feat(mnp)!: seal the upload under the group key

Downloads have been encrypted under a GEK-derived key since the beginning:
`file_chunk` and `stream_data` both go through `chunk_ciphertext`. Uploads
never were. `file_upload` carried the filename and the raw bytes in plain
msgpack, and `file_upload_ack` carried the name the node stored them under —
so the same file was ciphertext leaving a node and plaintext arriving at one.
There was no threat model behind that asymmetry.

Both halves now travel sealed under a third groupbox purpose,
HKDF(GEK, info="meshbay:upload:v1"). The filename, the destination folder and
the bytes are all inside the seal; only `upload_id` and `chunk_index` stay in
clear, because the node routes and orders on them before it can decrypt. This
direction seals *towards* the node — it holds the GEK for its own group — and
it opens the payload before it picks a destination or touches the disk.

What that forced, and why none of it is optional:

  - `filename` was the correlation key on both sides. It cannot be: matching an
    ack to its request by name would hand back exactly what the seal hides.
    `upload_id` replaces it — client-drawn, opaque to the node, unique within a
    connection, never an authorization input. The property it guarded (one
    refusal fails one upload, not every upload in flight) is unchanged.
  - Refusals can no longer quote what they refused. `No directory named 'X'`
    becomes `No such directory in this group` plus the `code` that was already
    there; the client knows what it sent.
  - No plaintext fallback. A path that still accepts plaintext is not a sealed
    path, so an unsealed `file_upload` is refused with `upload_not_sealed`.

Hardened while here, because what comes out of a seal is authenticated but not
validated — a member can seal anything: `filename` and `data` have their types
checked before any upload state is created, and `chunk_index`/`total_chunks`,
which are outside the seal by necessity, can no longer raise where a refusal
was meant.

Tests. `test_upload_sealed.py` pins the node half: nothing identifying on the
wire, tamper/wrong-key/wrong-group all refused with nothing written, and
multi-chunk reassembly unchanged. `test_upload_seal_client.py` drives the
shipped `uploadFile` over the shipped `crypto.js` under node and feeds its real
frames to the real `_do_file_upload` — the file lands intact, and the ack the
node actually produced comes back with the name it chose for a collision, which
is the half a source-reading test cannot see. Both upload purposes join the
JS/Python groupbox parity vectors.

BREAKING CHANGE: MNP 2.0. `file_upload`/`file_upload_ack` change shape on the
wire every deployed client speaks, which is MAJOR by the same rule 1.0 was —
but the break is confined to uploads. `MNP_MIN_SUPPORTED` stays at "1.0", so a
1.x peer still connects, browses, downloads, streams and chats; only its
uploads are refused, with a message saying which side is old. The client checks
the node's version before sending a chunk, so neither side meets this as a
timeout. This is the version negotiation shipped in 1.0 earning its keep: 1.0
cost a flag day, 2.0 costs a refusal code.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AsoWC3GmhNdwVFomW3QjH3

5 files changed, 464 insertions, 126 deletions

diff --git a/packages/meshbay-node/src/meshbay_node/transport/webrtc_server.py b/packages/meshbay-node/src/meshbay_node/transport/webrtc_server.py
index f8b6c03..5318c25 100644
--- a/packages/meshbay-node/src/meshbay_node/transport/webrtc_server.py
+++ b/packages/meshbay-node/src/meshbay_node/transport/webrtc_server.py
@@ -103,7 +103,13 @@ from meshbay_common.join import (
     ROLE_OPERATOR,
     join_transcript,
 )
-from meshbay_common.protocol import MNP, chunk_ciphertext, file_chunk_wire
+from meshbay_common.protocol import (
+    MNP,
+    chunk_ciphertext,
+    file_chunk_wire,
+    file_upload_ack_wire,
+    file_upload_payload,
+)
 from meshbay_node.transport.wire import index_sync_message
 from meshbay_node.indexer import GroupIndex
 from meshbay_node.indexer.indexer import DirectoryIndexer
@@ -3998,27 +4004,88 @@ class WebRTCPeerSession:
         self._send(resp)
 
