feat(hub): replace reCAPTCHA with a self-hosted proof of work

Sign-up and passphrase reset sent every visitor's address and browser
details to Google, which needed a consent the sign-up form could not ask
for freely. The hub now issues its own challenge (ALTCHA's scheme, signed
with a key derived from the hub's private key, single-use, 30 minutes)
and the page solves it with SubtleCrypto while the form is filled in.

- GET /v1/users/captcha issues a challenge; 404 captcha_disabled when off.
- [captcha] enabled / max_number replace site_key / secret_key, which are
  now ignored with a warning; MESHBAY_CAPTCHA_ENABLED replaces the old
  variables.
- No Google origin left in the CSP of the hub, the desktop shell or the
  Android shell; captcha_site_key leaves /v1/hub/info.

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

19 files changed, 233 insertions, 241 deletions

diff --git a/packages/meshbay-hub/src/meshbay_hub/api/hub.py b/packages/meshbay-hub/src/meshbay_hub/api/hub.py
index 0d2d39c..b47f076 100644
--- a/packages/meshbay-hub/src/meshbay_hub/api/hub.py
+++ b/packages/meshbay-hub/src/meshbay_hub/api/hub.py
@@ -7,19 +7,10 @@ from sqlalchemy.ext.asyncio import AsyncSession
 from meshbay_hub import __version__, hub_settings
 from meshbay_hub.api import federation
 from meshbay_hub.auth import hub_public_key_pem
-from meshbay_hub.config import HubConfig
 from meshbay_hub.db.engine import get_db, get_engine
 
 router = APIRouter(prefix="/v1/hub", tags=["hub"])
 
-_cfg: HubConfig | None = None
-
-
-def set_config(cfg: HubConfig) -> None:
-    global _cfg
-    _cfg = cfg
-
-
 @router.get("/info")
 async def hub_info(db: AsyncSession = Depends(get_db)):
     """Versions and the instance policy a client needs before signing in."""
@@ -40,7 +31,6 @@ async def hub_info(db: AsyncSession = Depends(get_db)):
         # which answers 503 — this is the same answer somewhere an operator can
         # read it without opening the source.
         "federation": federation.FEDERATION_ENABLED,
-        "captcha_site_key": _cfg.captcha.site_key if _cfg and _cfg.captcha.enabled else "",
         # How long a browser tab stays signed in with nobody at it. The page
         # measures it (static/idle.js); the hub only says how long.
         "browser_idle_hours":
diff --git a/packages/meshbay-hub/src/meshbay_hub/api/users.py b/packages/meshbay-hub/src/meshbay_hub/api/users.py
index 130240e..fbba6d7 100644
--- a/packages/meshbay-hub/src/meshbay_hub/api/users.py
+++ b/packages/meshbay-hub/src/meshbay_hub/api/users.py
@@ -100,18 +100,24 @@ def _generate_code() -> str:
 async def _verify_captcha_or_raise(token: str | None, request: Request) -> None:
     if not token:
         raise HTTPException(status_code=400, detail="captcha_required")
-    from meshbay_hub.captcha import verify_captcha
-    ok = await verify_captcha(
-        _cfg.captcha.secret_key,  # type: ignore[union-attr]
-        token,
-        request.client.host if request.client else None,
-        _cfg.captcha.host_check,  # type: ignore[union-attr]
-        _cfg.captcha.allow_unattributed_host,  # type: ignore[union-attr]
-    )
-    if not ok:
+    from meshbay_hub.captcha import verify
+    if not verify(token):
         raise HTTPException(status_code=400, detail="captcha_failed")
 
 
+@router.get("/captcha")
+@limiter.limit("30/minute")
+async def captcha_challenge(request: Request):
+    """A proof-of-work challenge for the register and reset-request forms.
+
+    404 when this hub has the check turned off, so the page sends no answer.
+    """
+    if not (_cfg and _cfg.captcha.enabled):
+        raise HTTPException(status_code=404, detail="captcha_disabled")
+    from meshbay_hub.captcha import issue
+    return issue(_cfg.captcha.max_number)
+
+
 # ── Models ────────────────────────────────────────────────────────────────────
 
