fix(security): sign the setup ceremony with startup-generated keys

REVIEW-001 (M0) M1s: the /setup wizard's pre-session double-submit ceremony
(A15) was signed with csrf_key/cookie_secret left at the shipped CHANGE_ME
placeholder until `finish` persisted generated values — a publicly-known
signing key for the whole bootstrap.

Generate strong ephemeral values at startup in setup mode, mirroring the
TOTP key (R-6/D-1): `finish` skips a non-placeholder, so the wizard persists
the very values the process already uses.

Also add cmd/myday's first test — REVIEW-001 M2c: the D-1 invariant (the
startup TOTP key is the persisted key) was covered only by a manual smoke. It
generates the key, enrolls a secret, persists via the wizard, reopens the DB
with the persisted key and asserts the secret still unseals.

cmd/myday/main.go

@@ -104,20 +104,9 @@ func run() error {
 	// R-6 already rules that a placeholder is replaced with a strong generated
 	// value, so generate it here; it is adopted (and thereby persisted by the
 	// wizard's `finish`, which skips a non-placeholder) only in setup mode.
-	sealerKey := cfg.Secrets.TOTPEncryptionKey
-	generatedKey := false
-	switch {
-	case config.SecretIsPlaceholder(sealerKey):
-		k, err := config.GenerateSecret(24) // 24 bytes -> 32 base64url chars == auth.KeySize
-		if err != nil {
-			return err
-		}
-		sealerKey, generatedKey = k, true
-	case len(sealerKey) != auth.KeySize:
-		// An operator-supplied key of the wrong size: fail closed and say so,
-		// rather than run a ceremony whose result we cannot store (A9).
-		return fmt.Errorf("myday: secrets.totp_encryption_key must be exactly %d bytes, got %d",
-			auth.KeySize, len(sealerKey))
+	sealerKey, generatedKey, err := prepareTOTPKey(cfg)
+	if err != nil {
+		return err
 	}
 	sealer, err := auth.NewAEADSealer([]byte(sealerKey))
 	if err != nil {
@@ -163,6 +152,21 @@ func run() error {
 			"the wizard enrolls TOTP before it persists, so the key must exist now")
 	}
 
+	// M1s: the CSRF and cookie signing keys drive the whole pre-session
+	// ceremony (A15), so they must be strong from the FIRST request too — not
+	// only after `finish` persists them (which would leave the wizard signing
+	// with the shipped, publicly-known placeholder until a restart). Generated
+	// here in setup mode only; `finish` then persists these very values.
+	if inSetup {
+		generated, err := prepareSigningKeys(cfg)
+		if err != nil {
+			return err
+		}
+		for _, name := range generated {
+			slog.Info("myday: generated a strong signing key for setup (R-6/M1s)", "name", name)
+		}
+	}
+
 	csrf, err := httpx.NewCSRF([]byte(cfg.Secrets.CSRFKey))
 	if err != nil {
 		return err
@@ -199,6 +203,7 @@ func run() error {
 		CSRF:          preSession,
 		Audit:         aud,
 		SuppressNudge: setupMode,
+		BaseURL:       cfg.Server.BaseURL,
 		Secure:        cfg.IsSecure(),
 		Logger:        slog.Default(),
 	})
@@ -221,6 +226,7 @@ func run() error {
 		Audit:          aud,
 		TOTPOnlyEnroll: true,
 		SuccessPath:    func(string) string { return "/setup" },
+		BaseURL:        cfg.Server.BaseURL,
 		Secure:         cfg.IsSecure(),
 		Logger:         slog.Default(),
 	})
@@ -392,6 +398,58 @@ func buildRegistry(authFlow *flow.Actions, setupFlow *setup.Actions, recoveryFlo
 	return reg
 }
 