     def _do_file_upload(self, msg: dict) -> None:
+        """
+        One chunk of an upload, sealed under the group key (MNP 2.0).
+
+        Sealing this direction is not symmetry for its own sake. Downloads have
+        been under a GEK-derived key since the beginning; uploads carried the
+        filename and the raw bytes in plain msgpack, so the same file was
+        ciphertext leaving a node and plaintext arriving at one. The node holds
+        the GEK for its own group, so it opens the payload here — before it
+        decides a destination, before it touches the disk — and refuses a chunk
+        that does not open.
+
+        `upload_id` is the correlation key and stays in clear; `filename`, `dir`
+        and `root` moved inside the seal, which is why every refusal below names
+        the upload rather than the file. A `code` says which refusal it is, and
+        the client already knows what it sent.
+        """
         ctx = self._group_ctx()
-        filename = msg.get("filename", "")
-        chunk_index = msg.get("chunk_index", 0)
-        total_chunks = msg.get("total_chunks", 1)
-        data = msg.get("data")
+        upload_id = str(msg.get("upload_id") or "")[:64]
+
+        gek = ctx.get("gek")
+        if not gek:
+            self._send({"type": "error", "detail": "Group encryption not initialized",
+                        "code": "no_group_key", "upload_id": upload_id})
+            return
+
+        try:
+            payload = file_upload_payload(gek, self._group_id or "", msg)
+        except Exception:
+            # Deliberately one answer for "not sealed at all" and "sealed wrong":
+            # distinguishing them tells a peer which of the two it got right.
+            # An MNP 1.x client lands here, which is the whole of the upgrade
+            # story — everything else it does still works.
+            self._audit("upload_refused", "unsealed")
+            self._send({
+                "type": "error",
+                "detail": "This upload did not open under the group key — the "
+                          "client may be running an older version",
+                "code": "upload_not_sealed",
+                "upload_id": upload_id,
+            })
+            return
+
+        filename = payload.get("filename") or ""
+        data = payload.get("data")
+        # From the clear part of the message, so peer-controlled and unchecked
+        # by the AEAD. Everything below compares and adds to them.
+        try:
+            chunk_index = int(msg.get("chunk_index", 0))
+            total_chunks = int(msg.get("total_chunks", 1))
+        except (TypeError, ValueError):
+            self._send({"type": "error", "detail": "Invalid chunk index",
+                        "code": "bad_chunk_index", "upload_id": upload_id})
+            return
+
+        def _refuse(detail: str, code: str = "") -> None:
+            """A refusal names the upload, never the file: the name is sealed."""
+            out = {"type": "error", "detail": detail, "upload_id": upload_id}
+            if code:
+                out["code"] = code
+            self._send(out)
 
-        if not filename or data is None:
-            self._send({"type": "error", "detail": "Missing filename or data",
-                        "filename": filename})
+        # Types first, and before any state is created. What comes out of a
+        # sealed payload is authenticated, not validated: it is msgpack a
+        # member wrote, and `SAFE_UPLOAD_NAME.match(123)` raises where a
+        # refusal was meant.
+        if not isinstance(filename, str) or not filename:
+            _refuse("Missing filename or data", "upload_incomplete")
             return
+        # Bytes, always: base64 was the shape of the old plaintext `data` field
+        # and there is no sealed message that can carry a string here.
+        if not isinstance(data, (bytes, bytearray)):
+            _refuse("Invalid chunk encoding", "bad_chunk_encoding")
+            return
+        chunk_bytes = bytes(data)
 
         if not SAFE_UPLOAD_NAME.match(filename):
-            self._send({"type": "error", "detail": "Invalid filename",
-                        "filename": filename})
+            _refuse("Invalid filename", "invalid_filename")
             return
 
         roots: RootSet | None = ctx.get("roots")
         if not roots:
-            self._send({"type": "error",
-                        "detail": "No directories configured for this group",
-                        "filename": filename})
+            _refuse("No directories configured for this group", "no_roots")
             return
 
