Aether/backend/internal/oidc/handler.go

package oidc

import (
	"context"
	"crypto/rand"
	"encoding/base64"
	"encoding/json"
	"fmt"
	"net/http"
	"strings"

	gooidc "github.com/coreos/go-oidc/v3/oidc"
	"golang.org/x/oauth2"

	"aether/internal/auth"
	"aether/pkg/middleware"
	"aether/pkg/response"
)

const stateCookie = "oidc_state"

// Config holds OIDC provider settings (all required if OIDC is enabled).
type Config struct {
	Issuer       string
	ClientID     string
	ClientSecret string
	RedirectURL  string
	// LinkByUsername: bestehendes Konto gleichen Namens übernehmen statt ein
	// zweites anzulegen. Siehe auth.Store.GetOrCreateByOIDC.
	LinkByUsername bool
}

// Handler implements /auth/oidc/login and /auth/oidc/callback.
type Handler struct {
	cfg      Config
	provider *gooidc.Provider
	oauth2   *oauth2.Config
	verifier *gooidc.IDTokenVerifier

	authStore *auth.Store
	sessions  *auth.SessionStore
}

// New creates an OIDC handler. Returns nil if cfg.Issuer is empty (OIDC disabled).
func New(ctx context.Context, cfg Config, authStore *auth.Store, sessions *auth.SessionStore) (*Handler, error) {
	if cfg.Issuer == "" {
		return nil, nil
	}
	provider, err := gooidc.NewProvider(ctx, cfg.Issuer)
	if err != nil {
		return nil, fmt.Errorf("oidc provider: %w", err)
	}
	oauthCfg := &oauth2.Config{
		ClientID:     cfg.ClientID,
		ClientSecret: cfg.ClientSecret,
		RedirectURL:  cfg.RedirectURL,
		Endpoint:     provider.Endpoint(),
		Scopes:       []string{gooidc.ScopeOpenID, "profile"},
	}
	return &Handler{
		cfg:      cfg,
		provider: provider,
		oauth2:   oauthCfg,
		verifier: provider.Verifier(&gooidc.Config{ClientID: cfg.ClientID}),

		authStore: authStore,
		sessions:  sessions,
	}, nil
}

// GET /api/v1/auth/oidc/login – redirect to OIDC provider.
func (h *Handler) Login(w http.ResponseWriter, r *http.Request) {
	state, err := randomState()
	if err != nil {
		response.Error(w, http.StatusInternalServerError, "state generation failed")
		return
	}
	// Secure wie bei Sitzungs- und CSRF-Cookie: Hinter dem Reverse Proxy sieht
	// die App selbst kein TLS, deshalb über IsHTTPS (das X-Forwarded-Proto
	// auswertet – aber nur von vertrauenswürdigen Proxys). Ohne das ginge der
	// Zustandswert auch über eine unverschlüsselte Verbindung mit.
	http.SetCookie(w, &http.Cookie{
		Name: stateCookie, Value: state, Path: "/",
		MaxAge: 300, HttpOnly: true, SameSite: http.SameSiteLaxMode,
		Secure: middleware.IsHTTPS(r),
	})
	http.Redirect(w, r, h.oauth2.AuthCodeURL(state), http.StatusFound)
}

// GET /api/v1/auth/oidc/callback – exchange code, create session, redirect to SPA.
func (h *Handler) Callback(w http.ResponseWriter, r *http.Request) {
	// Validate state.
	stateCk, err := r.Cookie(stateCookie)
	if err != nil || r.URL.Query().Get("state") != stateCk.Value {
		http.Redirect(w, r, "/?error=oidc_state", http.StatusFound)
		return
	}
	// Clear state cookie.
	http.SetCookie(w, &http.Cookie{Name: stateCookie, Value: "", Path: "/", MaxAge: -1})

	code := r.URL.Query().Get("code")
	if code == "" {
		http.Redirect(w, r, "/?error=oidc_no_code", http.StatusFound)
		return
	}

	// Exchange code for tokens.
	token, err := h.oauth2.Exchange(r.Context(), code)
	if err != nil {
		http.Redirect(w, r, "/?error=oidc_exchange", http.StatusFound)
		return
	}

	rawIDToken, ok := token.Extra("id_token").(string)
	if !ok {
		http.Redirect(w, r, "/?error=oidc_no_id_token", http.StatusFound)
		return
	}

	idToken, err := h.verifier.Verify(r.Context(), rawIDToken)
	if err != nil {
		http.Redirect(w, r, "/?error=oidc_verify", http.StatusFound)
		return
	}

	// Extract claims.
	var claims struct {
		Sub               string `json:"sub"`
		PreferredUsername string `json:"preferred_username"`
		Name              string `json:"name"`
	}
	if err := idToken.Claims(&claims); err != nil {
		http.Redirect(w, r, "/?error=oidc_claims", http.StatusFound)
		return
	}

	// Derive username: prefer preferred_username, fall back to sub.
	username := sanitizeUsername(claims.PreferredUsername)
	if username == "" {
		username = sanitizeUsername(claims.Sub)
	}
	if username == "" {
		http.Redirect(w, r, "/?error=oidc_no_username", http.StatusFound)
		return
	}

	// Find or create user.
	user, err := h.authStore.GetOrCreateByOIDC(r.Context(), claims.Sub, username, claims.Name, h.cfg.LinkByUsername)
	if err != nil {
		http.Redirect(w, r, "/?error=oidc_user", http.StatusFound)
		return
	}

	if err := h.sessions.SetUser(w, r, user.ID); err != nil {
		http.Redirect(w, r, "/?error=oidc_session", http.StatusFound)
		return
	}

	// Set CSRF cookie so the SPA can function immediately after redirect.
	_, _ = auth.SetCSRFCookie(w, r)

	http.Redirect(w, r, "/", http.StatusFound)
}

// PublicConfig returns whether OIDC is enabled (called without auth).
func (h *Handler) IsEnabled() bool { return h != nil }

// ConfigJSON writes the OIDC config state (for login page).
func ConfigJSON(enabled bool, cloudName string) []byte {
	b, _ := json.Marshal(map[string]any{
		"oidc_enabled": enabled,
		"cloud_name":   cloudName,
	})
	return b
}

// ── helpers ───────────────────────────────────────────────────────────────────

func randomState() (string, error) {
	b := make([]byte, 16)
	if _, err := rand.Read(b); err != nil {
		return "", err
	}
	return base64.URLEncoding.EncodeToString(b), nil
}

func sanitizeUsername(s string) string {
	s = strings.ToLower(strings.TrimSpace(s))
	var out strings.Builder
	for _, r := range s {
		if (r >= 'a' && r <= 'z') || (r >= '0' && r <= '9') || r == '_' || r == '-' || r == '.' {
			out.WriteRune(r)
		}
	}
	return out.String()
}