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>

1 files changed, 126 insertions, 22 deletions

diff --git a/packages/meshbay-hub/tests/test_register_captcha.py b/packages/meshbay-hub/tests/test_register_captcha.py
index 5f94b9c..b63b197 100644
--- a/packages/meshbay-hub/tests/test_register_captcha.py
+++ b/packages/meshbay-hub/tests/test_register_captcha.py
@@ -1,27 +1,49 @@
-"""Registration CAPTCHA is enforced for every fresh account when configured.
+"""Sign-up and passphrase reset are gated by the hub's own proof of work.
 
-The gate used to be skipped whenever the request carried an `auth_key` — which
-every real client sends (the password split) — so it protected nobody and a bot
-skipped it by including the field. It now runs on `captcha.enabled` alone; the
-desktop client is Chromium and renders the same widget.
+The gate replaced reCAPTCHA, which sent every visitor's address and browser
+details to Google and needed a consent the sign-up form could not ask for
+freely. What it must still be: unconditional when enabled — the old gate was
+once skipped for any request carrying an `auth_key`, which every real client
+sends — and an answer the hub issued, in date, and used once.
+
+The last test runs the page's own solver (`static/captcha.js`) against a
+challenge from `captcha.issue`, so the two halves cannot drift apart unseen.
 """
 
+import hashlib
+import json
+import shutil
+import subprocess
+import time
+from pathlib import Path
+
 import pytest
+from meshbay_hub import captcha
+from meshbay_hub.config import load_config
+
+STATIC = Path(captcha.__file__).parent / "static"
 
 
 @pytest.fixture
 def captcha_on(client, monkeypatch):
-    """Turn on a fake captcha: any config with both keys is `enabled`, and
-    verification succeeds only for the token 'good-token'."""
+    """The check on, with little enough work that the tests solve it at once."""
     from meshbay_hub.api.users import _cfg
-    monkeypatch.setattr(_cfg.captcha, "site_key", "test-site")
-    monkeypatch.setattr(_cfg.captcha, "secret_key", "test-secret")
+    monkeypatch.setattr(_cfg.captcha, "enabled", True)
+    monkeypatch.setattr(_cfg.captcha, "max_number", 2000)
+    monkeypatch.setattr(captcha, "_spent", {})
+
 
-    async def fake_verify(secret, token, remote_ip=None, allowed_hosts=None,
-                          allow_unattributed=False):
-        return token == "good-token"
+def _solve(ch: dict) -> str:
+    for n in range(ch["max_number"]):
+        if hashlib.sha256(f"{ch['prefix']}{n}".encode()).hexdigest() == ch["challenge"]:
+            return f"{ch['prefix']}{n}.{ch['signature']}"
+    raise AssertionError("no number matches the challenge")
 
-    monkeypatch.setattr("meshbay_hub.captcha.verify_captcha", fake_verify)
+
+async def _token(client) -> str:
+    r = await client.get("/v1/users/captcha")
+    assert r.status_code == 200, r.text
+    return _solve(r.json())
 
 
 def _body(**over):
@@ -31,29 +53,111 @@ def _body(**over):
 
 
 @pytest.mark.asyncio
-async def test_missing_captcha_rejected_even_with_auth_key(client, captcha_on):
+async def test_missing_answer_rejected_even_with_auth_key(client, captcha_on):
     r = await client.post("/v1/users/register", json=_body())
     assert r.status_code == 400
     assert r.json()["detail"] == "captcha_required"
 
 
 @pytest.mark.asyncio
-async def test_bad_captcha_rejected(client, captcha_on):
+async def test_solved_challenge_accepted(client, captcha_on):
     r = await client.post("/v1/users/register",
-                          json=_body(captcha_token="wrong"))
+                          json=_body(captcha_token=await _token(client)))
+    assert r.status_code == 201, r.text
+
+
+@pytest.mark.asyncio
+async def test_an_answer_is_good_for_one_form(client, captcha_on):
+    token = await _token(client)
+    r = await client.post("/v1/users/register", json=_body(captcha_token=token))
+    assert r.status_code == 201, r.text
+    r = await client.post("/v1/users/register", json=_body(
+        username="another_test", email="another@t.com", captcha_token=token))
     assert r.status_code == 400
     assert r.json()["detail"] == "captcha_failed"
 