         # The client names the root it is uploading into — it is browsing one,
@@ -4029,47 +4096,32 @@ class WebRTCPeerSession:
         # An unknown name is refused rather than falling back to a writable
         # root, because "the file went somewhere else" is discovered weeks
         # later — the same reason the old single upload root was never guessed.
-        # A client that names nothing is an MNP 1.0 one, and there was exactly
-        # one destination in its world: the first writable root.
         # `dir` is the folder being browsed, as a virtual path
         # (`Media/Films/1999`); `root` is the older, coarser form and is what
-        # its first segment means on its own.
-        target_rel = str(msg.get("dir") or "").strip().strip("/")
+        # its first segment means on its own. Both are sealed now, so a refusal
+        # below can no longer quote them back.
+        target_rel = str(payload.get("dir") or "").strip().strip("/")
         target_root_name = (target_rel.split("/")[0] if target_rel
-                            else str(msg.get("root") or "").strip())
+                            else str(payload.get("root") or "").strip())
         upload_root = None
         if target_root_name:
             upload_root = roots.by_name(target_root_name)
             if upload_root is None:
-                self._send({"type": "error",
-                            "detail": f"No directory named "
-                                      f"{target_root_name!r} in this group",
-                            "code": "no_such_root",
-                            "filename": filename})
+                _refuse("No such directory in this group", "no_such_root")
                 return
         else:
             writable = roots.writable_roots
             upload_root = writable[0] if writable else None
 
         if upload_root is None:
-            self._send({"type": "error",
-                        "detail": "No writable directory in this group",
-                        "code": "no_writable_root",
-                        "filename": filename})
+            _refuse("No writable directory in this group", "no_writable_root")
             return
         if not upload_root.writable:
-            self._send({"type": "error",
-                        "detail": f"Directory '{upload_root.name}' is read-only",
-                        "code": "root_read_only",
-                        "filename": filename})
+            _refuse("That directory is read-only", "root_read_only")
             self._audit("upload_refused", filename[:64])
             return
         if not upload_root.available:
-            self._send({"type": "error",
-                        "detail": f"Directory '{upload_root.name}' is "
-                                  f"currently unavailable",
-                        "code": "root_unavailable",
-                        "filename": filename})
+            _refuse("That directory is currently unavailable", "root_unavailable")
             return
 
         # The folder the sender is looking at, and no subdirectory of the node's
@@ -4093,23 +4145,17 @@ class WebRTCPeerSession:
         if target_rel:
             target_dir = roots.resolve(target_rel)
             if target_dir is None or not target_dir.is_dir():
-                self._send({"type": "error",
-                            "detail": "Not a directory in this group",
-                            "code": "no_such_directory",
-                            "filename": filename})
+                _refuse("Not a directory in this group", "no_such_directory")
                 return
             rel_dir = target_rel
         else:
-            # An MNP 1.0 client names nothing; the root itself is where its one
-            # destination now is.
+            # A client that names nothing: the first writable root is where its
+            # one destination is.
             target_dir = upload_root.path
             rel_dir = upload_root.name
             if not target_dir.is_dir():
-                self._send({"type": "error",
-                            "detail": f"Directory '{upload_root.name}' is "
-                                      f"currently unavailable",
-                            "code": "root_unavailable",
-                            "filename": filename})
+                _refuse("That directory is currently unavailable",
+                        "root_unavailable")
                 return
 
         upload_key = f"{rel_dir}/{filename}"
@@ -4125,33 +4171,24 @@ class WebRTCPeerSession:
             # Backstop: _free_name already guarantees this, and it stays because
             # it asserts the invariant where the write happens.
             if final_path.exists():
-                self._send({"type": "error", "detail": "File already exists",
-                        "filename": filename})
+                _refuse("File already exists", "already_exists")
                 return
             state = {"next_index": 0, "bytes": 0, "stored_name": stored_name}
             self._uploads[upload_key] = state
         elif state is None:
-            self._send({"type": "error", "detail": "Upload not started",
-                        "filename": filename})
+            _refuse("Upload not started", "not_started")
             return
 
