[guest mode] do not use same subject for all guests

internal/endpoints/guestmode/guestmode.go

 package guestmode
 
 import (
+	"encoding/base64"
+	"encoding/json"
+
 	"github.com/gofiber/fiber/v2"
 	"github.com/oidc-mytoken/utils/utils"
 
 	"github.com/oidc-mytoken/server/internal/config"
 	"github.com/oidc-mytoken/server/internal/server/paths"
 	"github.com/oidc-mytoken/server/internal/server/routes"
+	"github.com/oidc-mytoken/server/internal/utils/hashutils"
 )
 
 func Init(s fiber.Router) {
@@ -30,19 +34,40 @@ func handleConfig(ctx *fiber.Ctx) error {
 	return ctx.JSON(conf)
 }
 
+var stateDB map[string]string
+
+func init() {
+	stateDB = make(map[string]string)
+}
+
 func handleAuth(ctx *fiber.Ctx) error {
 	state := ctx.Query("state")
-	return ctx.Redirect(routes.RedirectURI + "?state=" + state + "&code=code")
+	stateDB[state] = ctx.IP()
+	return ctx.Redirect(routes.RedirectURI + "?state=" + state + "&code=" + state)
 }
 
 func handleToken(ctx *fiber.Ctx) error {
+	var body struct {
+		StateCode string `json:"code" xml:"code" form:"code"`
+	}
+	_ = ctx.BodyParser(&body)
+	sub := "guest-"
+	if body.StateCode != "" {
+		sub += hashutils.SHA3_256Str([]byte(stateDB[body.StateCode]))
+	} else {
+		sub += hashutils.SHA3_256Str([]byte(utils.RandASCIIString(32)))
+	}
+	data, err := json.Marshal(map[string]string{"sub": sub})
+	if err != nil {
+		return err
+	}
+	idToken := "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9." + base64.RawURLEncoding.EncodeToString(data) + "."
 	return ctx.JSON(
 		map[string]any{
 			"access_token":  utils.RandASCIIString(64),
 			"refresh_token": utils.RandASCIIString(64),
-			"id_token": `eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiJndWVzdCJ9.
-OI5skE5VAlQjI4rqAFUjqwGyEnmmQNXBTOvO7pukZoo`,
-			"expires_in": 600,
+			"id_token":      idToken,
+			"expires_in":    600,
 		},
 	)
 }