 # Mirrored by USERNAME_MIN_LEN in static/auth-page.js; test_username_floor.py
diff --git a/packages/meshbay-hub/src/meshbay_hub/api/webapp.py b/packages/meshbay-hub/src/meshbay_hub/api/webapp.py
index badbdda..adbd522 100644
--- a/packages/meshbay-hub/src/meshbay_hub/api/webapp.py
+++ b/packages/meshbay-hub/src/meshbay_hub/api/webapp.py
@@ -82,21 +82,19 @@ _NO_STORE = {"Cache-Control": "no-store"}
 # `<script>`. `'wasm-unsafe-eval'` is required for the Argon2id WASM. The hub's
 # own origin is deliberately absent from `script-src`: a response it returns is
 # never executed, which is the point of T3.
-_RECAPTCHA_SRC = "https://www.google.com https://www.gstatic.com"
 CSP = "; ".join([
     "default-src 'none'",
-    f"script-src 'self' 'wasm-unsafe-eval' {_RECAPTCHA_SRC}",
+    "script-src 'self' 'wasm-unsafe-eval'",
     "style-src 'self' 'unsafe-inline'",
-    f"img-src 'self' data: blob: {_RECAPTCHA_SRC}",
+    "img-src 'self' data: blob:",
     "media-src 'self' blob:",
     "font-src 'self'",
     # The page talks to its own origin and nothing else — the hub, the node
-    # arrives over WebRTC, which this directive does not govern. reCAPTCHA's
-    # two origins are the only others, kept so its script may call home from
-    # this page; the register and reset views were driven in Chrome and Firefox
-    # against Google's test keys with this line and reported no violation.
-    # `https: wss:` let an injected script post anything anywhere.
-    f"connect-src 'self' {_RECAPTCHA_SRC}",
+    # arrives over WebRTC, which this directive does not govern. No third
+    # party has an origin here: the sign-up check is a proof of work the hub
+    # issues itself (captcha.py). `https: wss:` let an injected script post
+    # anything anywhere.
+    "connect-src 'self'",
     "worker-src 'self'",
     # `object-src` exists for one thing, and `frame-src`'s `blob:` for the same
     # thing: previewing a PDF without writing it anywhere.
@@ -123,9 +121,10 @@ CSP = "; ".join([
     # response it is holding. Without it Chrome refuses the frame, the worker is
     # never asked, and the page waits out its timeout for a download that cannot
     # happen — on Firefox and Safari that is the *only* way to write a large
-    # file to disk, so the whole path was dead. Added when reCAPTCHA needed a
-    # frame, which is why nobody connected the two.
-    f"frame-src 'self' blob: {_RECAPTCHA_SRC}",
+    # file to disk, so the whole path was dead. It was missing because the
+    # directive had been written for a third-party captcha frame, and nobody
+    # connected the two.
+    "frame-src 'self' blob:",
     # `'self'`, not `'none'`, and the difference is one same-origin iframe.
     #
     # The threat frame-ancestors answers is clickjacking: a *foreign* page
diff --git a/packages/meshbay-hub/src/meshbay_hub/app.py b/packages/meshbay-hub/src/meshbay_hub/app.py
index 2ad71bd..6155470 100644
--- a/packages/meshbay-hub/src/meshbay_hub/app.py
+++ b/packages/meshbay-hub/src/meshbay_hub/app.py
@@ -23,7 +23,6 @@ from meshbay_hub.api.federation import router as federation_router
 from meshbay_hub.api.groups import router as groups_router
 from meshbay_hub.api.health import router as health_router
 from meshbay_hub.api.hub import router as hub_router
-from meshbay_hub.api.hub import set_config as hub_set_config
 from meshbay_hub.api.invite_links import redeem_router as invite_links_redeem_router
 from meshbay_hub.api.invite_links import router as invite_links_router
 from meshbay_hub.api.middleware import limiter
@@ -140,7 +139,6 @@ def create_app(cfg: HubConfig | None = None) -> FastAPI:
             generate_hub_keypair(kp)
         load_hub_keypair(kp, cfg.identity.id)
         users_set_config(cfg)
-        hub_set_config(cfg)
         set_admin_usernames(cfg.identity.admin_usernames)
 
         await _backfill_email_hashes()
diff --git a/packages/meshbay-hub/src/meshbay_hub/captcha.py b/packages/meshbay-hub/src/meshbay_hub/captcha.py
index cf33d52..e6faf1a 100644
--- a/packages/meshbay-hub/src/meshbay_hub/captcha.py
+++ b/packages/meshbay-hub/src/meshbay_hub/captcha.py
@@ -1,85 +1,86 @@
-"""reCAPTCHA v2 server-side verification."""
+"""Proof-of-work challenge gating registration and password reset.
 
-import logging
+The hub hands out a challenge; the page finds the number behind it by trying
+them in turn and sends the answer with the form. Nobody else is asked: no
+script, cookie or address leaves for a third party, which is what the
+reCAPTCHA this replaced could not say (docs/MESHBAY_DESIGN.md §7.7).
 
-import httpx
+What it buys is cost, not a verdict on who is human. A person never sees it —
+the page solves while the form is being filled in — and a script pays it once
+per attempt. The bounds that hold against a script with machines to spare are
+the per-IP limits on the two routes and, above all, the mail budget
+(`mail.py`): the sign-up and the reset are what make this hub send mail.
 
