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/mavweb/passkey_prf_test.go

@@ -0,0 +1,293 @@
+package main
+
+import (
+	"bytes"
+	"context"
+	"crypto/ecdsa"
+	"crypto/elliptic"
+	"crypto/rand"
+	"crypto/sha256"
+	"encoding/base64"
+	"encoding/binary"
+	"encoding/json"
+	"errors"
+	"net/http"
+	"net/http/httptest"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"github.com/kami/maven/internal/webauthn"
+)
+
+const prfTestOrigin = "https://maven.test"
+const prfTestRPID = "maven.test"
+
+// fakeKeyIPC stands in for the mavend socket and records exactly what secret
+// each call received — the point of the whole test file is that it is the PRF
+// output and never the credential public key.
+type fakeKeyIPC struct {
+	unlockSecret []byte
+	wrapSecret   []byte
+	unlockCalls  int
+	wrapCalls    int
+	unlockErr    error
+}
+
+func (f *fakeKeyIPC) Unlock(_ context.Context, secret []byte) error {
+	f.unlockCalls++
+	f.unlockSecret = bytes.Clone(secret)
+	return f.unlockErr
+}
+
+func (f *fakeKeyIPC) StoreEncryptionKey(_ context.Context, secret []byte) error {
+	f.wrapCalls++
+	f.wrapSecret = bytes.Clone(secret)
+	return nil
+}
+
+func b64u(b []byte) string { return base64.RawURLEncoding.EncodeToString(b) }
+
+// prfAuthenticator is a minimal software authenticator: a P-256 key plus the
+// COSE encoding of its public half.
+type prfAuthenticator struct {
+	key    *ecdsa.PrivateKey
+	credID []byte
+	cose   []byte
+}
+
+func newPRFAuthenticator(t *testing.T) *prfAuthenticator {
+	t.Helper()
+	key, err := ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
+	if err != nil {
+		t.Fatalf("generate key: %v", err)
+	}
+	x := key.PublicKey.X.FillBytes(make([]byte, 32))
+	y := key.PublicKey.Y.FillBytes(make([]byte, 32))
+	// COSE_Key: {1: 2 (EC2), 3: -7 (ES256), -1: 1 (P-256), -2: x, -3: y}
+	var c []byte
+	c = append(c, 0xa5)             // map(5)
+	c = append(c, 0x01, 0x02)       // 1: 2
+	c = append(c, 0x03, 0x26)       // 3: -7
+	c = append(c, 0x20, 0x01)       // -1: 1
+	c = append(c, 0x21, 0x58, 0x20) // -2: bytes(32)
+	c = append(c, x...)
+	c = append(c, 0x22, 0x58, 0x20) // -3: bytes(32)
+	c = append(c, y...)
+	return &prfAuthenticator{key: key, credID: []byte("prf-cred"), cose: c}
+}
+
+func (a *prfAuthenticator) authData(flags byte, counter uint32, attested bool) []byte {
+	h := sha256.Sum256([]byte(prfTestRPID))
+	d := append([]byte{}, h[:]...)
+	d = append(d, flags)
+	cb := make([]byte, 4)
+	binary.BigEndian.PutUint32(cb, counter)
+	d = append(d, cb...)
+	if attested {
+		d = append(d, make([]byte, 16)...) // aaguid
+		l := make([]byte, 2)
+		binary.BigEndian.PutUint16(l, uint16(len(a.credID)))
+		d = append(d, l...)
+		d = append(d, a.credID...)
+		d = append(d, a.cose...)
+	}
+	return d
+}
+
+func clientDataJSON(typ, challenge string) []byte {
+	b, _ := json.Marshal(map[string]string{"type": typ, "challenge": challenge, "origin": prfTestOrigin})
+	return b
+}
+
+// register drives POST /register/finish with a valid attestation.