         # Reject out-of-order or replayed chunks — otherwise chunk_index>0 appends
         # blindly to whatever .part file is already on disk.
         if chunk_index != state["next_index"]:
-            self._send({"type": "error", "detail": "Unexpected chunk index",
-                        "filename": filename})
+            _refuse("Unexpected chunk index", "bad_chunk_index")
             return
 
-        if isinstance(data, str):
-            chunk_bytes = base64.b64decode(data)
-        else:
-            chunk_bytes = bytes(data)
-
         if state["bytes"] + len(chunk_bytes) > MAX_UPLOAD_BYTES:
             self._uploads.pop(upload_key, None)
             tmp_path.unlink(missing_ok=True)
-            self._send({"type": "error", "detail": "Upload exceeds size limit",
-                        "filename": filename})
+            _refuse("Upload exceeds size limit", "too_large")
             return
 
         with open(tmp_path, "wb" if chunk_index == 0 else "ab") as f:
@@ -4159,16 +4196,16 @@ class WebRTCPeerSession:
         state["next_index"] = chunk_index + 1
         state["bytes"] += len(chunk_bytes)
 
-        self._send({
-            "type": MNP.FILE_UPLOAD_ACK,
-            "v": MNP_VERSION,
-            "chunk_index": chunk_index,
-            "filename": filename,
+        self._send(file_upload_ack_wire(
+            gek, self._group_id or "",
+            upload_id=upload_id,
+            chunk_index=chunk_index,
+            filename=filename,
             # What it is actually called on disk, which a chat attachment has to
             # reference and the uploader deserves to be told.
-            "stored_as": stored_name,
-            "dir": rel_dir,
-        })
+            stored_as=stored_name,
+            dir=rel_dir,
+        ))
 
         if chunk_index + 1 >= total_chunks:
             self._uploads.pop(upload_key, None)
diff --git a/packages/meshbay-node/tests/conftest.py b/packages/meshbay-node/tests/conftest.py
index 3dc9cd9..ba86c13 100644
--- a/packages/meshbay-node/tests/conftest.py
+++ b/packages/meshbay-node/tests/conftest.py
@@ -39,3 +39,34 @@ def one_root(path: Path, *, name: str = "", kind: str = "generic",
     """
     return RootSet.build([{"path": str(path), "name": name, "kind": kind,
                            "writable": writable}])
+
+
+def sealed_upload(session, *, filename: str, data: bytes,
+                  chunk_index: int = 0, total_chunks: int = 1,
+                  dir: str = "", root: str = "",
+                  upload_id: str = "up-test") -> dict:
+    """
+    A `file_upload` message as the shipping client builds one (MNP 2.0).
+
+    Built through `file_upload_wire`, not by hand: a test that assembles the
+    wire shape itself is a second encoder, and a second encoder is how
+    `file_chunk` and `index_sync` forked between the transports (finding C6)
+    with nobody noticing. The key and the AAD are taken off the session, so
+    these agree with the handler by construction rather than by copying.
+    """
+    from meshbay_common.protocol import file_upload_wire
+
+    ctx = session._group_ctx()
+    return file_upload_wire(
+        ctx["gek"], session._group_id or "",
+        upload_id=upload_id, chunk_index=chunk_index, total_chunks=total_chunks,
+        filename=filename, data=data, dir=dir, root=root,
+    )
+
+
+def opened_ack(session, msg: dict) -> dict:
+    """The payload of a `file_upload_ack` the node sent, opened as a client would."""
+    from meshbay_common.protocol import file_upload_ack_payload
+
+    ctx = session._group_ctx()
+    return file_upload_ack_payload(ctx["gek"], session._group_id or "", msg)
diff --git a/packages/meshbay-node/tests/test_root_writable_policy.py b/packages/meshbay-node/tests/test_root_writable_policy.py
index 7eb75fd..8345880 100644
--- a/packages/meshbay-node/tests/test_root_writable_policy.py
+++ b/packages/meshbay-node/tests/test_root_writable_policy.py
@@ -21,13 +21,14 @@ anything. A deprecated instruction that still works is not deprecated, and this
 one would reopen uploads group-wide.
 """
 