-log = logging.getLogger(__name__)
+The scheme is ALTCHA's, keyed to this hub. A challenge is the SHA-256 of
+`<nonce>.<expires>.<number>`, with `number` drawn below `max_number`; its HMAC
+is the hub's word that it issued it. The answer carries the nonce, the expiry,
+the number and that HMAC, so the hub recomputes the challenge rather than
+remembering it — and every worker, and the next restart, agrees on it, since
+the key is derived from the hub's own private key and not drawn per process.
+What is remembered is an answer once spent, so one solve is one form.
+"""
 
-VERIFY_URL = "https://www.google.com/recaptcha/api/siteverify"
+import hashlib
+import hmac
+import secrets
+import time
 
+from meshbay_hub.auth import hub_private_key_pem
 
-async def verify_captcha(
-    secret_key: str,
-    token: str,
-    remote_ip: str | None = None,
-    allowed_hosts: frozenset[str] | set[str] | None = None,
-    allow_unattributed: bool = False,
-) -> bool:
-    """True when Google accepts the token, and it came from a host we expect.
+# How long a challenge stays good. Long enough for somebody filling in the form
+# slowly; the page fetches a fresh one if it outlived this before submitting.
+CHALLENGE_TTL = 1800
 
-    `allowed_hosts` is the answer to a reCAPTCHA key whose own origin check is
-    turned off. That check compares the *page's hostname* against the key's
-    domain list, and the desktop client's page is served from `app://meshbay`
-    — Chromium refuses a service worker on a custom scheme and the interface
-    ships inside the package, so it cannot be `https://<hub>` and the hostname
-    cannot be the hub's. Google answers "Invalid domain for site key" inside
-    its own widget, which is not something the client can configure away.
+_KEY_LABEL = b"meshbay captcha v1"
+# Answers already accepted, until their challenge would have expired anyway.
+_spent: dict[str, int] = {}
 
-    Turning the key's origin check off moves that decision here, where the hub
-    knows which hosts are its own. `siteverify` reports the hostname it
-    observed, not one the caller asserts, so this is a real check and not a
-    formality — a bot embedding the site key on a page of its own is refused
-    on the hostname it actually served from.
 
-    `None` (the default) skips it, which is what a deployment leaving the
-    origin check with Google wants: it is then already done, one layer up.
+def _key() -> bytes:
+    return hmac.new(hub_private_key_pem(), _KEY_LABEL, hashlib.sha256).digest()
 
-    `allow_unattributed` covers the case that made the desktop client fail
-    anyway. A solve from a page Google cannot attribute to a domain comes back
-    with an **empty** hostname, not the host part of the origin — measured
-    live: `app://meshbay` reports `''`, never `'meshbay'`. So an allowlist
-    entry can never match it, and an empty string cannot be an allowlist entry
-    either: a blank in a TOML list is a typo far more often than it is an
-    intention, and the config parser drops blanks for that reason.
 
-    What it admits is every non-web client, not only ours — a `file://` page or
-    somebody else's Electron application report the same nothing. That is the
-    same bar the desktop client's own origin would have been (`app://meshbay`
-    is not a credential; any application can claim it), and it is a bar: the
-    captcha still has to be *solved*, per token. What is given up is the origin
-    restriction for non-web clients, not the captcha.
+def _sign(challenge: str) -> str:
+    return hmac.new(_key(), challenge.encode(), hashlib.sha256).hexdigest()
+
+
+def issue(max_number: int) -> dict:
+    """A fresh challenge, in the shape the page solves (`static/captcha.js`)."""
+    prefix = f"{secrets.token_hex(12)}.{int(time.time()) + CHALLENGE_TTL}."
+    number = secrets.randbelow(max_number)
+    challenge = hashlib.sha256(f"{prefix}{number}".encode()).hexdigest()
+    return {
+        "prefix": prefix,
+        "challenge": challenge,
+        "max_number": max_number,
+        "signature": _sign(challenge),
+        "expires_in": CHALLENGE_TTL,
+    }
+
+
+def verify(token: str) -> bool:
+    """True for an answer this hub issued, still in date and not spent before.
+
+    The token is `<nonce>.<expires>.<number>.<signature>`. Nothing in it is
+    trusted on its own: the challenge is recomputed from the first three, and
+    only this hub's key turns that into the signature.
     """
-    payload: dict[str, str] = {"secret": secret_key, "response": token}
-    if remote_ip:
-        payload["remoteip"] = remote_ip
-    try:
-        async with httpx.AsyncClient(timeout=5) as client:
-            resp = await client.post(VERIFY_URL, data=payload)
-            resp.raise_for_status()
-            result = resp.json()
-            if not result.get("success"):
-                log.info("captcha rejected: %s", result.get("error-codes", []))
-                return False
-            if allowed_hosts is not None:
-                # Logged at warning with the hostname spelled out: this is also
-                # how an operator finds what to allow after adding a client
-                # whose origin they have not seen before. It is how the empty
-                # one was found.
-                host = result.get("hostname") or ""
-                if not host:
-                    if not allow_unattributed:
-                        log.warning(
-                            "captcha solved on a host Google did not attribute; "
-                            "set captcha.allow_unattributed_host to admit "
-                            "non-web clients such as the desktop application")
-                        return False
-                elif host not in allowed_hosts:
-                    log.warning(
-                        "captcha solved on an unexpected host %r; allowed: %s",
-                        host, sorted(allowed_hosts))
-                    return False
-            return True
-    except Exception:
-        log.exception("captcha verification request failed")
+    parts = token.split(".")
+    if len(parts) != 4:
+        return False
+    nonce, expires_s, number_s, signature = parts
+    if not (expires_s.isdigit() and number_s.isdigit()):
+        return False
+    now = int(time.time())
+    if not now < int(expires_s) <= now + CHALLENGE_TTL:
+        return False
+    challenge = hashlib.sha256(f"{nonce}.{expires_s}.{number_s}".encode()).hexdigest()
+    if not hmac.compare_digest(_sign(challenge), signature):
+        return False
+    for spent, until in list(_spent.items()):
+        if until <= now:
+            del _spent[spent]
+    if signature in _spent:
         return False
+    _spent[signature] = int(expires_s)
+    return True
diff --git a/packages/meshbay-hub/src/meshbay_hub/config.py b/packages/meshbay-hub/src/meshbay_hub/config.py
index e618510..f710ead 100644
--- a/packages/meshbay-hub/src/meshbay_hub/config.py
+++ b/packages/meshbay-hub/src/meshbay_hub/config.py
@@ -66,33 +66,15 @@ class JWTConfig:
 
