Encrypt cookies

README.md

@@ -23,6 +23,12 @@ When you put it behind a reverse proxy that terminates TLS, set `--public-url`,
 and make sure the proxy neither buffers request bodies nor imposes its own
 upload limit.
 
+The data directory holds the uploads, `tokens.json`, and `session.key`, which
+seals the login cookie so that it carries a session rather than the token
+itself. The last two are credential material, written `0600`. Deleting
+`session.key` logs every browser session out and costs nothing else; a new one
+is generated on the next start.
+
 ## Options
 
 **Read `./uncensored-send --help` rather than this file.**

internal/server/admin.go

@@ -88,7 +88,7 @@ func compareExpiry(a, b *time.Time) int {
 }
 
 func (s *Server) handleFiles(w http.ResponseWriter, r *http.Request) {
-	lim, err := s.limitsFor(r, credential(r))
+	lim, err := s.limitsFor(r, s.credential(r))
 	if err != nil {
 		s.fail(w, r, http.StatusUnauthorized, "Unrecognised token.")
 		return

internal/server/cookie.go

@@ -21,22 +21,40 @@ const tokenCookie = "uncensored_send_token"
 const rememberFor = 365 * 24 * time.Hour
 
 // cookieCredential returns the remembered token, if any.
-func cookieCredential(r *http.Request) string {
+//
+// The cookie carries the token sealed, so this is also where an unreadable one
+// - another server's key, or the older plain format - quietly becomes "no
+// session" rather than a credential that cannot be resolved.
+func (s *Server) cookieCredential(r *http.Request) string {
 	c, err := r.Cookie(tokenCookie)
 	if err != nil {
 		return ""
 	}
-	return strings.TrimSpace(c.Value)
+	return s.sessions.open(strings.TrimSpace(c.Value))
+}
+
+// staleSession reports a cookie that is present but will not open: sealed with
+// another server's key, or written in the plain-text format this replaced.
+// There is no session in it, and leaving it in the browser means sending a dead
+// credential on every request for a year, so the page treats it exactly as it
+// treats a revoked token: say so once, and clear it.
+func (s *Server) staleSession(r *http.Request) bool {
+	c, err := r.Cookie(tokenCookie)
+	if err != nil {
+		return false
+	}
+	value := strings.TrimSpace(c.Value)
+	return value != "" && s.sessions.open(value) == ""
 }
 
 // credential resolves the caller's token from an explicit header first, then
 // from the remembered cookie. Upload additionally accepts a form field, which
 // takes precedence over both.
-func credential(r *http.Request) string {
+func (s *Server) credential(r *http.Request) string {
 	if t := bearer(r); t != "" {
 		return t
 	}
-	return cookieCredential(r)
+	return s.cookieCredential(r)
 }
 
 // logIn stores the token in a cookie.
@@ -48,10 +66,14 @@ func credential(r *http.Request) string {
 //
 // When persist is false the cookie carries no lifetime and the browser drops it
 // when it closes, which is the right default on a machine that is not yours.
-func (s *Server) logIn(w http.ResponseWriter, r *http.Request, token string, persist bool) {
+func (s *Server) logIn(w http.ResponseWriter, r *http.Request, token string, persist bool) error {
+	sealed, err := s.sessions.seal(token)
+	if err != nil {
+		return err
+	}
 	c := &http.Cookie{
 		Name:     tokenCookie,
-		Value:    token,
+		Value:    sealed,
 		Path:     s.cfg.BasePath,
 		HttpOnly: true,
 		Secure:   s.isHTTPS(r),
@@ -61,6 +83,7 @@ func (s *Server) logIn(w http.ResponseWriter, r *http.Request, token string, per
 		c.MaxAge = int(rememberFor.Seconds())
 	}
 	http.SetCookie(w, c)
+	return nil
 }
 
 // forget clears a remembered token.

