Is Authentication API a server side API?

[abitrolly]

From https://github.com/greenheartgames/greenworks/blob/master/docs/authentication.md I thought the Auth API could be used by client side (game on user's computer) to request user ID from Steam. But the API requires a private key that is generated per application, and which must be stored secret and must not be distributed with the app.

Does that mean I need to run my own server for use Greenworks auth? Which means I will have to implement additional API to call that server from the game.