How can I authenticate to GCP using a service account key that was uploaded (not generated)?

To authenticate to GCP using a service account key that was uploaded, you need to use the projects.serviceAccounts.keys.upload method.

Steps:

1. Generate a new service account key. You can do this in the Google Cloud console or using the gcloud command-line tool. When you generate the key, select the "JSON" format.
2. Upload the service account key to your GCP project. You can do this using the projects.serviceAccounts.keys.upload method.
3. Configure your application to use the service account key. You can do this by setting the GOOGLE_APPLICATION_CREDENTIALS environment variable to the path of the service account key file.

Example:

The following code sample shows you how to use the projects.serviceAccounts.keys.upload method to upload a service account key:

  // Imports the Google Cloud client libraries.
  import (
    "context"
    "fmt"
    "io"

    iam "google.golang.org/api/iam/v1"
  )

  // uploadServiceAccountKey uploads a service account key to a project.
  func uploadServiceAccountKey(w io.Writer, projectID, serviceAccountEmail, pathToKeyFile string) error {
    // projectID := "my-project-id"
    // serviceAccountEmail := "service-account@my-project.iam.gserviceaccount.com"
    // pathToKeyFile := "/path/to/key.json"

    ctx := context.Background()

    service, err := iam.NewService(ctx)
    if err != nil {
      return fmt.Errorf("iam.NewService: %v", err)
    }

    // Get the service account.
    serviceAccount, err := service.Projects.ServiceAccounts.Get(fmt.Sprintf("projects/%s/serviceAccounts/%s", projectID, serviceAccountEmail)).Do()
    if err != nil {
      return fmt.Errorf("Projects.ServiceAccounts.Get: %v", err)
    }

    // Create the service account key.
    key := &iam.ServiceAccountKey{
      PrivateKeyData: "YOUR_PRIVATE_KEY_DATA",
    }

    // Upload the service account key.
    key, err = service.Projects.ServiceAccounts.Keys.Upload(serviceAccount.Name, key).Do()
    if err != nil {
      return fmt.Errorf("Projects.ServiceAccounts.Keys.Upload: %v", err)
    }

    fmt.Fprintf(w, "Service account key uploaded: %v\n", key.Name)

    return nil
  }
  

Additional Resources:

* Creating and Managing Service Account Keys * Authenticating to Cloud Platform