fix(hub): the pepper and a device key take the passphrase, not a token

POST /me/bundle-pepper (was GET) and POST /users/devices require auth_key.
A refreshed or lifted token could otherwise fetch the pepper, or register a
device whose every sign-in carries it. Both callers have just been given the
passphrase.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

5 files changed, 67 insertions, 27 deletions

diff --git a/packages/meshbay-hub/src/meshbay_hub/api/users.py b/packages/meshbay-hub/src/meshbay_hub/api/users.py
index 92b3d3d..8e780df 100644
--- a/packages/meshbay-hub/src/meshbay_hub/api/users.py
+++ b/packages/meshbay-hub/src/meshbay_hub/api/users.py
@@ -351,6 +351,20 @@ async def _take_login_attempt(db: AsyncSession, username: str) -> None:
                             headers={"Retry-After": str(retry_after)})
 
 
+async def _prove_passphrase(db: AsyncSession, user: User, auth_key: str) -> None:
+    """Refuse with 403 unless `auth_key` is this account's, spending an attempt.
+
+    For what a live access token must not be enough for: a token may be a
+    refreshed one, or one lifted from a page, and what it would buy here outlives
+    the session or reopens the offline search the pepper exists to prevent.
+    """
+    await _take_login_attempt(db, user.username)
+    if not await verify_password_off_loop(auth_key, user.pw_hash, user.pw_salt,
+                                          user.pw_version):
+        raise HTTPException(status_code=403, detail="Passphrase does not match")
+    await login_throttle.clear(db, user.username)
+
+
 async def _login_failed(db: AsyncSession, username: str, ip: str,
                         user_id: str | None = None) -> None:
     """Record a wrong passphrase and answer 401. Always raises."""
@@ -367,12 +381,12 @@ async def _login_failed(db: AsyncSession, username: str, ip: str,
 # ── Bundle pepper ────────────────────────────────────────────────────────────
 #
 # Half of what opens this account's keypair bundles on nodes; the passphrase is
-# the other half. Handed out only where the caller proved the passphrase or a
-# device key — sign-in, device sign-in — or already holds a session that did
-# (`GET /me/bundle-pepper`, which a passphrase change uses: it re-seals every
-# bundle before the hub is asked to accept the new passphrase). Never on a token refresh, which
-# proves only possession of a refresh token; never to a node token; never in a
-# token or a log line.
+# the other half. Handed out only in the response to a call that proved the
+# passphrase or a device key: sign-in, device sign-in, a passphrase change, and
+# `POST /me/bundle-pepper` with the passphrase (a page that has a session but
+# not the key derived from it). A token alone never obtains it — not a refreshed
+# one, not a node's — because a bundle plus the pepper is an offline oracle for
+# the passphrase again. Never in a token or a log line.
 
 def _bundle_pepper(user: User) -> dict:
     """The pepper for a response, created on first use. The caller commits."""
@@ -384,15 +398,20 @@ def _bundle_pepper(user: User) -> dict:
             "bundle_pepper_version": user.bundle_pepper_version}
 
 
-@router.get("/me/bundle-pepper")
+class PepperRequest(BaseModel):
+    auth_key: str
+
+
+@router.post("/me/bundle-pepper")
 @limiter.limit("10/minute")
 async def get_bundle_pepper(
+    body: PepperRequest,
     request: Request,
     current_user: User = Depends(require_user_scope),
     db: AsyncSession = Depends(get_db),
 ):
-    """For a session opened before the pepper existed: asked once, then kept
-    only as part of the key derived from it."""
+    """The pepper, for a session that has just been given the passphrase again."""
+    await _prove_passphrase(db, current_user, body.auth_key)
     pepper = _bundle_pepper(current_user)
     await db.commit()
     return pepper
@@ -511,6 +530,7 @@ DEVICE_AUTH_TIMESTAMP_WINDOW = 60  # seconds, as for node auth
 class DeviceRegisterRequest(BaseModel):
     pk_auth_ed25519: str          # base64 raw 32 bytes
     label: str = ""
+    auth_key: str                 # the passphrase proof, as at sign-in
 
 
 class DeviceAuthRequest(BaseModel):
@@ -528,9 +548,13 @@ async def register_device(
     """
     Register a device's hub authentication key.
 