+func (a *prfAuthenticator) register(t *testing.T, h *PasskeyHandle) {
+	t.Helper()
+	_, chal, err := h.rp.CreationOptions([]byte("u"), "user")
+	if err != nil {
+		t.Fatalf("CreationOptions: %v", err)
+	}
+	// {"fmt":"none","attStmt":{},"authData":<bytes>}
+	att := []byte{0xa3}
+	att = append(att, 0x63, 'f', 'm', 't', 0x64, 'n', 'o', 'n', 'e')
+	att = append(att, 0x67, 'a', 't', 't', 'S', 't', 'm', 't', 0xa0)
+	ad := a.authData(1<<6|0x05, 0, true)
+	att = append(att, 0x68, 'a', 'u', 't', 'h', 'D', 'a', 't', 'a')
+	att = append(att, 0x59, byte(len(ad)>>8), byte(len(ad)))
+	att = append(att, ad...)
+
+	body, _ := json.Marshal(map[string]any{
+		"challenge": chal,
+		"credential": map[string]any{
+			"id":   b64u(a.credID),
+			"type": "public-key",
+			"response": map[string]any{
+				"clientDataJSON":    b64u(clientDataJSON("webauthn.create", chal)),
+				"attestationObject": b64u(att),
+			},
+		},
+	})
+	w := httptest.NewRecorder()
+	h.RegisterFinish(w, httptest.NewRequest(http.MethodPost, "/auth/webauthn/register/finish", bytes.NewReader(body)))
+	if w.Code != http.StatusOK {
+		t.Fatalf("RegisterFinish: %d %s", w.Code, w.Body.String())
+	}
+}
+
+// assert drives POST /assert/finish with a valid assertion and the given
+// base64url PRF result.
+func (a *prfAuthenticator) assert(t *testing.T, h *PasskeyHandle, prf string) *httptest.ResponseRecorder {
+	t.Helper()
+	_, chal, err := h.rp.AssertionOptions()
+	if err != nil {
+		t.Fatalf("AssertionOptions: %v", err)
+	}
+	ad := a.authData(0x05, 7, false)
+	cdj := clientDataJSON("webauthn.get", chal)
+	hash := sha256.Sum256(cdj)
+	sig, err := ecdsa.SignASN1(rand.Reader, a.key, append(append([]byte{}, ad...), hash[:]...))
+	if err != nil {
+		t.Fatalf("sign: %v", err)
+	}
+	body, _ := json.Marshal(map[string]any{
+		"challenge": chal,
+		"prf":       prf,
+		"credential": map[string]any{
+			"id":   b64u(a.credID),
+			"type": "public-key",
+			"response": map[string]any{
+				"clientDataJSON":    b64u(cdj),
+				"authenticatorData": b64u(ad),
+				"signature":         b64u(sig),
+			},
+		},
+	})
+	w := httptest.NewRecorder()
+	h.AssertFinish(w, httptest.NewRequest(http.MethodPost, "/auth/webauthn/assert/finish", bytes.NewReader(body)))
+	return w
+}
+
+func newPRFHandle(t *testing.T, key *fakeKeyIPC) *PasskeyHandle {
+	t.Helper()
+	store, err := newCredentialStore(filepath.Join(t.TempDir(), "passkeys.json"))
+	if err != nil {
+		t.Fatalf("credential store: %v", err)
+	}
+	return &PasskeyHandle{
+		rp:        webauthn.NewRP(webauthn.Config{Origin: prfTestOrigin, RPID: prfTestRPID, RPName: "maven"}),
+		encryptFn: key,
+		store:     store,
+		session:   webauthn.NewPasskeySession(0),
+	}
+}
+
+// The fix for Vikunja #14: what goes over IPC is the PRF secret from the
+// authenticator, not the credential public key sitting in passkeys.json.
+func TestAssertSendsPRFSecretNotPublicKey(t *testing.T) {
+	key := &fakeKeyIPC{}
+	h := newPRFHandle(t, key)
+	auth := newPRFAuthenticator(t)
+	auth.register(t, h)
+
+	// Enrolment must not wrap anything: create() yields no PRF result.
