merge: Phase 11.5 security remediation, invite redesign, per-node identity

Brings in the security remediation branch. Three bodies of work, and what they
changed about what this project may claim.

Phase 11.5 closed the gap between the documents and the code: the unauthenticated
node HTTP API and the TCP transport deleted, one handshake shared by the
remaining two transports, mutual authentication, structured admin transcripts,
upload confinement, group isolation, revocation that reaches nodes. Six critical
and seven high findings closed, bounded, or deferred by decision.

The invite redesign closed H3 and M3 — the last open High. The hub was the key
directory: an inviter fetched the invitee's key from it and wrapped the group key
for whatever came back, so a hub answering with its own key was handed the group
key by an honest member following the protocol exactly. That lookup is gone. The
node holds the group key and wraps it itself, for a key its recipient proves
possession of, bound to an account by a one-time code the hub never sees. M3 fell
out of the same work: node authority comes from a local roster, never from the
hub.

Per-node identity cut what remains of C4 down to one operator. A single keypair
used to be copied to every node its owner joined; each node now gets its own, so
cracking the bundle on one machine yields a key that is a stranger everywhere
else — and on that machine, one that unlocks nothing its holder did not already
serve. The bundle KDF moved to Argon2id 128 MB, and the hub stopped storing or
publishing user keys at all.

What this project may now say: the hub cannot read your content unless it ships
you malicious client code. T3 remains, accepted (D1), and is what the native
client removes. C4 is reduced, not closed, until 13.3. Chat is still plaintext at
rest until Phase 15. Draft-v5 §2 states each claim against the adversary it holds
against, which is the convention this branch exists to keep.

Four defects were found by deploying it and using a browser, none by the test
suite: a node going deaf on its hub socket, a token that predated group
membership, a client reading values before they were assigned, and identity keys
a browser held but never re-read. The lessons are recorded in CLAUDE.md.

Tests: 343 across the three packages, plus QE/deploy/e2e.py — register, pair,
invite, join, download, stream, second browser, revoke — run against the live
deployment on a wiped hub and node.

1 files changed, 75 insertions, 0 deletions

diff --git a/packages/meshbay-common/src/meshbay_common/adminop.py b/packages/meshbay-common/src/meshbay_common/adminop.py
new file mode 100644
index 0000000..2d90102
--- /dev/null
+++ b/packages/meshbay-common/src/meshbay_common/adminop.py
@@ -0,0 +1,75 @@
+"""
+Admin operation challenge transcripts (MNP).
+
+Destructive and privileged node operations are authorized by an Ed25519 signature
+from the node operator, not by a JWT — the hub controls JWT issuance, so a JWT can
+never establish node-level authority (see draft-v4 §4.2.x).
+
+Finding H5: the node used to challenge the client with 32 raw random bytes and the
+client signed them blind. That is an unbound signing oracle — the signed message
+named no operation, no subject, no node and no time, so a signature obtained for one
+purpose was structurally valid for any other, and a malicious node could ask a user
+to sign bytes meaningful in a different protocol.
+
+The transcript below fixes that:
+
+  - a fixed domain-separation prefix, so these signatures can never collide with
+    node_auth, revocation tokens, chunk signatures or anything added later;
+  - the operation and its subject, so the client can display and verify what it is
+    authorizing before signing;
+  - the node's public key, so a signature for node A is not valid on node B;
+  - the group, so authority does not leak across groups on a multi-group node;
+  - a node-chosen nonce, so signatures cannot be replayed;
+  - a timestamp, so stale challenges can be rejected.
+
+Every field is length-prefixed. Plain concatenation would let a crafted subject
+impersonate a following field (finding L4 applies the same rule to the GEK proof).
+
+Both sides MUST build the transcript with this function — the client from the
+fields it received, the node from the state it stored. They are compared by
+producing the same bytes, never by trusting a value off the wire.
+"""
+
+ADMIN_TRANSCRIPT_PREFIX = b"meshbay:admin:v1"
+
+# Operations that require node-operator authority.
+OP_FILE_DELETE = "file_delete"
+OP_INVITE_CREATE = "invite_create"
+# OP_GEK_BUNDLE_STORE is gone. Members no longer hand the node key material at
+# all: the node holds the GEK and wraps it itself, for a key the recipient proved
+# they hold (see `join.py` and docs/invite-pairing-v1.md). The operation existed
+# only to make member-supplied bundles safe, and deleting the message is a
+# stronger guarantee than authorizing it.
+
+# A challenge older than this is refused, so a signature captured from a stale
+# exchange cannot be replayed later.
+ADMIN_CHALLENGE_TTL = 120  # seconds
+
+
+def admin_transcript(
+    op: str,
+    node_pk_b64: str,
+    group_id: str,
+    subject: str,
+    nonce: bytes,
+    ts: int,
+) -> bytes:
+    """
+    Build the exact byte string signed for an admin operation.
+
+    `subject` identifies what is being acted on: a file_id for OP_FILE_DELETE, the
+    invitee's user_id for OP_INVITE_CREATE.
+    """
+    fields = [
+        op.encode(),
+        node_pk_b64.encode(),
+        group_id.encode(),
+        subject.encode(),
+        nonce,
+        str(ts).encode(),
+    ]
+    out = bytearray(ADMIN_TRANSCRIPT_PREFIX)
+    for field in fields:
+        out += len(field).to_bytes(4, "big")
+        out += field
+    return bytes(out)