internal/server/delete.go

@@ -98,7 +98,7 @@ func (s *Server) deleteCredentials(w http.ResponseWriter, r *http.Request) []str
 	}
 
 	add(bearer(r))
-	add(cookieCredential(r))
+	add(s.cookieCredential(r))
 
 	// A small form post; the cap keeps this from being a way to stream a body
 	// into memory. A non-form body simply fails to parse and is ignored.

internal/server/login.go

@@ -40,7 +40,7 @@ func (s *Server) handleLoginPage(w http.ResponseWriter, r *http.Request) {
 	next := destination(r.URL.Query().Get("next"))
 
 	// Already logged in: say so rather than showing an empty form.
-	if lim, err := s.limitsFor(r, cookieCredential(r)); err == nil && !lim.Anonymous() {
+	if lim, err := s.limitsFor(r, s.cookieCredential(r)); err == nil && !lim.Anonymous() {
 		s.render(w, http.StatusOK, "login.html", loginPage{
 			page:      s.page(r, "Log in", false),
 			Next:      next,
@@ -82,7 +82,14 @@ func (s *Server) handleLogin(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	s.logIn(w, r, token, r.PostFormValue("persist") != "")
+	if err := s.logIn(w, r, token, r.PostFormValue("persist") != ""); err != nil {
+		// Sealing needs nothing but randomness, so this is the machine failing
+		// rather than the caller: say so instead of leaving them logged out
+		// with no explanation.
+		s.log.Error("sealing the session", "err", err)
+		s.fail(w, r, http.StatusInternalServerError, "Could not start a session.")
+		return
+	}
 	s.log.Info("logged in", "name", lim.Name, "ip", clientIP(r, s.cfg))
 
 	if wantsJSON(r) {

internal/server/pages.go

@@ -61,13 +61,19 @@ type indexPage struct {
 func (s *Server) handleIndex(w http.ResponseWriter, r *http.Request) {
 	// A remembered token is resolved server-side, so the page can show the real
 	// limits without the cookie ever being readable by a script.
-	lim, err := s.limitsFor(r, cookieCredential(r))
+	lim, err := s.limitsFor(r, s.cookieCredential(r))
 	stale := false
-	if err != nil {
+	switch {
+	case err != nil:
 		// The token was revoked or the file was edited; end the session rather
 		// than leave the caller wondering why uploads fail.
 		s.forget(w, r)
 		lim, stale = auth.Anonymous(s.cfg), true
+	case s.staleSession(r):
+		// A cookie this server cannot open. Same treatment: it is not a
+		// session, and it should stop being sent.
+		s.forget(w, r)
+		stale = true
 	}
 
 	s.render(w, http.StatusOK, "index.html", indexPage{
@@ -122,7 +128,7 @@ func (s *Server) renderInfo(w http.ResponseWriter, r *http.Request, m *store.Met
 		Size:      config.FormatSize(m.Size),
 		Expires:   describeExpiry(m.Expires, s.now()),
 		URL:       s.objectURL(r, m.ID),
-		CanDelete: s.mayDelete(r, m, credential(r)),
+		CanDelete: s.mayDelete(r, m, s.credential(r)),
 		Error:     errMsg,
 	})
 }

internal/server/server.go

@@ -37,6 +37,10 @@ type Server struct {
 	// none. Resolved once: the assets cannot change while the process runs.
 	favicon string
 
+	// sessions seals the login cookie, so the token it remembers is not
+	// legible to anyone reading the browser's cookie jar.
+	sessions *sealer
+
 	now func() time.Time // swappable in tests
 }
 