 
 @pytest.mark.asyncio
-async def test_good_captcha_accepted(client, captcha_on):
-    r = await client.post("/v1/users/register",
-                          json=_body(captcha_token="good-token"))
-    assert r.status_code == 201
+async def test_a_wrong_number_is_refused(client, captcha_on):
+    nonce, expires, number, sig = (await _token(client)).split(".")
+    forged = f"{nonce}.{expires}.{int(number) + 1}.{sig}"
+    r = await client.post("/v1/users/register", json=_body(captcha_token=forged))
+    assert r.json()["detail"] == "captcha_failed"
+
+
+@pytest.mark.asyncio
+async def test_a_challenge_the_hub_did_not_sign_is_refused(client, captcha_on):
+    """Somebody minting their own easy challenge has no key to sign it with."""
+    forged = f"{'00' * 12}.{int(time.time()) + 60}.0.{'0' * 64}"
+    r = await client.post("/v1/users/register", json=_body(captcha_token=forged))
+    assert r.json()["detail"] == "captcha_failed"
 
 
 @pytest.mark.asyncio
-async def test_no_captcha_configured_still_registers(client):
-    # Default test config has no captcha keys — registration proceeds without one.
+async def test_an_expired_answer_is_refused(client, captcha_on, monkeypatch):
+    token = await _token(client)
+    later = time.time() + captcha.CHALLENGE_TTL + 1
+    monkeypatch.setattr(captcha.time, "time", lambda: later)
+    r = await client.post("/v1/users/register", json=_body(captcha_token=token))
+    assert r.json()["detail"] == "captcha_failed"
+
+
+@pytest.mark.asyncio
+async def test_password_reset_request_is_gated_too(client, captcha_on):
+    body = {"username": "nobody_test", "email": "nobody@t.com"}
+    r = await client.post("/v1/users/password/reset-request", json=body)
+    assert r.json()["detail"] == "captcha_required"
+    r = await client.post("/v1/users/password/reset-request",
+                          json={**body, "captcha_token": await _token(client)})
+    assert r.status_code == 200, r.text
+
+
+@pytest.mark.asyncio
+async def test_turned_off_it_issues_nothing_and_checks_nothing(client):
+    # The test configuration has it off (conftest.py).
+    r = await client.get("/v1/users/captcha")
+    assert r.status_code == 404
+    assert r.json()["detail"] == "captcha_disabled"
     r = await client.post("/v1/users/register", json=_body())
     assert r.status_code == 201
+
+
+def test_on_unless_the_configuration_says_otherwise(tmp_path):
+    cfg_file = tmp_path / "hub.toml"
+    cfg_file.write_text('[server]\nport = 8000\n', encoding="utf-8")
+    assert load_config(cfg_file).captcha.enabled
+    cfg_file.write_text('[captcha]\nenabled = false\n', encoding="utf-8")
+    assert not load_config(cfg_file).captcha.enabled
+
+
+def test_old_recaptcha_keys_are_ignored_with_a_warning(tmp_path, monkeypatch):
+    # Caught at the logger rather than through caplog: other tests in the suite
+    # reconfigure logging, and caplog then sees nothing.
+    from meshbay_hub import config
+    warned = []
+    monkeypatch.setattr(config.log, "warning", lambda msg, *a: warned.append(msg))
+    cfg_file = tmp_path / "hub.toml"
+    cfg_file.write_text('[captcha]\nsite_key = "6Lsite"\nsecret_key = "6Lsecret"\n',
+                        encoding="utf-8")
+    cfg = load_config(cfg_file)
+    assert cfg.captcha.enabled
+    assert any("no longer read" in m for m in warned)
+
+
+@pytest.mark.skipif(shutil.which("node") is None, reason="node is not available")
+@pytest.mark.asyncio
+async def test_the_page_solver_answers_the_hub(client, captcha_on, tmp_path):
+    ch = (await client.get("/v1/users/captcha")).json()
+    (tmp_path / "package.json").write_text('{"type":"module"}', encoding="utf-8")
+    (tmp_path / "captcha.js").write_text(
+        (STATIC / "captcha.js").read_text(encoding="utf-8"), encoding="utf-8")
+    (tmp_path / "case.js").write_text(
+        "import { solve } from './captcha.js';\n"
+        f"console.log(JSON.stringify(await solve({json.dumps(ch)})));\n", encoding="utf-8")
+    proc = subprocess.run(["node", "case.js"], cwd=tmp_path,
+                          capture_output=True, text=True, encoding="utf-8")
+    assert proc.returncode == 0, proc.stderr
+    token = json.loads(proc.stdout)
+    assert token == _solve(ch)
+    r = await client.post("/v1/users/register", json=_body(captcha_token=token))
+    assert r.status_code == 201, r.text