-    Requires an existing session, which in practice means the passphrase was
-    entered on this device a moment ago. A device cannot enrol itself.
+    Requires the passphrase, not only a session. A registered key signs in
+    without it for as long as it stays registered, and each such sign-in carries
+    the bundle pepper, so a bare token (refreshed, or lifted from a page) would
+    otherwise turn into a permanent credential. The caller has just typed the
+    passphrase to sign in, so it has the proof at hand.
     """
+    await _prove_passphrase(db, current_user, body.auth_key)
     try:
         raw = base64.b64decode(body.pk_auth_ed25519)
         Ed25519PublicKey.from_public_bytes(raw)
diff --git a/packages/meshbay-hub/src/meshbay_hub/static/app.js b/packages/meshbay-hub/src/meshbay_hub/static/app.js
index c101ec9..72dd123 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/app.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/app.js
@@ -1139,8 +1139,8 @@ function App() {
    * exactly what a browser does, and is a worse experience rather than a
    * broken one.
    */
-  const registerThisDevice = useCallback(async (token) => {
-    if (!platform.device.available) return;
+  const registerThisDevice = useCallback(async (token, authKey) => {
+    if (!platform.device.available || !authKey) return;
     try {
       const backend = await platform.secrets.backend();
       if (backend === 'unavailable') return;
@@ -1148,7 +1148,7 @@ function App() {
       if (!pk) return;
       await hubFetch('/v1/users/devices', {
         method: 'POST', token,
-        body: { pk_auth_ed25519: pk, label: t('device.this_device') },
+        body: { pk_auth_ed25519: pk, label: t('device.this_device'), auth_key: authKey },
       });
     } catch (err) {
       console.warn('device not registered:', err.message);
@@ -1158,11 +1158,12 @@ function App() {
   const authCtx = {
     user,
     login: async (username, password) => {
-      let token, refreshToken;
+      let token, refreshToken, authKey;
       if (window.MeshBayKeys) {
         const data = await window.MeshBayKeys.loginAndRecover(username, password);
         token = data.accessToken;
         refreshToken = data.refreshToken;
+        authKey = data.authKey;
         // The only thing sign-in produces: the key that opens a node's bundle.
         // Which identity we use is decided per node, when we get there.
         session.bundleKey = data.bundleKey;
@@ -1180,7 +1181,7 @@ function App() {
       // On a desktop build, remember this device so the next launch does not ask
       // for the passphrase again. The key is generated and held by the main
       // process; what travels here is only its public half.
-      await registerThisDevice(token);
+      await registerThisDevice(token, authKey);
       // Before the session lands, so the idle watch starting with it does not
       // read the last-active time of whoever used this browser before.
       markActive(true);
diff --git a/packages/meshbay-hub/src/meshbay_hub/static/group-page.js b/packages/meshbay-hub/src/meshbay_hub/static/group-page.js
index 4510848..fe858b2 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/group-page.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/group-page.js
@@ -257,10 +257,11 @@ function GroupPage({ groupId, group, token, username, userId, userPrefs,
     setError('');
     try {
       // Same derivation as sign-in — the token is already ours, only the key
-      // that opens node bundles is missing here, and the pepper that goes into
-      // it is asked for with that token. Persisted so this browser is set up
-      // from now on.
-      const { pepper, version } = await window.MeshBayKeys.fetchBundlePepper(token);
+      // that opens node bundles is missing here. The hub hands the pepper that
+      // goes into it only against the passphrase proof, never the token alone.
+      // Persisted so this browser is set up from now on.
+      const authKey = await window.MeshBayKeys.deriveAuthKey(pass, username);
+      const { pepper, version } = await window.MeshBayKeys.fetchBundlePepper(token, authKey);
       session.bundleKey = await window.MeshBayKeys.sessionBundleKey(
         pass, username, userId, pepper, version);
       await _storeBundleKey(session.bundleKey);
diff --git a/packages/meshbay-hub/src/meshbay_hub/static/keyderive.js b/packages/meshbay-hub/src/meshbay_hub/static/keyderive.js
index 9f28b5c..6bd5896 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/keyderive.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/keyderive.js
@@ -195,14 +195,25 @@ async function nodeBundleKey(sessionKey, nodePkB64) {
 }
 