@@ -45,6 +49,10 @@ func New(cfg *config.Config, st *store.Store, tokens *auth.File, log *slog.Logge
 	if err != nil {
 		return nil, err
 	}
+	sessions, err := newSealer(sessionKeyPath(cfg.DataDir))
+	if err != nil {
+		return nil, err
+	}
 	s := &Server{
 		cfg:         cfg,
 		store:       st,
@@ -56,6 +64,7 @@ func New(cfg *config.Config, st *store.Store, tokens *auth.File, log *slog.Logge
 		slots:       make(chan struct{}, cfg.MaxConcurrent),
 		now:         time.Now,
 		favicon:     faviconFor(web.Static()),
+		sessions:    sessions,
 	}
 	s.handler = s.routes()
 	return s, nil
@@ -245,7 +254,7 @@ func (s *Server) page(r *http.Request, title string, script bool) page {
 	if s.favicon != "" {
 		p.Favicon = s.cfg.BasePath + "static/" + s.favicon
 	}
-	if lim, err := s.limitsFor(r, cookieCredential(r)); err == nil {
+	if lim, err := s.limitsFor(r, s.cookieCredential(r)); err == nil {
 		p.User, p.Admin = lim.Name, lim.Admin
 	}
 	return p

internal/server/server_test.go

@@ -126,6 +126,17 @@ func (h *harness) uploadReader(t *testing.T, body io.Reader, headers map[string]
 	return resp
 }
 
+// session builds the cookie a browser would be holding for this token. The
+// value is sealed, so a test cannot simply write the token into it.
+func (h *harness) session(t *testing.T, token string) *http.Cookie {
+	t.Helper()
+	sealed, err := h.sessions.seal(token)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return &http.Cookie{Name: tokenCookie, Value: sealed}
+}
+
 func decode[T any](t *testing.T, resp *http.Response) T {
 	t.Helper()
 	defer resp.Body.Close()
@@ -709,8 +720,13 @@ func TestLoginStoresTheToken(t *testing.T) {
 	if cookie == nil {
 		t.Fatal("logging in set no cookie")
 	}
-	if cookie.Value != h.token {
+	// The whole point of sealing it: the credential is not sitting in the
-		t.Error("the cookie does not hold the token")
+	// browser's cookie jar for anyone glancing at a developer console.
+	if strings.Contains(cookie.Value, h.token) {
+		t.Error("the cookie carries the token in the clear")
+	}
+	if got := h.sessions.open(cookie.Value); got != h.token {
+		t.Errorf("the cookie does not open to the token (got %q)", got)
 	}
 	if !cookie.HttpOnly {
 		t.Error("the session cookie is readable by scripts")
@@ -815,7 +831,7 @@ func TestLoginRedirectIsAllowlisted(t *testing.T) {
 
 func TestLogoutEndsTheSession(t *testing.T) {
 	h := newHarness(t, nil)
-	resp := h.postForm(t, "/logout", url.Values{}, &http.Cookie{Name: tokenCookie, Value: h.token})
+	resp := h.postForm(t, "/logout", url.Values{}, h.session(t, h.token))
 	resp.Body.Close()
 
 	if resp.StatusCode != http.StatusSeeOther {
@@ -837,7 +853,7 @@ func TestUploadNeverTouchesTheSession(t *testing.T) {
 		t.Errorf("an upload with a one-off token set a session cookie: %v", c)
 	}
 
-	resp = h.formUploadWith(t, &http.Cookie{Name: tokenCookie, Value: h.token},
+	resp = h.formUploadWith(t, h.session(t, h.token),
 		map[string]string{}, "b.bin", "two")
 	resp.Body.Close()
 	if c := findCookie(resp, tokenCookie); c != nil {
@@ -848,7 +864,7 @@ func TestUploadNeverTouchesTheSession(t *testing.T) {
 // A one-off token on the form wins over the logged-in session.
 func TestExplicitTokenBeatsTheCookie(t *testing.T) {
 	h := newHarness(t, nil)
-	cookie := &http.Cookie{Name: tokenCookie, Value: h.token}
+	cookie := h.session(t, h.token)
 
 	resp := h.formUploadWith(t, cookie, map[string]string{"token": h.admin}, "b.bin", "y")
 	defer resp.Body.Close()
@@ -863,6 +879,116 @@ func TestExplicitTokenBeatsTheCookie(t *testing.T) {
 	}
 }
 
