Derive the cold-start unlock key from the passkey PRF, not the public key (#14)

Cold-start unlock wrapped the database key under the credential *public* key.
A public key is public: mavweb writes it verbatim to passkeys.json, normally in
the same state dir as db_key.wrapped, so anyone holding both files recovered the
database key offline with no authenticator involved. The wrapped blob was a
plaintext key with extra steps.

The secret is now the WebAuthn PRF extension output — 32 bytes the authenticator
computes over a fixed salt and never stores anywhere. The blob gains a version:

  v2:  "MVNKW2\x00" || salt || nonce || AES-256-GCM(key), magic as AAD
  v1:  salt || nonce || AES-256-GCM(key)                  (read-only)

v1 still opens so an existing deployment is not bricked, and reports itself so
the daemon can log a SECURITY line telling him to re-enroll. Nothing writes v1.
The magic is authenticated, so a v2 blob cannot be stripped and re-read as v1.

Four other defects on the same path:

  - The locked-boot store was opened on an IPC goroutine inside UnlockFn and
    never closed. Close is what re-encrypts the tmpfs working copy back over
    the ciphertext, so every write of a cold-started session was lost silently
    on the next boot. daemonLock now owns the store and seals it at shutdown.
  - MethodUnlock was reachable by anything on the box; the socket is same-uid
    and cannot authenticate its caller. It now requires a passkey assertion
    that mavweb verified first.
  - Concurrent unlocks would each open a store and wire a daemon. One at a
    time, and never a second one.
  - The hand-rolled HKDF keyed the expand step with the salt instead of the
    PRK. Replaced with crypto/hkdf.

Key wrapping moves from enrolment to the first assertion, because create() does
not produce a PRF result on most authenticators — only a support flag. An
authenticator without PRF now writes no wrapped file at all rather than one
that looks protected and is not, and the page says so.

Verified: make build, make test. New tests cover the v2 round trip, a wrong
secret, every single-bit tamper, truncation, the v1 downgrade attempt, legacy
v1 reads, non-32-byte and all-zero secrets, the ipc wire field, locked-mode
default-deny, a forged assertion never reaching the unlock path, seal-on-
shutdown after a cold start, and that nothing in the state dir contains the
plaintext key. The PRF round trip against real hardware is a QA step.