+// prepareTOTPKey returns the TOTP sealing key the process will use, generating
+// a strong one when the configured value is a placeholder (R-6/D-1). generated
+// reports that the value was minted here — run() adopts it only in setup mode,
+// where the process must still be able to seal a ceremony result.
+//
+// An operator-supplied key of the wrong size fails closed: rather than run a
+// ceremony whose result we cannot store (A9), say so and stop.
+func prepareTOTPKey(cfg *config.Config) (key string, generated bool, err error) {
+	key = cfg.Secrets.TOTPEncryptionKey
+	if config.SecretIsPlaceholder(key) {
+		// 24 bytes -> 32 base64url chars == auth.KeySize.
+		key, err = config.GenerateSecret(24)
+		if err != nil {
+			return "", false, err
+		}
+		return key, true, nil
+	}
+	if len(key) != auth.KeySize {
+		return "", false, fmt.Errorf("myday: secrets.totp_encryption_key must be exactly %d bytes, got %d",
+			auth.KeySize, len(key))
+	}
+	return key, false, nil
+}
+
+// prepareSigningKeys replaces placeholder CSRF / cookie signing keys with strong
+// generated values (R-6/M1s) and returns the names of the secrets it generated.
+//
+// run() calls it only in setup mode: the wizard's pre-session double-submit
+// ceremony (A15) is signed with these keys from the first request, so the
+// shipped, publicly-known placeholder must never be used. In productive mode
+// config.Validate already refuses a placeholder (fail closed, SPEC-003 §1).
+func prepareSigningKeys(cfg *config.Config) (generated []string, err error) {
+	for _, s := range []struct {
+		name string
+		dst  *string
+	}{
+		{"secrets.csrf_key", &cfg.Secrets.CSRFKey},
+		{"secrets.cookie_secret", &cfg.Secrets.CookieSecret},
+	} {
+		if !config.SecretIsPlaceholder(*s.dst) {
+			continue
+		}
+		v, err := config.GenerateSecret(32)
+		if err != nil {
+			return generated, err
+		}
+		*s.dst = v
+		generated = append(generated, s.name)
+	}
+	return generated, nil
+}
+
 // configTarget picks where the wizard persists settings: `config.yaml` beside
 // the loaded file. It NEVER overwrites the committed `config.example.yaml`
 // (D-010) — so a dev run on the example writes a real config.yaml next to it.

