fix(hub): the password verifier is Argon2id 64 MiB, and a hash's version names its parameters

`pw_version` 4: Argon2id 64 MiB, t=3, lanes=4 — RFC 9106's second recommended
setting. A v3 hash (256 MB) still verifies at its own parameters and is
rewritten at the new ones on the next sign-in, through the rehash path that
already existed.

Why not more. The verifier matters against an offline attacker holding the
database; online guessing is bounded by the sign-in lockout. That attacker pays
the client's 600 000 PBKDF2-SHA512 iterations and the hub's Argon2id per guess,
since `auth_key` is 256 bits and cannot be searched directly. Memory above
64 MiB multiplies that cost by a constant — at most 16 at 256 MB, less with
PBKDF2 counted — while the hub pays the same memory at every sign-in, one
derivation at a time. Measured on meshbay.org: 450 ms at 256 MB, 105 ms at
64 MiB, so a burst of sign-ins clears about four times faster.

Changing the current version exposed a latent lockout. `hash_password` always
used the current version's parameters, while the raw-password scheme recorded
`pw_version = 2` — harmless while versions 2 and 3 shared their parameters,
and with version 4 every legacy registration and v1→v2 rehash would have
stored a 64 MiB hash labelled 256 MB, which nothing could then verify. Seventeen
tests caught it. `hash_password` now takes the version it is hashing for.

The OpenSSL deadlock between two concurrent `lanes=4` derivations is the same at
64 MiB, so Argon2 stays on its single worker. The loop-stall test measures
against a v3 hash, because half of a 45 ms inline derivation is too close to
scheduling noise to be a reliable bound.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LcF3QKWii7uQ2kSyXErzCt

2 files changed, 64 insertions, 12 deletions

diff --git a/packages/meshbay-hub/tests/test_argon2_off_loop.py b/packages/meshbay-hub/tests/test_argon2_off_loop.py
index b156ebc..5c25a8e 100644
--- a/packages/meshbay-hub/tests/test_argon2_off_loop.py
+++ b/packages/meshbay-hub/tests/test_argon2_off_loop.py
@@ -1,10 +1,11 @@
 """
 Argon2 runs off the event loop, and never two at a time.
 
-One derivation is 256 MB and a quarter to half a second of CPU. On the loop it
-stopped the whole hub for that long at every sign-in. In a thread pool it would
-have been worse: two concurrent `lanes=4` derivations deadlock inside OpenSSL
-and never return (`auth._argon2_executor`). These pin both halves.
+A derivation is 64 MiB and ~0.1 s at the current version, 256 MB and ~0.45 s
+for a v3 hash not yet rewritten. On the loop it stopped the whole hub for that
+long at every sign-in. In a thread pool it would have been worse: two concurrent
+`lanes=4` derivations deadlock inside OpenSSL and never return, at 64 MiB as at
+256 MB (`auth._argon2_executor`). These pin both halves.
 """
 
 import asyncio
@@ -51,9 +52,13 @@ async def test_concurrent_derivations_all_return():
 
 @pytest.mark.asyncio
 async def test_the_loop_keeps_turning_while_argon2_runs():
-    pw_hash, salt = auth.hash_password("k" * 44)
+    # A v3 (256 MB) hash, the heaviest the hub still verifies: at 64 MiB an
+    # inline derivation is ~45 ms and half of it is too close to scheduling
+    # noise to tell a stalled loop from a busy machine.
+    version = 3
+    pw_hash, salt = auth.hash_password("k" * 44, version)
     started = time.perf_counter()
-    auth.verify_password("k" * 44, pw_hash, salt, auth.current_pw_version())
+    auth.verify_password("k" * 44, pw_hash, salt, version)
     inline = time.perf_counter() - started
 
     gaps, done = [], asyncio.Event()
@@ -68,8 +73,7 @@ async def test_the_loop_keeps_turning_while_argon2_runs():
 
     task = asyncio.create_task(ticker())
     await asyncio.sleep(0.02)
-    assert await auth.verify_password_off_loop(
-        "k" * 44, pw_hash, salt, auth.current_pw_version())
+    assert await auth.verify_password_off_loop("k" * 44, pw_hash, salt, version)
     done.set()
     await task
     # Inline, the loop stalls for the whole derivation; off it, for scheduling noise.
diff --git a/packages/meshbay-hub/tests/test_hub_api.py b/packages/meshbay-hub/tests/test_hub_api.py
index 5e9e6c2..37e8e4f 100644
--- a/packages/meshbay-hub/tests/test_hub_api.py
+++ b/packages/meshbay-hub/tests/test_hub_api.py
@@ -621,6 +621,54 @@ async def test_password_rehash_on_login(client, app):
     assert r.status_code == 200
 
