fix: tolerate clock skew when verifying JWTs (leeway 60s)

A client whose clock is a little fast could not connect at all: the MNP
handshake verified the hub-issued token with no leeway, so a token whose
`iat` was a few seconds ahead of the node's clock failed with "the token
is not yet valid (iat)". Seen against a freshly-resumed VM guest.

`meshbay_common.handshake.JWT_LEEWAY_SECONDS = 60` is the shared value;
applied to the handshake, the node's own hub-token decode, revocation-token
verification, and the hub's access-token decode. 60s absorbs NTP-level skew
without meaningfully widening the window on a stolen token (they already
carry a jti and an exp).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

4 files changed, 17 insertions, 3 deletions

diff --git a/packages/meshbay-common/src/meshbay_common/handshake.py b/packages/meshbay-common/src/meshbay_common/handshake.py
index 2f3d641..f7d4911 100644
--- a/packages/meshbay-common/src/meshbay_common/handshake.py
+++ b/packages/meshbay-common/src/meshbay_common/handshake.py
@@ -77,6 +77,13 @@ ROLE_NODE = "node"
 
 NONCE_LEN = 32
 
+# Clock-skew tolerance for JWT `iat`/`exp`/`nbf`. The token is issued by the
+# hub and verified by a node, on two machines whose clocks are only as close
+# as their NTP — and a VM guest that has just resumed can be tens of seconds
+# out. Without this a slightly-fast client cannot connect at all
+# ("token is not yet valid (iat)").
+JWT_LEEWAY_SECONDS = 60
+
 
 class HandshakeError(Exception):
     """
@@ -230,7 +237,8 @@ def authorize_token(
     check entirely and fell back to the node's first group (M1).
     """
     try:
-        decoded = jwt.decode(token, hub_pk_pem, algorithms=["EdDSA"])
+        decoded = jwt.decode(token, hub_pk_pem, algorithms=["EdDSA"],
+                             leeway=JWT_LEEWAY_SECONDS)
     except Exception as exc:
         raise HandshakeError(f"Invalid JWT: {exc}") from exc
 
diff --git a/packages/meshbay-hub/src/meshbay_hub/auth.py b/packages/meshbay-hub/src/meshbay_hub/auth.py
index 2bf59db..34baf45 100644
--- a/packages/meshbay-hub/src/meshbay_hub/auth.py
+++ b/packages/meshbay-hub/src/meshbay_hub/auth.py
@@ -166,7 +166,9 @@ def decode_access_token(token: str) -> dict:
     """Verify and decode an access token. Raises on failure."""
     if _hub_pk_pem is None:
         raise RuntimeError("Hub keypair not loaded")
-    return jwt.decode(token, _hub_pk_pem, algorithms=["EdDSA"])
+    # Clock-skew tolerance (meshbay_common.handshake.JWT_LEEWAY_SECONDS): a
+    # client whose clock is a little fast must still be able to call the API.
+    return jwt.decode(token, _hub_pk_pem, algorithms=["EdDSA"], leeway=60)
 
 
 # ── Email encryption at rest ──────────────────────────────────────────────────
diff --git a/packages/meshbay-node/src/meshbay_node/hub_client.py b/packages/meshbay-node/src/meshbay_node/hub_client.py
index 1e975fa..8873958 100644
--- a/packages/meshbay-node/src/meshbay_node/hub_client.py
+++ b/packages/meshbay-node/src/meshbay_node/hub_client.py
@@ -129,7 +129,9 @@ class HubClient:
 
         access_token = data["access_token"]
 
-        decoded = jwt.decode(access_token, hub_pk_pem, algorithms=["EdDSA"])
+        from meshbay_common.handshake import JWT_LEEWAY_SECONDS
+        decoded = jwt.decode(access_token, hub_pk_pem, algorithms=["EdDSA"],
+                             leeway=JWT_LEEWAY_SECONDS)
         # No pk_user claim to check any more: tokens carry no key. What binds this
         # token to this node is the Ed25519 challenge it was issued against.
         assert "jti" in decoded, "Hub token missing jti — hub is outdated"
diff --git a/packages/meshbay-node/src/meshbay_node/revocation.py b/packages/meshbay-node/src/meshbay_node/revocation.py
index abbff4d..d3ee18f 100644
--- a/packages/meshbay-node/src/meshbay_node/revocation.py
+++ b/packages/meshbay-node/src/meshbay_node/revocation.py
@@ -66,7 +66,9 @@ class RevocationSubscriber:
     def verify_and_apply(self, token: str) -> bool:
         """Verify a revocation token and apply it. Returns True if valid."""
         try:
+            from meshbay_common.handshake import JWT_LEEWAY_SECONDS
             payload = jwt.decode(token, self._hub_pk_pem, algorithms=["EdDSA"],
+                                 leeway=JWT_LEEWAY_SECONDS,
                                  options={"verify_exp": False})
             if payload.get("type") != "revocation":
                 return False