Vikunja #14

cmd/mavend/coldstart_test.go

@@ -0,0 +1,190 @@
+package main
+
+import (
+	"bytes"
+	"context"
+	"crypto/rand"
+	"io"
+	"os"
+	"path/filepath"
+	"testing"
+	"time"
+
+	"github.com/kami/maven/internal/store"
+	"github.com/kami/maven/internal/webauthn"
+)
+
+func randBytes(t *testing.T, n int) []byte {
+	t.Helper()
+	b := make([]byte, n)
+	if _, err := io.ReadFull(rand.Reader, b); err != nil {
+		t.Fatalf("rand: %v", err)
+	}
+	b[0] |= 1
+	return b
+}
+
+func TestDaemonLockStartsLockedAndFlips(t *testing.T) {
+	dl := newDaemonLock(true)
+	if !dl.isLocked() {
+		t.Fatal("newDaemonLock(true) is not locked")
+	}
+	dl.unlock(nil)
+	if dl.isLocked() {
+		t.Fatal("still locked after unlock")
+	}
+	if newDaemonLock(false).isLocked() {
+		t.Fatal("newDaemonLock(false) reports locked")
+	}
+}
+
+// closeStore must be safe on a daemon that never unlocked and safe twice —
+// shutdown runs it unconditionally.
+func TestDaemonLockCloseStoreIsSafeWhenNeverUnlocked(t *testing.T) {
+	dl := newDaemonLock(true)
+	if err := dl.closeStore(); err != nil {
+		t.Fatalf("closeStore with no store: %v", err)
+	}
+	if err := dl.closeStore(); err != nil {
+		t.Fatalf("second closeStore: %v", err)
+	}
+}
+
+// The data-loss bug: in locked mode the store is opened on an IPC goroutine
+// inside UnlockFn, and shutdown runs on main. Without the handoff nothing
+// calls Close, and Close is what re-encrypts the tmpfs working copy back over
+// the ciphertext file — so every write of a cold-started session vanished.
+func TestDaemonLockSealsTheStoreOpenedAfterUnlock(t *testing.T) {
+	dir := t.TempDir()
+	dbPath := filepath.Join(dir, "maven.db")
+	tmpfs := filepath.Join(dir, "work")
+	key := randBytes(t, 32)
+	// Store.Close zeroes the key slice it was handed (encState.key is the
+	// caller's backing array), so the next boot needs its own copy — exactly
+	// as mavend keeps envKeyBytes separate from the config's key.
+	nextBoot := bytes.Clone(key)
+	ctx := context.Background()
+
+	// Cold start: locked, no store.
+	dl := newDaemonLock(true)
+
+	// ... unlock arrives, opens the store and hands it over.
+	st, err := store.OpenEncrypted(ctx, dbPath, tmpfs, key)
+	if err != nil {
+		t.Fatalf("OpenEncrypted: %v", err)
+	}
+	dl.unlock(st)
+	if _, err := st.WriteNote(ctx, time.Now(), "заметка после холодного старта", nil, "test"); err != nil {
+		t.Fatalf("WriteNote: %v", err)
+	}
+
+	// Shutdown.
+	if err := dl.closeStore(); err != nil {
+		t.Fatalf("closeStore: %v", err)
+	}
+	if err := dl.closeStore(); err != nil {
+		t.Fatalf("second closeStore after a real store: %v", err)
+	}
+
+	// Next boot with the same key must see the write.
+	st2, err := store.OpenEncrypted(ctx, dbPath, tmpfs, nextBoot)
+	if err != nil {
+		t.Fatalf("reopen: %v", err)
+	}
+	defer st2.Close()
+	notes, err := st2.RecentNotes(ctx, 10)
+	if err != nil {
+		t.Fatalf("RecentNotes: %v", err)
+	}
+	if len(notes) != 1 {
+		t.Fatalf("got %d notes after a cold-started session, want 1 — the session was lost", len(notes))
+	}
+}
+
+// The whole point of the wrapped blob: what sits in the state dir must not let
+// anyone open the database. Nothing written there may contain the key, and the
+// ciphertext must not be readable with a wrong one.
+func TestColdStartLeavesNoPlaintextKeyOnDisk(t *testing.T) {
+	dir := t.TempDir()
+	dbPath := filepath.Join(dir, "maven.db")
+	tmpfs := filepath.Join(dir, "work")
+	wrappedPath := filepath.Join(dir, "db_key.wrapped")
+	key := randBytes(t, 32)
+	secret := randBytes(t, 32)
+	ctx := context.Background()
+
+	blob, err := webauthn.WrapKey(key, secret)
+	if err != nil {
+		t.Fatalf("WrapKey: %v", err)
+	}
+	if err := os.WriteFile(wrappedPath, blob, 0o600); err != nil {
+		t.Fatalf("write wrapped key: %v", err)
+	}
+
+	st, err := store.OpenEncrypted(ctx, dbPath, tmpfs, key)
+	if err != nil {
+		t.Fatalf("OpenEncrypted: %v", err)
+	}
+	if _, err := st.WriteNote(ctx, time.Now(), "секрет", nil, "test"); err != nil {
+		t.Fatalf("WriteNote: %v", err)
+	}
+	if err := st.Close(); err != nil {
+		t.Fatalf("Close: %v", err)
+	}
+
+	// Walk everything in the state dir; none of it may contain the key.
+	err = filepath.Walk(dir, func(p string, info os.FileInfo, err error) error {
+		if err != nil || info.IsDir() {
+			return err
+		}
+		b, rerr := os.ReadFile(p)
+		if rerr != nil {
+			return nil // unreadable is not a leak
+		}
+		if bytes.Contains(b, key) {
+			t.Errorf("%s contains the plaintext encryption key", p)
+		}
+		return nil
+	})
+	if err != nil {
+		t.Fatalf("walk: %v", err)
+	}
+
+	// The wrapped file must have owner-only permissions.
+	fi, err := os.Stat(wrappedPath)
+	if err != nil {
+		t.Fatalf("stat: %v", err)
+	}
+	if perm := fi.Mode().Perm(); perm != 0o600 {
+		t.Errorf("wrapped key file mode = %o, want 600", perm)
+	}
+
+	// A wrong passkey must not open the store.
+	if _, _, err := webauthn.UnwrapKey(blob, randBytes(t, 32)); err == nil {
+		t.Fatal("a wrong PRF secret unwrapped the key")
+	}
+	if _, err := store.OpenEncrypted(ctx, dbPath, filepath.Join(dir, "work2"), randBytes(t, 32)); err == nil {
+		t.Fatal("the encrypted store opened under a wrong key")
+	}
+
+	// And the right one round-trips back to a readable database.
+	got, version, err := webauthn.UnwrapKey(blob, secret)
+	if err != nil {
+		t.Fatalf("UnwrapKey: %v", err)
+	}
+	if version != webauthn.BlobV2 {
+		t.Errorf("blob version = %v, want v2", version)
+	}
+	st2, err := store.OpenEncrypted(ctx, dbPath, tmpfs, got)
+	if err != nil {
+		t.Fatalf("reopen with the unwrapped key: %v", err)
+	}
+	defer st2.Close()
+	notes, err := st2.RecentNotes(ctx, 10)
+	if err != nil {
+		t.Fatalf("RecentNotes: %v", err)
+	}
+	if len(notes) != 1 {
+		t.Fatalf("got %d notes, want 1", len(notes))
+	}
+}