 
+@pytest.mark.asyncio
+async def test_a_v3_auth_key_hash_is_rewritten_at_the_current_parameters(client, db_session):
+    """Every account registered before the 64 MiB change holds a v3 (256 MB)
+    hash. It must keep signing in, and be rewritten on the way through —
+    otherwise the old cost is paid at every sign-in for ever."""
+    import os
+
+    from cryptography.hazmat.primitives.kdf.argon2 import Argon2id
+    from meshbay_hub.auth import (
+        _ARGON2_KEY_LEN,
+        _ARGON2_LANES,
+        _ARGON2_VERSIONS,
+        current_pw_version,
+    )
+    from meshbay_hub.db.models import User
+    from sqlalchemy import select
+
+    assert current_pw_version() > 3
+    assert _ARGON2_VERSIONS[current_pw_version()]["memory_cost"] == 65536
+
+    key = "dGVzdGF1dGhrZXl0ZXN0YXV0aGtleXRlc3RhdXRo"
+    r = await client.post("/v1/users/register", json={
+        "username": "v3holder", "email": "v3holder@test.com", "auth_key": key})
+    assert r.status_code == 201, r.text
+
+    salt = os.urandom(16)
+    v3 = _ARGON2_VERSIONS[3]
+    user = (await db_session.execute(
+        select(User).where(User.username == "v3holder"))).scalar_one()
+    user.pw_hash = Argon2id(salt=salt, length=_ARGON2_KEY_LEN, iterations=v3["iterations"],
+                            lanes=_ARGON2_LANES, memory_cost=v3["memory_cost"]).derive(key.encode())
+    user.pw_salt, user.pw_version = salt, 3
+    await db_session.commit()
+
+    r = await client.post("/v1/users/login", json={"username": "v3holder", "auth_key": key})
+    assert r.status_code == 200, r.text
+
+    db_session.expire_all()
+    user = (await db_session.execute(
+        select(User).where(User.username == "v3holder"))).scalar_one()
+    assert user.pw_version == current_pw_version()
+    assert user.pw_hash != Argon2id(salt=salt, length=_ARGON2_KEY_LEN, iterations=3,
+                                    lanes=_ARGON2_LANES, memory_cost=65536).derive(key.encode())
+
+    r = await client.post("/v1/users/login", json={"username": "v3holder", "auth_key": key})
+    assert r.status_code == 200, r.text
+
+
 # ── WebRTC signaling (9.2) ──────────────────────────────────────────────────
 
 @pytest.mark.asyncio
@@ -764,7 +812,8 @@ async def test_webapp_html_includes_scripts(client):
 
 @pytest.mark.asyncio
 async def test_register_with_auth_key(client, app):
-    """Registration with auth_key sets pw_version 3."""
+    """Registration with auth_key sets the current auth_key pw_version."""
+    from meshbay_hub.auth import current_pw_version
     from meshbay_hub.db.engine import get_db
     from meshbay_hub.db.models import User
     from sqlalchemy import select
@@ -783,7 +832,7 @@ async def test_register_with_auth_key(client, app):
     async for db in get_db():
         result = await db.execute(select(User).where(User.username == "authuser"))
         user = result.scalar_one()
-        assert user.pw_version == 3
+        assert user.pw_version == current_pw_version()
         break
 
 
@@ -932,12 +981,11 @@ async def test_login_legacy_migration(client, app):
     })
     assert r.status_code == 200
 
-    # Verify pw_version is now 3 (migrated)
+    # Verify pw_version is now the current auth_key version (migrated)
     async for db in get_db():
         result = await db.execute(select(User).where(User.username == "migrateuser"))
         user = result.scalar_one()
         assert user.pw_version == current_pw_version()
-        assert user.pw_version == 3
         break
 
     # Login again with auth_key only → should succeed (migrated account)