Document Steam Session Tickets authentication with SpacetimeAuth (#4908)

# Description of Changes

- Add documentation for Steam Session Ticket authentication with
SpacetimeAuth

# API and ABI breaking changes

None.

# Expected complexity level and risk

1

# Testing

Ran the steps manually

Author: JulienLavocat

docs/docs/00200-core-concepts/00500-authentication/00100-spacetimeauth/00600-steam.md

@@ -0,0 +1,122 @@
+---
+title: Steam Session Ticket Authentication
+---
+
+SpacetimeAuth supports authentication using Steam's Session Ticket system. This allows
+users to authenticate with your application using their Steam account.
+This method is particularly useful for gaming applications that want to distribute
+their applications on Steam.
+
+Once a user authenticates with Steam, SpacetimeAuth will create an account for them
+in your SpacetimeAuth project and issue an ID token that your application can use
+to authenticate with SpacetimeDB. The ID token will contain claims with information
+about the user and game ownership, which you can use for authorization in your application.
+
+The ID token will notably contain the following claims:
+
+- `sub`: The unique identifier for the user in SpacetimeAuth
+- `provider_id`: The unique identifier for the user in Steam
+- `login_method`: The authentication provider (in this case, "steam")
+- `preferred_username`: The user's Steam display name
+- `picture`: The URL to the user's Steam avatar (full size)
+- `steam_owned_games`: An array of all the applications you published owned by the
+  user on Steam
+
+## Creating a Steam Publisher Key
+
+To use Steam Session Ticket authentication, you need to create a
+[Steam Publisher Key](https://partner.steamgames.com/doc/webapi_overview/auth)
+in the Steamworks dashboard. This key will be used to verify the authenticity of
+the session tickets provided by Steam during the authentication process and to
+retrieve information about the user's owned games.
+To create a Steam Publisher Key, follow [Valve's official documentation](https://partner.steamgames.com/doc/webapi_overview/auth#create_publisher_key)
+
+## Adding your Steam Publisher Key and allowed app IDs to SpacetimeAuth
+
+Once you have your Steam Publisher Key, you need to add it to your SpacetimeAuth
+project along with the list of allowed app IDs that you want to check for ownership.
+You can do this in the SpacetimeAuth dashboard under the "Settings" section.
+
+![Dashboard settings page](/images/spacetimeauth/steam-dashboard.png)
+
+Any app IDs that will authenticate users through Steam Session Tickets must be added
+to the allowed app IDs list in SpacetimeAuth. This ensures that only session tickets
+issued for those specific app IDs will be accepted during authentication. This is
+an important security measure to prevent unauthorized access using session tickets
+from other applications. Make sure to add all the app IDs for the games or DLCs you
+want to check ownership for in your application.
+
+## Getting a Steam Session Ticket
+
+Now that you've set up your Steam Publisher Key and allowed app IDs, you can implement
+the client-side logic to get a Steam Session Ticket and authenticate with SpacetimeAuth.
+You can use the Steamworks SDK to retrieve a session ticket for the user and then
+send it to your backend server for authentication.
+
+Depending on the platform you're developing for, the process of obtaining a Steam
+Session Ticket may vary.
+
+Here are some resources to help you get started with retrieving a Steam Session Ticket:
+
+- [Steamworks SDK documentation (C++/Unreal Engine)](https://partner.steamgames.com/doc/api/ISteamUser#GetAuthTicketForWebApi)
+- [Steamworks .Net (C#/Unity)](https://steamworks.github.io/gettingstarted/)
+- [GodotSteam (Godot Engine)](https://godotsteam.com/classes/user/#getauthticketforwebapi)
+- [Steamworks.js (JavaScript/Node.js)](https://github.com/ceifa/steamworks.js)
+
+:::warning
+Make sure to request a session ticket with `spacetimeauth` as the identity parameter.
+:::
+
+## Exchanging the Steam Session Ticket for a SpacetimeAuth ID Token
+
+Once you have the Steam Session Ticket, you can send it to the token endpoint of
+SpacetimeAuth to exchange it for an ID token that you can use to authenticate with
+SpacetimeDB.
+
+Here is an example of how to exchange the Steam Session Ticket using `curl`:
+
+```bash
+curl -X POST https://auth.spacetimedb.com/oidc/token \
+  -H "content-type: application/x-www-form-urlencoded" \
+  -d "client_id=client_032xAh3p8o3zGzDghXsO5x" \
+  -d "grant_type=urn:spacetimeauth:steam-ticket" \
+  -d "steam_ticket=<YOUR STEAM TICKET>" \
+  -d "steam_app_id=<APP ID FROM WHICH YOU REQUESTED THE TICKET>"
+```
+
+## Checking App or DLC Ownership
+
+You can check if a user owns a specific app or DLC by inspecting the `steam_owned_games`
+claim in the ID token. This claim contains an array of all the applications you published
+that the user owns on Steam. Each entry in the array includes the `appid` and a boolean
+`ownsapp` indicating whether the user owns the app or not. You can use this information
+to implement authorization logic in your application, such as granting access to
+certain features or content based on game ownership.
+
+```rust
+#[derive(serde::Deserialize)]
+struct SteamGame {
+    appid: u64,
+    ownsapp: bool,
+}
+
+#[derive(serde::Deserialize)]
+struct CustomClaims {
+    steam_owned_games: Vec<SteamGame>,
+}
+
+#[reducer(client_connected)]
+pub fn connect(ctx: &ReducerContext) -> Result<(), String> {
+    let jwt = ctx.sender_auth().jwt().ok_or("JWT required")?;
+    let claims: CustomClaims = serde_json::from_slice(jwt.raw_payload().as_bytes())
+        .map_err(|e| format!("Invalid JWT: {}", e))?;
+
+    const APP_ID: u64 = 2717550;
+    let owns = claims.steam_owned_games.iter().any(|g| g.appid == APP_ID && g.ownsapp);
+
+    if !owns {
+        return Err(format!("Unauthorized: does not own app {}", APP_ID));
+    }
+    Ok(())
+}
+```

docs/docs/00200-core-concepts/00500-authentication/00100-spacetimeauth/index.md

@@ -26,6 +26,7 @@ authorize users with the SpacetimeDB server.
 ### Authentication methods
 
 - Magic link
+- Steam (Session Ticket)
 - Github
 - Google
 - Discord