Currently Steam works well (I believe) for single-player games, but I would like to use Steam authorization and Steam purchase/subscription validation on my Game Server.

Here is the link that describes how to do user authentication using Steam callbacks (the post is about Unity but it doesn't matter):
https://forum.unity.com/threads/authenti...et.546487/

So, at least the following Steam calls should be implemented (maybe some other calls are needed too, needs further investigation):

for authentication
https://partner.steamgames.com/doc/api/I...sionTicket
https://partner.steamgames.com/doc/api/I...uthSession
https://partner.steamgames.com/doc/api/I...uthSession

for subscriptions/purchases (I guess)
https://partner.steamgames.com/doc/webap...n#QueryTxn

I know that purchases/subscriptions query is implemented, but it is locked to current user only and not ready for Game Server usage.

Here's additional information about that:
https://partner.steamgames.com/doc/featu...end_webapi