+// The cookie is sealed with a key this server holds, so one from anywhere else
+// is not a session. This is also the upgrade path: every cookie written in the
+// old plain-text format arrives here.
+func TestCookieFromAnotherKeyIsNotASession(t *testing.T) {
+	h := newHarness(t, nil)
+
+	// A cookie holding the raw token, exactly as the previous format wrote it.
+	plain := &http.Cookie{Name: tokenCookie, Value: h.token}
+	req, _ := http.NewRequest("GET", h.ts.URL+"/", nil)
+	req.AddCookie(plain)
+	resp, err := h.ts.Client().Do(req)
+	if err != nil {
+		t.Fatal(err)
+	}
+	page, _ := io.ReadAll(resp.Body)
+	resp.Body.Close()
+
+	if strings.Contains(string(page), ">friend<") {
+		t.Error("a plain-text cookie was accepted as a session")
+	}
+	if c := findCookie(resp, tokenCookie); c == nil || c.MaxAge >= 0 {
+		t.Error("the unusable cookie was not cleared, so the browser keeps sending it")
+	}
+
+	// Nor does it work anywhere the cookie is a credential.
+	res := h.formUploadWith(t, plain, map[string]string{"vanity": "should-not-work"}, "f.txt", "x")
+	defer res.Body.Close()
+	if res.StatusCode != http.StatusForbidden {
+		t.Errorf("upload with a plain-text cookie = %s, want 403", res.Status)
+	}
+
+	// A cookie sealed by a different server is just as dead.
+	other, err := newSealer(filepath.Join(t.TempDir(), "session.key"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	sealed, err := other.seal(h.token)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got := h.sessions.open(sealed); got != "" {
+		t.Errorf("a cookie from another key opened to %q", got)
+	}
+}
+
+// The key outlives the process: a restart must not log everyone out.
+func TestSessionKeyIsReusedAcrossRestarts(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "session.key")
+
+	first, err := newSealer(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	sealed, err := first.seal("a-token")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	second, err := newSealer(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got := second.open(sealed); got != "a-token" {
+		t.Errorf("after a restart the cookie opened to %q, want the token back", got)
+	}
+
+	info, err := os.Stat(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if perm := info.Mode().Perm(); perm != 0o600 {
+		t.Errorf("session key mode = %o, want 600: it is credential material", perm)
+	}
+
+	// A key that is present but unusable must stop the server rather than be
+	// replaced, which would silently log out every session.
+	if err := os.WriteFile(path, []byte("not a key"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := newSealer(path); err == nil {
+		t.Error("a corrupt session key was accepted")
+	}
+}
+
+// Two seals of the same token must differ, or the cookie becomes a stable
+// fingerprint of which token a visitor holds.
+func TestSealingIsNotDeterministic(t *testing.T) {
+	s, err := newSealer(filepath.Join(t.TempDir(), "session.key"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	first, err := s.seal("a-token")
+	if err != nil {
+		t.Fatal(err)
+	}
+	second, err := s.seal("a-token")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if first == second {
+		t.Error("sealing the same token twice gave the same cookie")
+	}
+	for _, c := range []string{first, second} {
+		if got := s.open(c); got != "a-token" {
+			t.Errorf("cookie opened to %q, want the token", got)
+		}
+	}
+}
