Feature 03 task 1: OIDC login and callback against Authentik (#16)

Co-authored-by: dev_agent <dev_agent@cubecraftlabs.com>
Co-committed-by: dev_agent <dev_agent@cubecraftlabs.com>

internal/auth/cookie.go

@@ -0,0 +1,124 @@
+package auth
+
+import (
+	"crypto/hmac"
+	"crypto/sha256"
+	"encoding/base64"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"net/http"
+	"net/url"
+	"strings"
+	"time"
+)
+
+const (
+	loginCookieName = "__Host-mealquest_login"
+	loginCookieTTL  = 10 * time.Minute
+	// loginCookieKeyLabel separates the login cookie's signing key from other
+	// keys derived from the same server secret.
+	loginCookieKeyLabel = "mealquest auth login cookie v1"
+)
+
+// loginState travels from login to callback in the signed login cookie.
+type loginState struct {
+	State    string `json:"s"`
+	Nonce    string `json:"n"`
+	Verifier string `json:"v"`
+	Return   string `json:"r"`
+	Expires  int64  `json:"e"` // Unix seconds
+}
+
+// loginCookie signs and verifies the login cookie with HMAC-SHA256.
+type loginCookie struct {
+	key []byte
+}
+
+func newLoginCookie(secretKey []byte) loginCookie {
+	mac := hmac.New(sha256.New, secretKey)
+	mac.Write([]byte(loginCookieKeyLabel))
+	return loginCookie{key: mac.Sum(nil)}
+}
+
+func (c loginCookie) sign(payload string) string {
+	mac := hmac.New(sha256.New, c.key)
+	mac.Write([]byte(payload))
+	return base64.RawURLEncoding.EncodeToString(mac.Sum(nil))
+}
+
+func (c loginCookie) set(w http.ResponseWriter, s loginState, now time.Time) error {
+	s.Expires = now.Add(loginCookieTTL).Unix()
+	data, err := json.Marshal(s)
+	if err != nil {
+		return err
+	}
+	payload := base64.RawURLEncoding.EncodeToString(data)
+	http.SetCookie(w, &http.Cookie{
+		Name:     loginCookieName,
+		Value:    payload + "." + c.sign(payload),
+		Path:     "/",
+		MaxAge:   int(loginCookieTTL / time.Second),
+		Secure:   true,
+		HttpOnly: true,
+		// Lax so the cookie rides the provider's top-level redirect back.
+		SameSite: http.SameSiteLaxMode,
+	})
+	return nil
+}
+
+func (c loginCookie) clear(w http.ResponseWriter) {
+	http.SetCookie(w, &http.Cookie{
+		Name:     loginCookieName,
+		Path:     "/",
+		MaxAge:   -1,
+		Secure:   true,
+		HttpOnly: true,
+		SameSite: http.SameSiteLaxMode,
+	})
+}
+
+func (c loginCookie) read(r *http.Request, now time.Time) (loginState, error) {
+	cookie, err := r.Cookie(loginCookieName)
+	if err != nil {
+		return loginState{}, errors.New("login cookie missing")
+	}
+	payload, sig, ok := strings.Cut(cookie.Value, ".")
+	if !ok || !hmac.Equal([]byte(sig), []byte(c.sign(payload))) {
+		return loginState{}, errors.New("login cookie signature invalid")
+	}
+	data, err := base64.RawURLEncoding.DecodeString(payload)
+	if err != nil {
+		return loginState{}, fmt.Errorf("login cookie encoding: %w", err)
+	}
+	var s loginState
+	if err := json.Unmarshal(data, &s); err != nil {
+		return loginState{}, fmt.Errorf("login cookie payload: %w", err)
+	}
+	if now.Unix() >= s.Expires {
+		return loginState{}, errors.New("login cookie expired")
+	}
+	return s, nil
+}
+
+// returnPath validates the optional post-login return path. Empty means "/".
+// Only same-origin absolute paths are allowed: no scheme, host, protocol-
+// relative "//" prefix, backslashes or control characters.
+func returnPath(raw string) (string, error) {
+	if raw == "" {
+		return "/", nil
+	}
+	if !strings.HasPrefix(raw, "/") || strings.HasPrefix(raw, "//") || strings.ContainsRune(raw, '\\') {
+		return "", errors.New("return path must be a same-origin path")
+	}
+	for _, c := range raw {
+		if c < 0x20 || c == 0x7f {
+			return "", errors.New("return path contains a control character")
+		}
+	}
+	u, err := url.Parse(raw)
+	if err != nil || u.Scheme != "" || u.Host != "" || u.User != nil {
+		return "", errors.New("return path must be a same-origin path")
+	}
+	return raw, nil
+}