 @dataclass
 class CaptchaConfig:
-    site_key:   str = ""
-    secret_key: str = ""
-    # Hostnames a solved captcha may have come from, checked against the one
-    # `siteverify` reports. Empty means "do not check", which is right while
-    # the reCAPTCHA key does its own origin check — it is then already done.
-    #
-    # Set this when that check is turned off in the reCAPTCHA console, which is
-    # what the desktop client needs: its page is served from `app://meshbay`,
-    # so the hostname Google sees is not the hub's and never can be. See
-    # docs/MESHBAY_DESIGN.md §7.7.
-    allowed_hosts: list[str] = field(default_factory=list)
-    # A solve from a page Google cannot attribute to a domain reports an empty
-    # hostname — `app://meshbay` does, measured live, and so does any other
-    # non-web client. No `allowed_hosts` entry can match that, and a blank
-    # entry is not the answer: the parser drops blanks because in a TOML list
-    # a blank is a typo far more often than an intention. Hence a named flag,
-    # which also states the trade at the place it is made.
-    allow_unattributed_host: bool = False
-
-    @property
-    def enabled(self) -> bool:
-        return bool(self.site_key and self.secret_key)
-
-    @property
-    def host_check(self) -> frozenset[str] | None:
-        """The set to hand `verify_captcha`, or None for "do not check"."""
-        return frozenset(self.allowed_hosts) if self.allowed_hosts else None
+    # The proof of work on registration and password reset (captcha.py). On
+    # unless a configuration turns it off, which is for development and for a
+    # hub nobody can reach.
+    enabled: bool = True
+    # The work asked of the page: it tries numbers from 0 until one matches, so
+    # half of this on average and all of it at worst. Sized for the slowest
+    # phone that signs up — about three seconds there, solved while the form is
+    # being filled in — not for the desktop, which takes a fraction of that.
+    max_number: int = 500_000
 
 
 @dataclass
@@ -183,12 +165,12 @@ def load_config(path: Path | None = None) -> HubConfig:
                 ):
                     setattr(cfg.mail, name, ml.get(name, getattr(cfg.mail, name)))
             if cap := raw.get("captcha", {}):
-                cfg.captcha.site_key   = cap.get("site_key",   cfg.captcha.site_key)
-                cfg.captcha.secret_key = cap.get("secret_key", cfg.captcha.secret_key)
-                if hosts := cap.get("allowed_hosts"):
-                    cfg.captcha.allowed_hosts = [str(h).strip() for h in hosts if str(h).strip()]
-                if "allow_unattributed_host" in cap:
-                    cfg.captcha.allow_unattributed_host = bool(cap["allow_unattributed_host"])
+                cfg.captcha.enabled    = bool(cap.get("enabled", cfg.captcha.enabled))
+                cfg.captcha.max_number = max(1, int(cap.get("max_number", cfg.captcha.max_number)))
+                if "site_key" in cap or "secret_key" in cap:
+                    log.warning("captcha.site_key and captcha.secret_key are no longer "
+                                "read: registration is gated by a proof of work, with "
+                                "no third party; remove them from the configuration")
             break
 
     # Env var overrides
@@ -204,13 +186,7 @@ def load_config(path: Path | None = None) -> HubConfig:
         cfg.identity.private_key_path = Path(kp).expanduser()
     if admin_users := os.environ.get("MESHBAY_ADMIN_USERS"):
         cfg.identity.admin_usernames = [u.strip() for u in admin_users.split(",") if u.strip()]
-    if captcha_site := os.environ.get("MESHBAY_CAPTCHA_SITE_KEY"):
-        cfg.captcha.site_key = captcha_site
-    if captcha_secret := os.environ.get("MESHBAY_CAPTCHA_SECRET_KEY"):
-        cfg.captcha.secret_key = captcha_secret
-    if captcha_hosts := os.environ.get("MESHBAY_CAPTCHA_ALLOWED_HOSTS"):
-        cfg.captcha.allowed_hosts = [h.strip() for h in captcha_hosts.split(",") if h.strip()]
-    if unattributed := os.environ.get("MESHBAY_CAPTCHA_ALLOW_UNATTRIBUTED_HOST"):
-        cfg.captcha.allow_unattributed_host = unattributed.lower() in ("1", "true", "yes")
+    if captcha := os.environ.get("MESHBAY_CAPTCHA_ENABLED"):
+        cfg.captcha.enabled = captcha.lower() in ("1", "true", "yes")
 