cmd/mavend/main.go

@@ -66,12 +66,19 @@ import (
 
 var errLocked = errors.New("mavend: daemon locked — complete passkey assertion first")
 
-// daemonLock tracks whether the daemon is in locked (pre-unlock) mode.
-// In locked mode, all CoreAPI methods return errLocked. The unlock path
-// replaces the CoreAPI with the real store adapter and flips the flag.
+// daemonLock tracks whether the daemon is in locked (pre-unlock) mode, and
+// owns the store handle the unlock path creates.
+//
+// The store matters here because of who runs when. In locked mode there is no
+// store at boot; one is opened inside UnlockFn, on an IPC goroutine, minutes
+// or days later. Shutdown runs on the main goroutine. Without a handoff the
+// main goroutine has nothing to close, and store.Close is what re-encrypts
+// the tmpfs working copy back over the ciphertext file — so a daemon that
+// cold-started lost every write of that session, silently, on the next boot.
 type daemonLock struct {
 	mu     sync.Mutex
 	locked bool
+	st     *store.Store
 }
 
 func newDaemonLock(locked bool) *daemonLock {
@@ -84,10 +91,25 @@ func (l *daemonLock) isLocked() bool {
 	return l.locked
 }
 
-func (l *daemonLock) unlock() {
+// unlock flips the flag and takes ownership of the store opened by UnlockFn.
+func (l *daemonLock) unlock(st *store.Store) {
 	l.mu.Lock()
 	defer l.mu.Unlock()
 	l.locked = false
+	l.st = st
+}
+
+// closeStore seals the store the unlock path opened, if any. Safe to call
+// when the daemon never unlocked, and safe to call twice.
+func (l *daemonLock) closeStore() error {
+	l.mu.Lock()
+	st := l.st
+	l.st = nil
+	l.mu.Unlock()
+	if st == nil {
+		return nil
+	}
+	return st.Close()
 }
 
 func main() {
@@ -155,6 +177,14 @@ func run(args []string) error {
 			return fmt.Errorf("open store: %w", err)
 		}
 		defer st.Close()
+	} else {
+		// Locked boot: the store does not exist yet. Seal whatever UnlockFn
+		// opened, at shutdown, on this goroutine.
+		defer func() {
+			if err := dl.closeStore(); err != nil {
+				log.Printf("mavend: seal store on shutdown: %v", err)
+			}
+		}()
 	}
 
