Each project has a service account associated with it. You can create a service account for your project in the Google Cloud Platform Console. When you create a service account, you obtain a key in P12 file format that can be used for authentication.
Procedure
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Create a service account.
For instructions about creating a service account and obtaining a P12 key type, go to Google Developers website, Creating a service account.
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Save your PKCS12 key securely and remember the key location.
You require the key location later when you create an app.