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/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)
+			}
 		}
 	}