     return cfg
diff --git a/packages/meshbay-hub/src/meshbay_hub/static/auth-page.js b/packages/meshbay-hub/src/meshbay_hub/static/auth-page.js
index e014f61..c5374f8 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/auth-page.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/auth-page.js
@@ -8,6 +8,7 @@ import {
 import * as platform from './platform.js';
 import { Icon } from './icon.js';
 import { loadPending } from './invite-link.js';
+import { solve } from './captcha.js';
 
 const PASSWORD_MIN_BITS = 60;
 const PASSWORD_MIN_LEN = 12;
@@ -44,73 +45,63 @@ function PasswordInput({
   `;
 }
 
-// ── reCAPTCHA v2 helper ──────────────────────────────────────────────────────
-
-let _captchaSiteKey = null;
-let _captchaKeyFetched = false;
-
-async function fetchCaptchaSiteKey() {
-  if (_captchaKeyFetched) return _captchaSiteKey;
-  try {
-    const info = await hubFetch('/v1/hub/info');
-    _captchaSiteKey = info.captcha_site_key || null;
-  } catch { _captchaSiteKey = null; }
-  _captchaKeyFetched = true;
-  return _captchaSiteKey;
-}
-
-function loadRecaptchaScript() {
-  if (document.getElementById('recaptcha-script')) return;
-  const s = document.createElement('script');
-  s.id = 'recaptcha-script';
-  s.src = 'https://www.google.com/recaptcha/api.js?render=explicit';
-  s.async = true;
-  s.defer = true;
-  document.head.appendChild(s);
-}
+// ── Proof of work (captcha.js) ───────────────────────────────────────────────
+//
+// Fetched and solved as soon as the form is on screen, so by the time somebody
+// has typed a name, an address and a passphrase twice there is nothing left to
+// wait for. Nothing is drawn: there is nothing for a person to do. A hub with
+// the check turned off answers 404, and the form then sends no answer at all.
 
 function useCaptcha() {
-  const [siteKey, setSiteKey] = useState(_captchaSiteKey);
-  const [token, setToken] = useState(null);
-  const containerRef = useRef(null);
-  const widgetId = useRef(null);
+  const pending = useRef(null);
+  const abort = useRef(null);
 
-  useEffect(() => {
-    fetchCaptchaSiteKey().then(k => {
-      if (k) { setSiteKey(k); loadRecaptchaScript(); }
-    });
+  const start = useCallback(() => {
+    if (abort.current) abort.current.abort();
+    const ctl = new AbortController();
+    abort.current = ctl;
+    pending.current = (async () => {
+      let challenge;
+      try {
+        challenge = await hubFetch('/v1/users/captcha');
+      } catch (err) {
+        return err.message === 'captcha_disabled' ? { token: null, until: Infinity } : null;
+      }
+      // A minute short of the hub's own limit, so an answer is never sent
+      // that expires on its way there.
+      const until = Date.now() + (challenge.expires_in - 60) * 1000;
+      return { token: await solve(challenge, ctl.signal), until };
+    })();
   }, []);
 
   useEffect(() => {
-    if (!siteKey || !containerRef.current) return;
-    const poll = setInterval(() => {
-      if (window.grecaptcha && window.grecaptcha.render && widgetId.current === null) {
-        clearInterval(poll);
-        widgetId.current = window.grecaptcha.render(containerRef.current, {
-          sitekey: siteKey,
-          callback: (tk) => setToken(tk),
-          'expired-callback': () => setToken(null),
-          theme: document.documentElement.getAttribute('data-theme') === 'dark'
-            ? 'dark' : 'light',
-        });
-      }
-    }, 100);
-    return () => clearInterval(poll);
-  }, [siteKey]);
+    start();
+    return () => abort.current && abort.current.abort();
+  }, [start]);
 
-  const reset = useCallback(() => {
-    if (widgetId.current !== null && window.grecaptcha) {
-      window.grecaptcha.reset(widgetId.current);
-      setToken(null);
+  // The answer for this submission — waited for if it is still being worked
+  // out, and worked out again if the form sat open past its expiry or the
+  // first fetch failed. Null only when the hub has no check, or cannot be
+  // reached, in which case the hub's own refusal is what the person sees.
+  const take = useCallback(async () => {
+    let got = await pending.current;
+    if (!got || got.until < Date.now()) {
+      start();
+      got = await pending.current;
     }
-  }, []);
+    return got ? got.token : null;
+  }, [start]);
 
-  const widget = siteKey
-    ? html`<div ref=${containerRef}
-                style="display:flex;justify-content:center;margin:12px 0"></div>`
-    : null;
+  // An answer is spent once the hub has checked it, whatever happened after —
+  // a name already taken is refused past that point. Call after a failure.
+  return { take, renew: start };
+}
 