 	// ----- daemon components (only wired when unlocked) -----
@@ -346,12 +376,16 @@ func run(args []string) error {
 		wireSpeaker(srv, st, cfg)
 	}
 
-	// WrapKeyFn — wraps the env key with a passkey credential public key and
-	// persists the wrapped blob. Only wired when the daemon has the key in
-	// memory (env key mode). Called by mavweb after passkey enrollment.
+	// WrapKeyFn — wraps the env key under the passkey PRF secret and persists
+	// the wrapped blob. Only wired when the daemon has the key in memory (env
+	// key mode). Called by mavweb after passkey enrollment.
+	//
+	// webauthn.WrapKey refuses anything that is not a 32-byte PRF output, so
+	// an authenticator without PRF support produces no wrapped file at all
+	// rather than a file that looks protected and is not.
 	if envKeyBytes != nil {
-		srv.WrapKeyFn = func(ctx context.Context, publicKey []byte) error {
-			blob, err := webauthn.WrapKey(envKeyBytes, publicKey)
+		srv.WrapKeyFn = func(ctx context.Context, secret []byte) error {
+			blob, err := webauthn.WrapKey(envKeyBytes, secret)
 			if err != nil {
 				return fmt.Errorf("wrap encryption key: %w", err)
 			}
@@ -367,20 +401,42 @@ func run(args []string) error {
 		}
 	}
 
-	// UnlockFn — cold-start unlock: unwraps the encryption key from the wrapped
-	// blob using the passkey credential public key, opens the store, wires all
+	// UnlockFn — cold-start unlock: unwraps the encryption key from the
+	// wrapped blob using the passkey PRF secret, opens the store, wires all
 	// daemon components, and replaces the locked API.
 	if locked {
-		srv.UnlockFn = func(ctx context.Context, publicKey []byte) error {
+		var unlockMu sync.Mutex
+		srv.UnlockFn = func(ctx context.Context, secret []byte) error {
+			// One unlock at a time, and never a second one. Without this a
+			// concurrent pair of Unlock calls would each open a store and
+			// wire a full daemon, and the loser's goroutines would run
+			// against a store nobody closes.
+			unlockMu.Lock()
+			defer unlockMu.Unlock()
+			if !dl.isLocked() {
+				return nil // already unlocked; the caller does not need to know
+			}
+
+			// The wire cannot authenticate its caller — the socket is
+			// same-uid — so the unlock path requires a passkey assertion
+			// that mavweb verified cryptographically first. Without this,
+			// MethodUnlock is reachable by anything on the box.
+			if !passkeySess.IsStepUp() {
+				return errors.New("unlock: no verified passkey assertion (assert first)")
+			}
+
 			wp := *wrappedKeyPath
 			blob, err := os.ReadFile(wp)
 			if err != nil {
 				return fmt.Errorf("read wrapped key: %w", err)
 			}
-			key, err := webauthn.UnwrapKey(blob, publicKey)
+			key, version, err := webauthn.UnwrapKey(blob, secret)
 			if err != nil {
 				return fmt.Errorf("unwrap key: %w", err)
 			}
+			if version == webauthn.BlobV1 {
+				log.Printf("SECURITY: %s was unwrapped from a %s blob. The wrapping key is derived from the credential PUBLIC key, which mavweb also writes to its passkeys.json — anyone holding both files can recover the database key with no authenticator. Re-enroll the passkey on an authenticator that supports the PRF extension to rewrite it as v2.", wp, version)
+			}
 			// Open the store with the unwrapped key.
 			st, err = store.OpenEncrypted(ctx, cfg.DBPath, cfg.DBTmpfs, key)
 			if err != nil {
@@ -543,7 +599,7 @@ func run(args []string) error {
 				go voiceW.mcp.run(ctx)
 			}
 
-			dl.unlock()
+			dl.unlock(st)
 			log.Printf("mavend: unlocked via passkey assertion")
 			return nil
 		}