+
 // A session whose token has since been revoked must not wedge the page.
 func TestStaleCookieIsDropped(t *testing.T) {
 	h := newHarness(t, nil)
@@ -893,7 +1019,7 @@ func TestStaleCookieIsDropped(t *testing.T) {
 func TestIndexShowsWhoIsLoggedIn(t *testing.T) {
 	h := newHarness(t, nil)
 	req, _ := http.NewRequest("GET", h.ts.URL+"/", nil)
-	req.AddCookie(&http.Cookie{Name: tokenCookie, Value: h.token})
+	req.AddCookie(h.session(t, h.token))
 	resp, err := h.ts.Client().Do(req)
 	if err != nil {
 		t.Fatal(err)
@@ -918,7 +1044,7 @@ func TestCookieDoesNotShadowTheDeleteToken(t *testing.T) {
 	req, _ := http.NewRequest("POST", h.ts.URL+"/d/"+res.ID+"/delete", form)
 	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
 	req.Header.Set("Accept", "application/json")
-	req.AddCookie(&http.Cookie{Name: tokenCookie, Value: h.token})
+	req.AddCookie(h.session(t, h.token))
 	resp, err := h.ts.Client().Do(req)
 	if err != nil {
 		t.Fatal(err)
@@ -1305,7 +1431,7 @@ func TestOwnerDeletesFromTheListing(t *testing.T) {
 	req, _ := http.NewRequest("POST", h.ts.URL+"/d/"+res.ID+"/delete", strings.NewReader("from=files"))
 	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
 	req.Header.Set("Accept", "text/html")
-	req.AddCookie(&http.Cookie{Name: tokenCookie, Value: h.token})
+	req.AddCookie(h.session(t, h.token))
 	resp, err := client.Do(req)
 	if err != nil {
 		t.Fatal(err)
@@ -1327,7 +1453,7 @@ func TestOwnerDeletesFromTheListing(t *testing.T) {
 func TestFilesPageAcceptsTheSession(t *testing.T) {
 	h := newHarness(t, nil)
 	req, _ := http.NewRequest("GET", h.ts.URL+"/files", nil)
-	req.AddCookie(&http.Cookie{Name: tokenCookie, Value: h.admin})
+	req.AddCookie(h.session(t, h.admin))
 	resp, err := h.ts.Client().Do(req)
 	if err != nil {
 		t.Fatal(err)
@@ -1406,7 +1532,7 @@ func TestAdminDeleteReturnsToTheListing(t *testing.T) {
 	req, _ := http.NewRequest("POST", h.ts.URL+"/d/"+res.ID+"/delete", form)
 	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
 	req.Header.Set("Accept", "text/html")
-	req.AddCookie(&http.Cookie{Name: tokenCookie, Value: h.admin})
+	req.AddCookie(h.session(t, h.admin))
 	resp, err := client.Do(req)
 	if err != nil {
 		t.Fatal(err)
@@ -1433,7 +1559,7 @@ func TestAdminDeleteStillRequiresAdmin(t *testing.T) {
 	req, _ := http.NewRequest("POST", h.ts.URL+"/d/"+res.ID+"/delete", form)
 	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
 	req.Header.Set("Accept", "application/json")
-	req.AddCookie(&http.Cookie{Name: tokenCookie, Value: h.token})
+	req.AddCookie(h.session(t, h.token))
 	resp, err := h.ts.Client().Do(req)
 	if err != nil {
 		t.Fatal(err)
@@ -1460,7 +1586,7 @@ func TestFilesLinkIsShownToEveryoneLoggedIn(t *testing.T) {
 	} {
 		req, _ := http.NewRequest("GET", h.ts.URL+"/", nil)
 		if c.token != "" {
-			req.AddCookie(&http.Cookie{Name: tokenCookie, Value: c.token})
+			req.AddCookie(h.session(t, c.token))
 		}
 		resp, err := h.ts.Client().Do(req)
 		if err != nil {
@@ -1654,7 +1780,7 @@ func TestInfoPageOffersADirectButtonToAnOwner(t *testing.T) {
 	} {
 		req, _ := http.NewRequest("GET", h.ts.URL+"/i/"+res.ID, nil)
 		if c.token != "" {
-			req.AddCookie(&http.Cookie{Name: tokenCookie, Value: c.token})
+			req.AddCookie(h.session(t, c.token))
 		}
 		resp, err := h.ts.Client().Do(req)
 		if err != nil {
@@ -1670,7 +1796,7 @@ func TestInfoPageOffersADirectButtonToAnOwner(t *testing.T) {
 