-  return { token, widget, reset, enabled: !!siteKey };
+// The hub's refusal of the proof of work, which a person can do nothing about
+// but try again, in words rather than as the code it is sent as.
+function errorText(err) {
+  return err.message === 'captcha_required' || err.message === 'captcha_failed'
+    ? t('captcha.failed') : err.message;
 }
 
 function passwordBits(pw) {
@@ -411,11 +402,9 @@ export function RegisterPage() {
         const rk = window.MeshBayKeys.generateRecoveryKey();
         // `name` (trimmed), not the raw field: the hub stores the trimmed
         // username and every key derivation must fold in the same string.
-        // `captcha.token` rides along — the submit button is already disabled
-        // until it is set when a captcha is configured (see the form below).
         const reg = await window.MeshBayKeys.registerUser(
           name, email, password, emailRecovery ? rk.mnemonic : null,
-          captcha.token);
+          await captcha.take());
         // An account made in the desktop application starts without browser
         // access: its identities stay on this device and nothing of them is
         // left on nodes until the person turns it on, here, natively.
@@ -433,17 +422,14 @@ export function RegisterPage() {
           body: {
             username: name, email, password,
             pk_user_ed25519: '', pk_user_x25519: '',
-            captcha_token: captcha.token,
+            captcha_token: await captcha.take(),
           },
         });
         setPhase('verify');
       }
     } catch (err) {
-      setError(err.message);
-      // A reCAPTCHA token is single-use: after a failed attempt (name taken,
-      // e-mail in use…) it is spent, so clear it and make the user solve a
-      // fresh one before the next try. No-op when no captcha is configured.
-      captcha.reset();
+      setError(errorText(err));
+      captcha.renew();
     } finally {
       setLoading(false);
     }
@@ -609,9 +595,8 @@ export function RegisterPage() {
                    onChange=${e => setEmailRecovery(e.target.checked)} />
             <span>${t('register.recovery_email_opt')}</span>
           </label>
-          ${captcha.widget}
           ${error && html`<div class="error-msg">${error}</div>`}
-          <button type="submit" disabled=${loading || (captcha.enabled && !captcha.token)}>
+          <button type="submit" disabled=${loading}>
             ${loading ? t('register.loading') : t('register.submit')}
           </button>
           <p class="register-legal">
@@ -658,12 +643,13 @@ export function ResetPasswordPage({ onLogin }) {
         method: 'POST',
         body: {
           username: username.trim(), email: email.trim(),
-          captcha_token: captcha.token,
+          captcha_token: await captcha.take(),
         },
       });
       setPhase('form');
     } catch (err) {
-      setError(err.message);
+      setError(errorText(err));
+      captcha.renew();
     } finally {
       setBusy(false);
     }
@@ -771,9 +757,8 @@ export function ResetPasswordPage({ onLogin }) {
           <input type="email" placeholder="${t('register.email')}" value=${email}
                  onInput=${e => setEmail(e.target.value)}
                  autocomplete="email" required />
-          ${captcha.widget}
           ${error && html`<div class="error-msg">${error}</div>`}
-          <button type="submit" disabled=${busy || (captcha.enabled && !captcha.token)}>
+          <button type="submit" disabled=${busy}>
             ${t('reset.send_code')}</button>
         </form>`}
 
diff --git a/packages/meshbay-hub/src/meshbay_hub/static/captcha.js b/packages/meshbay-hub/src/meshbay_hub/static/captcha.js
new file mode 100644
index 0000000..bc174a0
--- /dev/null
+++ b/packages/meshbay-hub/src/meshbay_hub/static/captcha.js
@@ -0,0 +1,47 @@
+// The proof of work that gates sign-up and passphrase reset (the hub's
+// captcha.py). The hub sends the SHA-256 of `<prefix><number>` and the page
+// finds the number by trying them from 0, so a person waits for nothing: it is
+// solved while the form is being filled in. Nothing leaves for a third party.
+//
+// On the main thread, in batches handed to SubtleCrypto, rather than in a Web
+// Worker: the Android shell serves the interface through its own request
+// interceptor, and a worker's script is a request that interceptor is not
+// guaranteed to see. Each batch is a turn of the event loop, so typing in the
+// form is not held up by it.
+
+const BATCH = 1000;
+
+function fromHex(hex) {
+  const out = new Uint8Array(hex.length / 2);
+  for (let i = 0; i < out.length; i++) out[i] = parseInt(hex.substr(i * 2, 2), 16);
+  return out;
+}
+
+function same(buf, want) {
+  const got = new Uint8Array(buf);
+  for (let i = 0; i < want.length; i++) if (got[i] !== want[i]) return false;
+  return true;
+}
+
+/**
+ * The answer to a challenge from `GET /v1/users/captcha`, as the token the
+ * register and reset-request bodies carry — or null if no number below
+ * `max_number` matches, which only a challenge that is not the hub's produces,
+ * or once `signal` is aborted: the form was left before it was needed.
+ */
+export async function solve({ prefix, challenge, max_number: max, signature }, signal) {
+  const enc = new TextEncoder();
+  const want = fromHex(challenge);
+  for (let start = 0; start < max; start += BATCH) {
+    if (signal && signal.aborted) return null;
+    const end = Math.min(start + BATCH, max);
+    const pending = [];
+    for (let n = start; n < end; n++) {
+      pending.push(crypto.subtle.digest('SHA-256', enc.encode(prefix + n)));
+    }
+    const digests = await Promise.all(pending);
+    const hit = digests.findIndex(d => same(d, want));
+    if (hit >= 0) return `${prefix}${start + hit}.${signature}`;
+  }
+  return null;
+}
diff --git a/packages/meshbay-hub/src/meshbay_hub/static/keyderive.js b/packages/meshbay-hub/src/meshbay_hub/static/keyderive.js
index edf4df9..bd5282c 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/keyderive.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/keyderive.js
@@ -401,9 +401,9 @@ async function registerUser(username, email, password, recoveryMnemonic, captcha
   // appends it to the verification e-mail and stores it nowhere
   // (docs/MESHBAY_DESIGN.md §3.6). Omitted when they chose to save it themselves.
   if (recoveryMnemonic) payload.recovery_key = recoveryMnemonic;
-  // reCAPTCHA response, when the hub has a captcha configured. The widget lives
-  // in RegisterPage (auth-page.js); this function just forwards its token. A
-  // hub with no captcha configured sends nothing and the server does not check.
+  // The proof-of-work answer, when the hub asks for one. RegisterPage
+  // (auth-page.js) works it out; this function just forwards it. A hub with
+  // the check turned off gets nothing and does not look.
   if (captchaToken) payload.captcha_token = captchaToken;
 