+	if key.wrapCalls != 0 || key.unlockCalls != 0 {
+		t.Fatalf("registration touched the key IPC (wrap=%d unlock=%d)", key.wrapCalls, key.unlockCalls)
+	}
+
+	secret := make([]byte, 32)
+	for i := range secret {
+		secret[i] = byte(i + 1)
+	}
+	if w := auth.assert(t, h, b64u(secret)); w.Code != http.StatusOK {
+		t.Fatalf("AssertFinish: %d %s", w.Code, w.Body.String())
+	}
+
+	if key.unlockCalls != 1 || key.wrapCalls != 1 {
+		t.Fatalf("unlock=%d wrap=%d, want 1 and 1", key.unlockCalls, key.wrapCalls)
+	}
+	if !bytes.Equal(key.unlockSecret, secret) {
+		t.Errorf("Unlock got %x, want the PRF secret %x", key.unlockSecret, secret)
+	}
+	if !bytes.Equal(key.wrapSecret, secret) {
+		t.Errorf("StoreEncryptionKey got %x, want the PRF secret %x", key.wrapSecret, secret)
+	}
+	// And explicitly: not the credential public key.
+	pub, _, err := h.store.Lookup(b64u(auth.credID))
+	if err != nil {
+		t.Fatalf("lookup: %v", err)
+	}
+	if bytes.Equal(key.unlockSecret, pub) {
+		t.Fatal("the credential public key was sent as the unlock secret")
+	}
+}
+
+// An authenticator without PRF must produce no unlock attempt at all — the
+// assertion still succeeds (step-up works), but cold-start unlock stays off
+// rather than falling back to something weaker.
+func TestAssertWithoutPRFDoesNotUnlock(t *testing.T) {
+	for _, prf := range []string{"", "!!!not-base64!!!", b64u(make([]byte, 32)), b64u(make([]byte, 16))} {
+		key := &fakeKeyIPC{}
+		h := newPRFHandle(t, key)
+		auth := newPRFAuthenticator(t)
+		auth.register(t, h)
+
+		w := auth.assert(t, h, prf)
+		if w.Code != http.StatusOK {
+			t.Fatalf("prf=%q: AssertFinish %d %s", prf, w.Code, w.Body.String())
+		}
+		if key.unlockCalls != 0 || key.wrapCalls != 0 {
+			t.Errorf("prf=%q: unlock=%d wrap=%d, want no key IPC at all", prf, key.unlockCalls, key.wrapCalls)
+		}
+	}
+}
+
+// A failed unlock must not fail the assertion: step-up is independently valid,
+// and a locked daemon degrades rather than breaking the login.
+func TestAssertSucceedsWhenUnlockFails(t *testing.T) {
+	key := &fakeKeyIPC{unlockErr: errors.New("wrong credential")}
+	h := newPRFHandle(t, key)
+	auth := newPRFAuthenticator(t)
+	auth.register(t, h)
+
+	secret := bytes.Repeat([]byte{3}, 32)
+	if w := auth.assert(t, h, b64u(secret)); w.Code != http.StatusOK {
+		t.Fatalf("AssertFinish: %d %s", w.Code, w.Body.String())
+	}
+	if key.unlockCalls != 1 {
+		t.Errorf("unlock attempted %d times, want 1", key.unlockCalls)
+	}
+}
+
+// A forged assertion must never reach the unlock path.
+func TestForgedAssertionNeverUnlocks(t *testing.T) {
+	key := &fakeKeyIPC{}
+	h := newPRFHandle(t, key)
+	auth := newPRFAuthenticator(t)
+	auth.register(t, h)
+
+	// A different key signing over the same credential id.
+	attacker := newPRFAuthenticator(t)
+	attacker.credID = auth.credID
+	w := attacker.assert(t, h, b64u(bytes.Repeat([]byte{4}, 32)))
+	if w.Code == http.StatusOK {
+		t.Fatal("an assertion signed by the wrong key was accepted")
+	}
+	if key.unlockCalls != 0 || key.wrapCalls != 0 {
+		t.Fatalf("a forged assertion reached the key IPC (unlock=%d wrap=%d)", key.unlockCalls, key.wrapCalls)
+	}
+}
+
+// The browser side is the only place the PRF result exists. If the page stops
+// asking for it or stops reading it back, cold-start unlock silently dies with
+// nothing failing, so the page source is asserted directly.
