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, 25 insertions, 10 deletions

diff --git a/packages/meshbay-hub/src/meshbay_hub/api/users.py b/packages/meshbay-hub/src/meshbay_hub/api/users.py
index 991a4f7..7b2fc8c 100644
--- a/packages/meshbay-hub/src/meshbay_hub/api/users.py
+++ b/packages/meshbay-hub/src/meshbay_hub/api/users.py
@@ -222,8 +222,8 @@ async def register(
     if not credential:
         raise HTTPException(status_code=400, detail="auth_key or password required")
 
-    pw_hash, pw_salt = await hash_password_off_loop(credential)
     pw_ver = current_pw_version() if body.auth_key else 2
+    pw_hash, pw_salt = await hash_password_off_loop(credential, pw_ver)
     hub_id = _cfg.identity.id if _cfg else "meshbay.org"
     user = User(
         username=body.username,
@@ -399,7 +399,7 @@ async def login(
             user.pw_version = current_pw_version()
         elif user.pw_version < 2:
             # Legacy rehash: upgrade Argon2 params within the password scheme (v1 -> v2)
-            new_hash, new_salt = await hash_password_off_loop(body.password)
+            new_hash, new_salt = await hash_password_off_loop(body.password, 2)
             user.pw_hash = new_hash
             user.pw_salt = new_salt
             user.pw_version = 2
diff --git a/packages/meshbay-hub/src/meshbay_hub/auth.py b/packages/meshbay-hub/src/meshbay_hub/auth.py
index 58e2310..7045197 100644
--- a/packages/meshbay-hub/src/meshbay_hub/auth.py
+++ b/packages/meshbay-hub/src/meshbay_hub/auth.py
@@ -31,10 +31,17 @@ _ARGON2_KEY_LEN = 32
 
 _ARGON2_VERSIONS = {
     1: {"iterations": 3, "memory_cost": 65536},     # 64 MB  — initial
-    2: {"iterations": 3, "memory_cost": 262144},    # 256 MB — production target
+    2: {"iterations": 3, "memory_cost": 262144},    # 256 MB — raw password (legacy)
     3: {"iterations": 3, "memory_cost": 262144},    # 256 MB — auth_key input (password split)
+    # 64 MiB, t=3, p=4 — RFC 9106's second recommended setting. What this hashes
+    # is already PBKDF2-SHA512 at 600 000 iterations of the passphrase, done by
+    # the client, and online guessing is bounded by the sign-in lockout, so the
+    # memory above this bought a constant factor against an offline attacker
+    # with the database, at four times the cost of every sign-in. Versions 3 and
+    # above are the auth_key scheme; a v3 hash is rewritten at its next sign-in.
+    4: {"iterations": 3, "memory_cost": 65536},
 }
-_ARGON2_CURRENT_VERSION = 3
+_ARGON2_CURRENT_VERSION = 4
 
 # Module-level hub keypair (loaded once at startup)
 _hub_sk_pem: bytes | None = None
@@ -112,10 +119,16 @@ def hub_id() -> str:
 
 # ── Password ──────────────────────────────────────────────────────────────────
 
-def hash_password(password: str) -> tuple[bytes, bytes]:
-    """Hash a password with Argon2id (current version). Returns (hash, salt)."""
+def hash_password(password: str, version: int = _ARGON2_CURRENT_VERSION) -> tuple[bytes, bytes]:
+    """Hash with the parameters of `version`, which is what the caller stores.
+
+    The version is an argument because the stored `pw_version` is what
+    verification reads the parameters from: hashing at one version's parameters
+    and recording another makes an account nobody can sign in to. That mistake
+    sat unseen while versions 2 and 3 shared their parameters.
+    """
     salt = os.urandom(16)
-    params = _ARGON2_VERSIONS[_ARGON2_CURRENT_VERSION]
+    params = _ARGON2_VERSIONS[version]
     pw_hash = Argon2id(
         salt=salt,
         length=_ARGON2_KEY_LEN,
@@ -143,7 +156,8 @@ def verify_password(password: str, pw_hash: bytes, salt: bytes, version: int = 2
 
 # ── Argon2 off the event loop, one at a time ─────────────────────────────────
 #
-# One derivation is 256 MB and a quarter to half a second of CPU. Called from an
+# One derivation is 64 MiB and ~0.1 s on meshbay.org (0.45 s for an old 256 MB
+# hash, until its account next signs in). Called from an
 # async handler it stops the whole hub for that long — every request, every node
 # socket, every offer relayed — once per sign-in, passphrase change, reset and
 # registration.
@@ -164,9 +178,10 @@ def verify_password(password: str, pw_hash: bytes, salt: bytes, version: int = 2
 _argon2_executor = ThreadPoolExecutor(max_workers=1, thread_name_prefix="argon2")
 
 
-async def hash_password_off_loop(password: str) -> tuple[bytes, bytes]:
+async def hash_password_off_loop(password: str,
+                                 version: int = _ARGON2_CURRENT_VERSION) -> tuple[bytes, bytes]:
     loop = asyncio.get_running_loop()
-    return await loop.run_in_executor(_argon2_executor, hash_password, password)
+    return await loop.run_in_executor(_argon2_executor, hash_password, password, version)
 
 
 async def verify_password_off_loop(password: str, pw_hash: bytes, salt: bytes,