   const resp = await hubCall('/v1/users/register', {
diff --git a/packages/meshbay-hub/src/meshbay_hub/static/locales/de.js b/packages/meshbay-hub/src/meshbay_hub/static/locales/de.js
index 1fe2b36..bf16bca 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/locales/de.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/locales/de.js
@@ -139,8 +139,7 @@ export default {
   'register.strength': 'Stärke: etwa {bits} Bit. Sie schützt die Kopie Ihrer '
     + 'Schlüssel, die auf den Nodes liegt, denen Sie beitreten — es lohnt sich also, '
     + 'sie gut zu wählen.',
-  'captcha.required': 'Bitte das Captcha ausfüllen.',
-  'captcha.failed': 'Captcha-Überprüfung fehlgeschlagen — bitte erneut versuchen.',
+  'captcha.failed': "Die Prüfung gegen Roboter ist fehlgeschlagen. Bitte versuchen Sie es erneut.",
 
   // Home
   'home.welcome': 'Willkommen bei MeshBay',
diff --git a/packages/meshbay-hub/src/meshbay_hub/static/locales/en.js b/packages/meshbay-hub/src/meshbay_hub/static/locales/en.js
index 642d3e2..c7dc47e 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/locales/en.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/locales/en.js
@@ -140,8 +140,7 @@ export default {
     + 'your keys where they are stored — a few unrelated words work well.',
   'register.strength': 'Strength: about {bits} bits. This protects the copy of '
     + 'your keys kept on the nodes you join, so it is worth getting right.',
-  'captcha.required': 'Please complete the captcha.',
-  'captcha.failed': 'Captcha verification failed — please try again.',
+  'captcha.failed': "The anti-robot check failed. Please try again.",
 
   // Home
   'home.welcome': 'Welcome to MeshBay',
diff --git a/packages/meshbay-hub/src/meshbay_hub/static/locales/es.js b/packages/meshbay-hub/src/meshbay_hub/static/locales/es.js
index fe66af4..0831bac 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/locales/es.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/locales/es.js
@@ -137,8 +137,7 @@ export default {
     + 'relación entre sí funcionan muy bien.',
   'register.strength': 'Solidez: unos {bits} bits. Protege la copia de sus claves que '
     + 'se guarda en los nodes a los que se une, así que conviene acertar.',
-  'captcha.required': 'Por favor, complete el captcha.',
-  'captcha.failed': 'Verificación del captcha fallida — inténtelo de nuevo.',
+  'captcha.failed': "La verificación anti-robots ha fallado. Inténtelo de nuevo.",
 
   // Home
   'home.welcome': 'Bienvenido a MeshBay',
diff --git a/packages/meshbay-hub/src/meshbay_hub/static/locales/fr.js b/packages/meshbay-hub/src/meshbay_hub/static/locales/fr.js
index 6296a8b..2a52848 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/locales/fr.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/locales/fr.js
@@ -138,8 +138,7 @@ export default {
   'register.strength': 'Robustesse : environ {bits} bits. Elle protège la copie de '
     + 'vos clés conservée sur les nodes que vous rejoignez, cela vaut donc la peine '
     + 'de la soigner.',
-  'captcha.required': 'Veuillez compléter le captcha.',
-  'captcha.failed': 'Vérification du captcha échouée — veuillez réessayer.',
+  'captcha.failed': "La vérification anti-robot a échoué. Veuillez réessayer.",
 