cmd/myday/main_test.go

@@ -0,0 +1,166 @@
+package main
+
+import (
+	"context"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"net/url"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"git.lohmar.co.uk/cclohmar/NextMyDay/internal/core/audit"
+	"git.lohmar.co.uk/cclohmar/NextMyDay/internal/core/auth"
+	"git.lohmar.co.uk/cclohmar/NextMyDay/internal/core/config"
+	"git.lohmar.co.uk/cclohmar/NextMyDay/internal/core/httpx"
+	"git.lohmar.co.uk/cclohmar/NextMyDay/internal/core/setup"
+	"git.lohmar.co.uk/cclohmar/NextMyDay/internal/core/store"
+	"git.lohmar.co.uk/cclohmar/NextMyDay/internal/core/users"
+)
+
+// stubEnroller satisfies setup.Enroller — this test never runs a ceremony.
+type stubEnroller struct{}
+
+func (stubEnroller) TOTPBegin(http.ResponseWriter, *http.Request, string) error  { return nil }
+func (stubEnroller) TOTPVerify(http.ResponseWriter, *http.Request, string) error { return nil }
+
+// setupYAML is a fresh-instance config: setup mode, every secret a placeholder.
+const setupYAML = `setup: true
+server:
+  listen: "127.0.0.1:8080"
+  base_url: "https://myday.example.com"
+app:
+  locale: "en-GB"
+database:
+  path: "DBPATH"
+secrets:
+  cookie_secret: "CHANGE_ME_32_BYTES_RANDOM"
+  csrf_key: "CHANGE_ME_32_BYTES_RANDOM"
+  totp_encryption_key: "CHANGE_ME_32_BYTES_RANDOM"
+`
+
+// TestTOTPKeySurvivesSetupAndRestart pins the D-1 invariant (M2c): the TOTP key
+// the process GENERATES AT STARTUP is the key the wizard persists, so a restart
+// adopts the same key and the secrets enrolled during setup still unseal. A
+// regression here silently destroys every future TOTP secret.
+func TestTOTPKeySurvivesSetupAndRestart(t *testing.T) {
+	ctx := context.Background()
+	dir := t.TempDir()
+	cfgPath := filepath.Join(dir, config.FileName)
+	dbPath := filepath.Join(dir, "myday.db")
+	body := strings.Replace(setupYAML, "DBPATH", dbPath, 1)
+	if err := os.WriteFile(cfgPath, []byte(body), 0o600); err != nil {
+		t.Fatalf("write config: %v", err)
+	}
+
+	cfg, err := config.Load(cfgPath)
+	if err != nil {
+		t.Fatalf("config.Load: %v", err)
+	}
+
+	// Startup (D-1 + M1s): every placeholder is replaced with a strong value
+	// the PROCESS will use — the wizard's ceremony must not run on a shipped
+	// placeholder key.
+	key, generated, err := prepareTOTPKey(cfg)
+	if err != nil {
+		t.Fatalf("prepareTOTPKey: %v", err)
+	}
+	if !generated || config.SecretIsPlaceholder(key) || len(key) != auth.KeySize {
+		t.Fatalf("the TOTP key was not generated (generated=%v len=%d)", generated, len(key))
+	}
+	cfg.Secrets.TOTPEncryptionKey = key // run() adopts it in setup mode
+	if _, err := prepareSigningKeys(cfg); err != nil {
+		t.Fatalf("prepareSigningKeys: %v", err)
+	}
+
+	// First run: the wizard enrolls the admin and seals its TOTP secret with
+	// the startup key.
+	db, err := store.Open(cfg.Database.Path)
+	if err != nil {
+		t.Fatalf("store.Open: %v", err)
+	}
+	if err := db.Migrate(); err != nil {
+		t.Fatalf("Migrate: %v", err)
+	}
+	aud := audit.New(db.DB)
+	us := users.New(db.DB, aud)
+	if _, err := us.Create(ctx, 0, "admin@example.com", "Claus", users.RoleAdmin); err != nil {
+		t.Fatalf("create admin: %v", err)
+	}
+	sealer, err := auth.NewAEADSealer([]byte(key))
+	if err != nil {
+		t.Fatalf("NewAEADSealer: %v", err)
+	}
+	ids := auth.New(db.DB, aud, auth.WithSealer(sealer)).Identities()
+	idID, _, err := ids.Mint(ctx, 1)
+	if err != nil {
+		t.Fatalf("Mint: %v", err)
+	}
+	if err := ids.Activate(ctx, idID); err != nil {
+		t.Fatalf("Activate: %v", err)
+	}
+	const secret = "JBSWY3DPEHPK3PXP"
+	if err := ids.PutTOTPSecret(ctx, idID, []byte(secret)); err != nil {
+		t.Fatalf("PutTOTPSecret: %v", err)
+	}
+
+	// The wizard persists the config (`finish`).
+	pre, err := httpx.NewPreSessionCSRF([]byte(cfg.Secrets.CookieSecret), false)
+	if err != nil {
+		t.Fatalf("NewPreSessionCSRF: %v", err)
+	}
+	wiz, err := setup.New(setup.Deps{
+		Config: cfg, ConfigPath: cfgPath, Users: us, Identities: ids,
+		Enroll: stubEnroller{}, CSRF: pre, Audit: aud, Printout: io.Discard,
+	})
+	if err != nil {
+		t.Fatalf("setup.New: %v", err)
+	}
+	form := url.Values{"app.locale": {"en-GB"}, "app.timezone": {"Europe/London"}}
+	req := httptest.NewRequest("POST", "/setup?do=setup&action=finish", strings.NewReader(form.Encode()))
+	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
+	rec := httptest.NewRecorder()
+	if err := wiz.Finish(rec, req); err != nil {
+		t.Fatalf("Finish: %v", err)
+	}
+	if rec.Code != http.StatusSeeOther {
+		t.Fatalf("Finish = %d, want 303", rec.Code)
+	}
+	if err := db.Close(); err != nil {
+		t.Fatalf("close db: %v", err)
+	}
+
+	// Restart: the persisted config must carry the SAME TOTP key, and the
+	// sealed secret must still open with it (D-1 — the whole point).
+	restored, err := config.Load(cfgPath)
+	if err != nil {
+		t.Fatalf("the persisted config does not load after setup: %v", err)
+	}
+	if restored.Setup {
+		t.Error("the persisted config still says setup: true")
+	}
+	if restored.Secrets.TOTPEncryptionKey != key {
+		t.Fatalf("the persisted TOTP key differs from the runtime key (D-1):\n got %q\nwant %q",
+			restored.Secrets.TOTPEncryptionKey, key)
+	}
+
+	db2, err := store.Open(restored.Database.Path)
+	if err != nil {
+		t.Fatalf("reopen db: %v", err)
+	}
+	defer func() { _ = db2.Close() }()
+	sealer2, err := auth.NewAEADSealer([]byte(restored.Secrets.TOTPEncryptionKey))
+	if err != nil {
+		t.Fatalf("restart sealer: %v", err)
+	}
+	ids2 := auth.New(db2.DB, nil, auth.WithSealer(sealer2)).Identities()
+	got, err := ids2.TOTPSecret(ctx, idID)
+	if err != nil {
+		t.Fatalf("the TOTP secret no longer unseals after a restart (D-1/M2c): %v", err)
+	}
+	if string(got) != secret {
+		t.Errorf("TOTP secret after restart = %q, want %q", got, secret)
+	}
+}