Hello, I want to know is possible somehow to if you have any type application which works with steam keys, after user buys or activates gift or activates key from third party resellers to send key to publisher server so internal authentifaction system could recognize key or should I instead save user steam id and then identificate if he has my app. Probably it looks like noobie question, but this is my first time with steam so thank you very much for explanation. 

Since steam does not fully reveal how keys are managed by steam to publisher, so I would like to know if they have such event after key is activated in steam

use steamworks api to do stuff like user auth: https://partner.steamgames.com/doc/sdk

1 minute ago, ninnghazad said:

use steamworks api to do stuff like user auth: https://partner.steamgames.com/doc/sdk

My question is steam provides event which fires up when any user activates my app key and does steam servers send key data to publisher server(which is mine) about key so that then user logs in in my app so I could recognize that key is valid and activated from steam.

you can query keys from steam. but checking if a user has a valid license is done by steam's api to which i linked. you don't use the key to check that - the api checks upon init if the logged in (in steam) user has a valid license for your software.

i think it happens when you call https://partner.steamgames.com/doc/api/steam_api#SteamAPI_Init