 /**
- * GET the pepper for a session that is already open — a stored session from
- * before the pepper existed, a passphrase change. Sign-in carries it already.
+ * The pepper for a session that is already open but lacks the key derived from
+ * it — a restored session, a passphrase change. Sign-in carries it already. The
+ * hub asks for the passphrase proof: a token alone never obtains the pepper.
  */
-async function fetchBundlePepper(token) {
+async function fetchBundlePepper(token, authKey) {
   const resp = await hubCall('/v1/users/me/bundle-pepper', {
-    headers: { Authorization: `Bearer ${token}` },
+    method: 'POST',
+    headers: { Authorization: `Bearer ${token}`, 'Content-Type': 'application/json' },
+    body: JSON.stringify({ auth_key: authKey }),
   });
-  if (!resp.ok) throw new Error(`bundle pepper: ${resp.status}`);
+  if (!resp.ok) {
+    // The hub's own words: a wrong passphrase and a locked account are what a
+    // person can act on, a bare status is not.
+    let detail = `bundle pepper: ${resp.status}`;
+    try { const j = await resp.json(); detail = j.detail || j.error || detail; } catch { /* not JSON */ }
+    const err = new Error(String(detail));
+    err.status = resp.status;
+    throw err;
+  }
   const data = await resp.json();
   return { pepper: data.bundle_pepper, version: data.bundle_pepper_version };
 }
@@ -446,6 +457,9 @@ async function loginAndRecover(username, password) {
   const result = {
     accessToken:  data.access_token,
     refreshToken: data.refresh_token,
+    // Kept for the one call that follows a sign-in and must prove the
+    // passphrase again: registering this device's key. Not stored.
+    authKey,
     // The pepper rides on the sign-in response, so this costs no extra call;
     // it is folded into the key here and not kept.
     bundleKey: await sessionBundleKey(
diff --git a/packages/meshbay-hub/src/meshbay_hub/static/profile-page.js b/packages/meshbay-hub/src/meshbay_hub/static/profile-page.js
index 7c36951..1800d72 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/profile-page.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/profile-page.js
@@ -147,7 +147,8 @@ export function ProfilePage({ user, onLogout }) {
       // In the desktop application both are kept by its main process, the new
       // one set aside until the hub has accepted the change.
       const K = window.MeshBayKeys;
-      const { pepper, version } = await K.fetchBundlePepper(user.token);
+      const oldAuthKey = await K.deriveAuthKey(cpOld, user.username);
+      const { pepper, version } = await K.fetchBundlePepper(user.token, oldAuthKey);
       const oldKey = await K.sessionBundleKey(
         cpOld, user.username, user.userId, pepper, version);
       const newKey = await K.sessionBundleKey(
@@ -160,7 +161,6 @@ export function ProfilePage({ user, onLogout }) {
           bundleKey: oldKey, newBundleKey: newKey,
           onProgress: setCpProgress,
         });
-        const oldAuthKey = await window.MeshBayKeys.deriveAuthKey(cpOld, user.username);
         const newAuthKey = await window.MeshBayKeys.deriveAuthKey(cpNew, user.username);
         resp = await hubFetch('/v1/users/password', {
           method: 'POST', token: user.token,