 	// And that button actually works with no token field at all.
 	resp := h.postForm(t, "/d/"+res.ID+"/delete",
-		url.Values{"from": {"info"}}, &http.Cookie{Name: tokenCookie, Value: h.token})
+		url.Values{"from": {"info"}}, h.session(t, h.token))
 	resp.Body.Close()
 	if resp.StatusCode != http.StatusOK {
 		t.Fatalf("owner delete => %s", resp.Status)
@@ -1787,7 +1913,7 @@ func TestHeaderReflectsTheSession(t *testing.T) {
 		for _, path := range []string{"/", "/login"} {
 			req, _ := http.NewRequest("GET", h.ts.URL+path, nil)
 			if c.token != "" {
-				req.AddCookie(&http.Cookie{Name: tokenCookie, Value: c.token})
+				req.AddCookie(h.session(t, c.token))
 			}
 			resp, err := h.ts.Client().Do(req)
 			if err != nil {
@@ -2010,7 +2136,7 @@ func TestPassphraseSessionsAreMemoised(t *testing.T) {
 	resp.Body.Close()
 
 	req, _ := http.NewRequest("GET", h.ts.URL+"/", nil)
-	req.AddCookie(&http.Cookie{Name: tokenCookie, Value: passphrase})
+	req.AddCookie(h.session(t, passphrase))
 	first, err := h.ts.Client().Do(req)
 	if err != nil {
 		t.Fatal(err)
@@ -2028,7 +2154,7 @@ func TestRotationEndsLiveSessions(t *testing.T) {
 	h := newHarness(t, nil)
 
 	// A logged-in browser, and a page render proving the session works.
-	session := &http.Cookie{Name: tokenCookie, Value: h.token}
+	session := h.session(t, h.token)
 	req, _ := http.NewRequest("GET", h.ts.URL+"/", nil)
 	req.AddCookie(session)
 	resp, err := h.ts.Client().Do(req)