+func TestPasskeyPageRequestsAndPostsPRF(t *testing.T) {
+	for _, want := range []string{
+		"getClientExtensionResults",
+		"ext.prf.results.first",
+		"body:JSON.stringify({challenge,prf,",
+	} {
+		if !strings.Contains(passkeyPageHTML, want) {
+			t.Errorf("the passkey page no longer contains %q", want)
+		}
+	}
+}

cmd/mavweb/webauthn.go

@@ -23,8 +23,8 @@ type assertIPC interface {
 // is *ipc.Client; in-process CoreAPI adapters do not implement it. When nil,
 // StoreEncryptionKey and Unlock are silently skipped.
 type keyIPC interface {
-	StoreEncryptionKey(ctx context.Context, publicKey []byte) error
-	Unlock(ctx context.Context, publicKey []byte) error
+	StoreEncryptionKey(ctx context.Context, secret []byte) error
+	Unlock(ctx context.Context, secret []byte) error
 }
 
 // PasskeyHandle holds the WebAuthn relying party, a local in-memory credential
@@ -102,17 +102,31 @@ async function enroll(){try{
  const r=await fetch('/auth/webauthn/register/finish',{method:'POST',headers:{'content-type':'application/json'},
   body:JSON.stringify({challenge,credential:{id:c.id,type:c.type,response:{
    clientDataJSON:b64u(c.response.clientDataJSON),attestationObject:b64u(c.response.attestationObject)}}})});
- say(r.ok?'enrolled ✓':'enroll failed: '+await r.text(),r.ok);
+ if(!r.ok){say('enroll failed: '+await r.text(),false);return;}
+ // The wrapped key can only be written from an assertion: PRF results are
+ // not produced at create() time on most authenticators. Enrolment reports
+ // whether PRF is available at all so he is not told cold-start works when
+ // it cannot.
+ const ext=c.getClientExtensionResults?c.getClientExtensionResults():{};
+ const prfOK=!!(ext.prf&&ext.prf.enabled);
+ say(prfOK?'enrolled ✓ — now assert once to write the cold-start key':
+  'enrolled ✓ — but this authenticator has no PRF: cold-start unlock unavailable',true);
 }catch(e){say('enroll error: '+e,false);}}
 async function assert(){try{
  const {challenge,options}=await (await fetch('/auth/webauthn/assert/begin')).json();
  options.challenge=ub64(options.challenge);
  const c=await navigator.credentials.get({publicKey:options});
+ // The PRF result is the cold-start secret. It never touches localStorage
+ // and is posted once, over the same request as the assertion.
+ const ext=c.getClientExtensionResults?c.getClientExtensionResults():{};
+ const prf=ext.prf&&ext.prf.results&&ext.prf.results.first?b64u(ext.prf.results.first):'';
  const r=await fetch('/auth/webauthn/assert/finish',{method:'POST',headers:{'content-type':'application/json'},
-  body:JSON.stringify({challenge,credential:{id:c.id,type:c.type,response:{
+  body:JSON.stringify({challenge,prf,credential:{id:c.id,type:c.type,response:{
    clientDataJSON:b64u(c.response.clientDataJSON),authenticatorData:b64u(c.response.authenticatorData),
    signature:b64u(c.response.signature)}}})});
- say(r.ok?'stepped up ✓ — enable tools now':'assert failed: '+await r.text(),r.ok);
+ if(!r.ok){say('assert failed: '+await r.text(),false);return;}
+ say(prf?'stepped up ✓ — enable tools now':
+  'stepped up ✓ — no PRF from this authenticator, so cold-start unlock stayed unavailable',true);
 }catch(e){say('assert error: '+e,false);}}
 </script>`
 
@@ -140,9 +154,7 @@ func (h *PasskeyHandle) RegisterFinish(w http.ResponseWriter, r *http.Request) {
 		http.Error(w, "bad request: "+err.Error(), http.StatusBadRequest)
 		return
 	}
-	var enrolledPublicKey []byte
 	save := func(id string, publicKey []byte, _ []byte, _ string) error {
-		enrolledPublicKey = publicKey
 		return h.store.Save(id, publicKey)
 	}
 	credID, err := h.rp.FinishRegistration(save, body.Challenge, body.Credential)
@@ -153,19 +165,15 @@ func (h *PasskeyHandle) RegisterFinish(w http.ResponseWriter, r *http.Request) {
 	}
 	log.Printf("webauthn: registered credential %s", credID)
 
-	// If mavend is reachable and supports key wrapping, store the encryption
-	// key wrapped with this credential's public key — enables cold-start unlock.
