Add an optional site password for visitors

Settings → Privacy can set a password for the whole public site. Until a
visitor enters it, every public page shows a prompt over a blurred
made-up page, and images, preview clips and exports return 401; nothing
real is sent. Unlocking sets a 90-day cookie derived from the password
hash, so changing the password asks everyone again. The owner is never
asked while signed in. Wrong guesses are rate limited, and a locked site
sends noindex.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/server/gate.go

@@ -0,0 +1,167 @@
+package server
+
+import (
+	"context"
+	"crypto/sha256"
+	"crypto/subtle"
+	"encoding/hex"
+	"net/http"
+	"strings"
+	"time"
+
+	"github.com/cdnutter/canon/internal/auth"
+	"github.com/cdnutter/canon/internal/store"
+)
+
+// The site password: when the owner sets one, visitors see a password
+// prompt over a blurred placeholder instead of the site. Nothing from the
+// real page is sent until they unlock it.
+
+const (
+	accessCookie       = "canon_access"
+	accessTTL          = 90 * 24 * time.Hour
+	MinSitePasswordLen = 4
+)
+
+type gatePage struct {
+	Error string
+	Next  string
+}
+
+func (s *Server) sitePasswordHash(ctx context.Context) string {
+	set, err := s.store.Settings(ctx)
+	if err != nil {
+		s.log.Error("loading settings", "err", err)
+		return ""
+	}
+	return set[store.SettingSitePassword]
+}
+
+// accessToken is what the access cookie holds. It's derived from the bcrypt
+// hash, which never leaves the server, so it can't be forged, and changing
+// the password signs every visitor out.
+func accessToken(hash string) string {
+	sum := sha256.Sum256([]byte("canon-access\x00" + hash))
+	return hex.EncodeToString(sum[:])
+}
+
+// hasAccess reports whether r may see the public site, and whether the site
+// is locked at all.
+func (s *Server) hasAccess(r *http.Request) (ok, locked bool) {
+	hash := s.sitePasswordHash(r.Context())
+	if hash == "" {
+		return true, false
+	}
+	if _, ok := s.currentUser(r); ok {
+		return true, true
+	}
+	c, err := r.Cookie(accessCookie)
+	if err != nil {
+		return false, true
+	}
+	return subtle.ConstantTimeCompare([]byte(c.Value), []byte(accessToken(hash))) == 1, true
+}
+
+// gated guards a public page, showing the password prompt instead.
+func (s *Server) gated(h http.HandlerFunc) http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		ok, locked := s.hasAccess(r)
+		if locked {
+			w.Header().Set("X-Robots-Tag", "noindex, nofollow")
+		}
+		if !ok {
+			s.render(w, r, http.StatusUnauthorized, "gate", "", gatePage{Next: r.URL.RequestURI()})
+			return
+		}
+		h(w, r)
+	}
+}
+
+// gatedAsset guards images, clips and downloads.
+func (s *Server) gatedAsset(h http.HandlerFunc) http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		if ok, _ := s.hasAccess(r); !ok {
+			http.Error(w, "This site is password protected", http.StatusUnauthorized)
+			return
+		}
+		h(w, r)
+	}
+}
+
+// safePublicNext is safeNext for visitors: it falls back to the home page.
+func safePublicNext(next string) string {
+	if !strings.HasPrefix(next, "/") || strings.HasPrefix(next, "//") || strings.HasPrefix(next, "/\\") {
+		return "/"
+	}
+	return next
+}
+
+func (s *Server) handleUnlock(w http.ResponseWriter, r *http.Request) {
+	if !s.sameOrigin(r) {
+		http.Error(w, "Cross-site request blocked", http.StatusForbidden)
+		return
+	}
+	r.Body = http.MaxBytesReader(w, r.Body, 64<<10)
+	password := r.PostFormValue("password")
+	next := safePublicNext(r.PostFormValue("next"))
+	hash := s.sitePasswordHash(r.Context())
+	if hash == "" {
+		http.Redirect(w, r, next, http.StatusSeeOther)
+		return
+	}
+	page := gatePage{Next: next}
+	ip := s.clientIP(r)
+	if s.loginLimit.Blocked("unlock:" + ip) {
+		page.Error = "Too many wrong tries. Wait a few minutes and try again."
+		s.render(w, r, http.StatusTooManyRequests, "gate", "", page)
+		return
+	}
+	if !auth.CheckPassword(hash, password) {
+		s.loginLimit.Fail("unlock:" + ip)
+		page.Error = "That's not the password."
+		s.render(w, r, http.StatusUnauthorized, "gate", "", page)
+		return
+	}
+	s.loginLimit.Reset("unlock:" + ip)
+	http.SetCookie(w, &http.Cookie{
+		Name:     accessCookie,
+		Value:    accessToken(hash),
+		Path:     "/",
+		Expires:  time.Now().Add(accessTTL),
+		HttpOnly: true,
+		Secure:   s.isHTTPS(r),
+		SameSite: http.SameSiteLaxMode,
+	})
+	http.Redirect(w, r, next, http.StatusSeeOther)
+}
+
+func (s *Server) apiSitePassword(w http.ResponseWriter, r *http.Request) {
+	var in struct {
+		Password string `json:"password"`
+	}
+	if err := decodeJSON(r, &in); err != nil {
+		s.writeErr(w, err)
+		return
+	}
+	hash := ""
+	if in.Password != "" {
+		if len([]rune(in.Password)) < MinSitePasswordLen {
+			s.writeErr(w, store.ValidationError{Msg: "Use at least 4 characters"})
+			return
+		}
+		if len(in.Password) > 72 {
+			s.writeErr(w, store.ValidationError{Msg: "Use at most 72 characters"})
+			return
+		}
+		var err error
+		if hash, err = auth.HashPassword(in.Password); err != nil {
+			s.writeErr(w, err)
+			return
+		}
+	}
+	if err := s.store.SetSettings(r.Context(), map[string]string{store.SettingSitePassword: hash}); err != nil {
+		s.writeErr(w, err)
+		return
+	}
+	writeJSON(w, http.StatusOK, map[string]bool{"sitePassword": hash != ""})
+}