internal/server/session.go

@@ -0,0 +1,133 @@
+package server
+
+import (
+	"crypto/aes"
+	"crypto/cipher"
+	"crypto/rand"
+	"encoding/base64"
+	"encoding/hex"
+	"errors"
+	"fmt"
+	"io/fs"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+// sessionKeyName is the file holding the key that seals session cookies. It
+// sits beside the token file and is written just as tightly: anyone who can
+// read it can mint a session for any token they already know.
+const sessionKeyName = "session.key"
+
+// sessionKeyPerm matches the token file rather than the rest of the data
+// directory, which is group-writable by design.
+const sessionKeyPerm fs.FileMode = 0o600
+
+// sealer turns a token into an opaque cookie value and back.
+//
+// The point is not to defend the cookie from its own browser - a stolen cookie
+// is a working session either way, exactly as it was when the token sat there
+// in the clear. The point is that the token itself no longer appears in it, so
+// reading the cookie jar over someone's shoulder, or in a screenshot of a
+// developer console, does not hand over a credential that also works against
+// the API from anywhere else.
+type sealer struct {
+	aead cipher.AEAD
+}
+
+// newSealer loads the key at path, creating it on first run.
+//
+// A key that is present but unusable is an error rather than a reason to
+// generate a new one: silently replacing it would log out every session, and
+// an operator who wants that can delete the file and say so.
+func newSealer(path string) (*sealer, error) {
+	key, err := readSessionKey(path)
+	if errors.Is(err, os.ErrNotExist) {
+		if key, err = createSessionKey(path); err != nil {
+			return nil, err
+		}
+	} else if err != nil {
+		return nil, err
+	}
+
+	block, err := aes.NewCipher(key)
+	if err != nil {
+		return nil, fmt.Errorf("session key: %w", err)
+	}
+	aead, err := cipher.NewGCM(block)
+	if err != nil {
+		return nil, fmt.Errorf("session key: %w", err)
+	}
+	return &sealer{aead: aead}, nil
+}
+
+func sessionKeyPath(dataDir string) string {
+	return filepath.Join(dataDir, sessionKeyName)
+}
+
+func readSessionKey(path string) ([]byte, error) {
+	raw, err := os.ReadFile(path)
+	if err != nil {
+		return nil, err
+	}
+	key, err := hex.DecodeString(strings.TrimSpace(string(raw)))
+	if err != nil {
+		return nil, fmt.Errorf("%s is not a hex key: %w", path, err)
+	}
+	if len(key) != 32 {
+		return nil, fmt.Errorf("%s holds a %d-byte key, want 32", path, len(key))
+	}
+	return key, nil
+}
+
+// createSessionKey writes a new key, refusing to clobber one that appeared in
+// the meantime: two servers started at once must not end up with the file
+// holding the key only one of them is using.
+func createSessionKey(path string) ([]byte, error) {
+	key := make([]byte, 32)
+	if _, err := rand.Read(key); err != nil {
+		return nil, fmt.Errorf("generating a session key: %w", err)
+	}
+	f, err := os.OpenFile(path, os.O_WRONLY|os.O_CREATE|os.O_EXCL, sessionKeyPerm)
+	if err != nil {
+		if errors.Is(err, os.ErrExist) {
+			return readSessionKey(path)
+		}
+		return nil, fmt.Errorf("creating %s: %w", path, err)
+	}
+	defer f.Close()
+	if _, err := fmt.Fprintf(f, "%x\n", key); err != nil {
+		return nil, fmt.Errorf("writing %s: %w", path, err)
+	}
+	// The umask may have widened the mode; say what it has to be.
+	if err := f.Chmod(sessionKeyPerm); err != nil {
+		return nil, fmt.Errorf("securing %s: %w", path, err)
+	}
+	return key, f.Sync()
+}
+
+// seal returns the cookie value carrying token.
+func (s *sealer) seal(token string) (string, error) {
+	nonce := make([]byte, s.aead.NonceSize())
+	if _, err := rand.Read(nonce); err != nil {
+		return "", err
+	}
+	sealed := s.aead.Seal(nonce, nonce, []byte(token), nil)
+	return base64.RawURLEncoding.EncodeToString(sealed), nil
+}
+
+// open recovers the token from a cookie value. Anything that does not decrypt
+// is treated as absent: a cookie from an older format or another key is not an
+// error to report, it is simply not a session.
+func (s *sealer) open(value string) string {
+	raw, err := base64.RawURLEncoding.DecodeString(value)
+	if err != nil || len(raw) < s.aead.NonceSize() {
+		return ""
+	}
+	nonce, body := raw[:s.aead.NonceSize()], raw[s.aead.NonceSize():]
+	token, err := s.aead.Open(nil, nonce, body, nil)
+	if err != nil {
+		return ""
+	}
+	return string(token)
+}

internal/server/upload.go

@@ -76,7 +76,7 @@ func (s *Server) uploadRaw(w http.ResponseWriter, r *http.Request, ip string) {
 		filename: filenameFromDisposition(r.Header.Get("Content-Disposition")),
 	}
 	if req.token == "" {
-		req.token = cookieCredential(r)
+		req.token = s.cookieCredential(r)
 	}
 	s.storeUpload(w, r, req, r.Body, ip)
 }
@@ -128,7 +128,7 @@ func (s *Server) uploadMultipart(w http.ResponseWriter, r *http.Request, boundar
 			// remembered. This happens after the fields precisely so a typed
 			// token still wins.
 			if req.token == "" {
-				req.token = cookieCredential(r)
+				req.token = s.cookieCredential(r)
 			}
 			s.storeUpload(w, r, req, part, ip)
 			return

web/templates/index.html

@@ -1,6 +1,6 @@
 {{define "content"}}
 {{if .Stale}}
-<p class="notice">Your login is no longer valid, that token has been removed. You have been logged out.</p>
+<p class="notice">Your login is no longer valid. You have been logged out; log in again to carry on.</p>
 {{end}}
 
 <form id="upload" class="card" method="post" action="{{.Base}}upload"