   // Home
   'home.welcome': 'Bienvenue sur MeshBay',
diff --git a/packages/meshbay-hub/src/meshbay_hub/static/locales/it.js b/packages/meshbay-hub/src/meshbay_hub/static/locales/it.js
index 0751692..1dd3f52 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/locales/it.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/locales/it.js
@@ -138,8 +138,7 @@ export default {
     + 'legame tra loro funziona bene.',
   'register.strength': 'Robustezza: circa {bits} bit. Protegge la copia delle sue '
     + 'chiavi conservata sui node a cui aderisce, quindi vale la pena sceglierla con cura.',
-  'captcha.required': 'Completa il captcha.',
-  'captcha.failed': 'Verifica captcha fallita — riprova.',
+  'captcha.failed': "La verifica anti-robot non è riuscita. Riprovi.",
 
   // Home
   'home.welcome': 'Benvenuto in MeshBay',
diff --git a/packages/meshbay-hub/src/meshbay_hub/static/locales/ja.js b/packages/meshbay-hub/src/meshbay_hub/static/locales/ja.js
index c3d4fa8..0d4aadf 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/locales/ja.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/locales/ja.js
@@ -136,8 +136,7 @@ export default {
     + 'その鍵を守るものです。互いに関係のない単語をいくつか並べると効果的です。',
   'register.strength': '強度：約 {bits} ビット。参加している各 node に保管される鍵の'
     + 'コピーを守るものですので、しっかり決めておく価値があります。',
-  'captcha.required': 'キャプチャを完了してください。',
-  'captcha.failed': 'キャプチャの検証に失敗しました — もう一度お試しください。',
+  'captcha.failed': "ロボット対策の確認に失敗しました。もう一度お試しください。",
 
   // Home
   'home.welcome': 'MeshBay へようこそ',
diff --git a/packages/meshbay-hub/src/meshbay_hub/static/locales/nl.js b/packages/meshbay-hub/src/meshbay_hub/static/locales/nl.js
index c02be99..31fc7b3 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/locales/nl.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/locales/nl.js
@@ -139,8 +139,7 @@ export default {
   'register.strength': 'Sterkte: ongeveer {bits} bits. Ze beschermt de kopie van uw '
     + 'sleutels die op de nodes staat waar u zich bij aansluit, dus het loont om er '
     + 'even bij stil te staan.',
-  'captcha.required': 'Vul de captcha in.',
-  'captcha.failed': 'Captchaverificatie mislukt — probeer het opnieuw.',
+  'captcha.failed': "De controle tegen robots is mislukt. Probeer het opnieuw.",
 
   // Home
   'home.welcome': 'Welkom bij MeshBay',
diff --git a/packages/meshbay-hub/src/meshbay_hub/static/locales/pl.js b/packages/meshbay-hub/src/meshbay_hub/static/locales/pl.js
index 97e8b53..e88272d 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/locales/pl.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/locales/pl.js
@@ -143,8 +143,7 @@ export default {
     + 'bardzo dobrze.',
   'register.strength': 'Siła: około {bits} bitów. Chroni kopię kluczy przechowywaną na '
     + 'nodes, do których się dołącza, więc warto ją dobrać starannie.',
-  'captcha.required': 'Proszę wypełnić captcha.',
-  'captcha.failed': 'Weryfikacja captcha nie powiodła się — spróbuj ponownie.',
+  'captcha.failed': "Weryfikacja antyrobotowa nie powiodła się. Spróbuj ponownie.",
 
   // Home
   'home.welcome': 'Witamy w MeshBay',
diff --git a/packages/meshbay-hub/src/meshbay_hub/static/locales/pt-BR.js b/packages/meshbay-hub/src/meshbay_hub/static/locales/pt-BR.js
index 8ff36a5..5b2d80f 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/locales/pt-BR.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/locales/pt-BR.js
@@ -139,8 +139,7 @@ export default {
     + 'entre si funcionam muito bem.',
   'register.strength': 'Robustez: cerca de {bits} bits. Ela protege a cópia das suas '
     + 'chaves mantida nos nodes dos quais você participa, então vale a pena caprichar.',
-  'captcha.required': 'Por favor, complete o captcha.',
-  'captcha.failed': 'Verificação do captcha falhou — tente novamente.',
+  'captcha.failed': "A verificação anti-robô falhou. Tente novamente.",
 
   // Home
   'home.welcome': 'Bem-vindo ao MeshBay',
diff --git a/packages/meshbay-hub/src/meshbay_hub/static/locales/zh-CN.js b/packages/meshbay-hub/src/meshbay_hub/static/locales/zh-CN.js
index d4ad64f..a6ea535 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/locales/zh-CN.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/locales/zh-CN.js
@@ -136,8 +136,7 @@ export default {
     + '几个互不相关的词就很好用。',
   'register.strength': '强度：约 {bits} 位。它保护的是您所加入的各个 node 上保存的密钥副本，'
     + '因此值得认真设置。',
-  'captcha.required': '请完成验证码。',
-  'captcha.failed': '验证码验证失败——请重试。',
+  'captcha.failed': "防机器人验证失败，请重试。",
 
   // Home
   'home.welcome': '欢迎使用 MeshBay',