxxxxxxxxxxx.p12" file associated with the service account credential in the Google Developer Console. A new private key may be generated for a service account if you no longer have the original.

To create a new service account:

To generate a new key for an existing service account, follow the above instructions until the "Create new Client ID" step and instead find the Service Account for which you need a new key and click "Generate new P12 key".