-	if h.encryptFn != nil && enrolledPublicKey != nil {
-		ctx, cancel := context.WithTimeout(r.Context(), 10*time.Second)
-		defer cancel()
-		if err := h.encryptFn.StoreEncryptionKey(ctx, enrolledPublicKey); err != nil {
-			log.Printf("webauthn: store encryption key: %v", err)
-			// Non-fatal: enrollment still succeeded, the wrapped key can be
-			// created later via the same endpoint.
-		} else {
-			log.Printf("webauthn: encryption key wrapped with credential %s", credID)
-		}
-	}
+	// Note what does NOT happen here: the encryption key is not wrapped at
+	// enrolment. Wrapping needs the authenticator's PRF output, and create()
+	// does not produce one on most authenticators — it only reports whether
+	// the extension is supported. The wrapped key is written on the first
+	// assertion instead (see AssertFinish).
+	//
+	// This used to wrap the key under the credential *public* key, which is
+	// written to passkeys.json next to the wrapped blob. See the header of
+	// internal/webauthn/keywrap.go.
 
 	json.NewEncoder(w).Encode(map[string]string{"credential_id": credID})
 }
@@ -189,6 +197,11 @@ func (h *PasskeyHandle) AssertFinish(w http.ResponseWriter, r *http.Request) {
 	var body struct {
 		Challenge  string         `json:"challenge"`
 		Credential map[string]any `json:"credential"`
+		// PRF is the base64url WebAuthn PRF output the browser read out of
+		// getClientExtensionResults(). Empty when the authenticator has no
+		// PRF extension: cold-start unlock is then unavailable and we say so
+		// rather than falling back to something weaker.
+		PRF string `json:"prf"`
 	}
 	if err := json.NewDecoder(r.Body).Decode(&body); err != nil {
 		http.Error(w, "bad request: "+err.Error(), http.StatusBadRequest)
@@ -222,26 +235,32 @@ func (h *PasskeyHandle) AssertFinish(w http.ResponseWriter, r *http.Request) {
 		}
 	}
 
-	// If the daemon is locked (cold-start), send the credential's public key
-	// over IPC so mavend can unwrap its encryption key and open the store.
-	// The public key comes from the local credential store (it was stored
-	// during enrollment). Non-fatal: if IPC doesn't support Unlock or the
-	// daemon is already unlocked, the call is a no-op on the server side.
+	// Cold-start unlock and key wrapping, both keyed on the PRF secret that
+	// this assertion just produced. The secret is used here and dropped; it is
+	// never stored on this side.
+	//
+	// Order matters: unlock first (if the daemon is locked there is nothing to
+	// wrap yet), then re-wrap, which writes the blob on the first assertion
+	// after enrolment and is a harmless rewrite afterwards. Both are
+	// best-effort — the assertion itself is valid either way.
 	if h.encryptFn != nil {
-		publicKey, _, err := h.store.Lookup(credID)
-		if err == nil && publicKey != nil {
+		secret, err := webauthn.DecodePRFResult(body.PRF)
+		switch {
+		case err != nil:
+			log.Printf("webauthn: no usable PRF secret from credential %s: %v", credID, err)
+		default:
 			ctx, cancel := context.WithTimeout(r.Context(), 10*time.Second)
 			defer cancel()
-			if err := h.encryptFn.Unlock(ctx, publicKey); err != nil {
+			if err := h.encryptFn.Unlock(ctx, secret); err != nil {
 				log.Printf("webauthn: unlock via credential %s: %v", credID, err)
-				// Non-fatal: assertion succeeded; if the daemon stays locked
-				// the user will see errors on subsequent pages, but the
-				// assertion itself is valid.
 			} else {
 				log.Printf("webauthn: daemon unlocked via credential %s", credID)
 			}
-		} else if err != nil {
-			log.Printf("webauthn: lookup credential %s for unlock: %v", credID, err)
+			if err := h.encryptFn.StoreEncryptionKey(ctx, secret); err != nil {
+				log.Printf("webauthn: wrap encryption key: %v", err)
+			} else {
+				log.Printf("webauthn: encryption key wrapped for credential %s", credID)
+			}
 		}
 	}