-import base64
 from pathlib import Path
 
 import pytest
 from cryptography.hazmat.primitives.asymmetric.ed25519 import Ed25519PrivateKey
 
+from conftest import sealed_upload
 from meshbay_common.adminop import OP_ROOT_UPDATE, OP_ROOT_EJECT, OP_ROOT_PLUG
+from meshbay_common.crypto import generate_gek
 from meshbay_common.protocol import MNP
 from meshbay_node.indexer.group_index import GroupIndex
 from meshbay_node.roots import RootSet
@@ -47,6 +48,8 @@ def _session(tmp_path: Path, user_id: str, *,
         "index": index,
         "sk_node": index.sk_node,
         "node_user_id": operator,
+        # Uploads are sealed under the group key since MNP 2.0.
+        "gek": generate_gek(),
     }
     session = WebRTCPeerSession.__new__(WebRTCPeerSession)
     session._ctx = ctx
@@ -61,11 +64,8 @@ def _session(tmp_path: Path, user_id: str, *,
 
 
 def _upload(session, filename="clip.mp4", body=b"bytes"):
-    session._do_file_upload({
-        "filename": filename, "dir": "shared",
-        "chunk_index": 0, "total_chunks": 1,
-        "data": base64.b64encode(body).decode(),
-    })
+    session._do_file_upload(sealed_upload(
+        session, filename=filename, data=body, dir="shared"))
 
 
 def _uploads_dir(session) -> Path:
diff --git a/packages/meshbay-node/tests/test_security_regressions.py b/packages/meshbay-node/tests/test_security_regressions.py
index fa821d9..988aa46 100644
--- a/packages/meshbay-node/tests/test_security_regressions.py
+++ b/packages/meshbay-node/tests/test_security_regressions.py
@@ -16,10 +16,11 @@ from pathlib import Path
 import pytest
 from cryptography.hazmat.primitives.asymmetric.ed25519 import Ed25519PrivateKey
 
+from meshbay_common.crypto import generate_gek
 from meshbay_common.protocol import IndexEntry
 from meshbay_node.indexer.group_index import GroupIndex
 from meshbay_node.roots import RootSet
-from conftest import one_root
+from conftest import one_root, opened_ack, sealed_upload
 from meshbay_node.transport.webrtc_server import WebRTCPeerSession
 
 
@@ -156,7 +157,10 @@ def _session(tmp_path: Path, user_id: str) -> WebRTCPeerSession:
     shared_root = tmp_path / "shared"
     shared_root.mkdir(exist_ok=True)
     index = GroupIndex(group_id="g" * 32, sk_node=Ed25519PrivateKey.generate())
-    ctx = {"roots": one_root(shared_root), "index": index, "sk_node": index.sk_node}
+    # A group key, because uploads are sealed under it since MNP 2.0 — the
+    # handler opens the payload before it has a filename to refuse.
+    ctx = {"roots": one_root(shared_root), "index": index,
+           "sk_node": index.sk_node, "gek": generate_gek()}
 
     session = WebRTCPeerSession.__new__(WebRTCPeerSession)
     session._ctx = ctx
@@ -188,12 +192,8 @@ def test_upload_cannot_overwrite_another_members_file(tmp_path):
     original.write_bytes(b"operator's original content")
 
     attacker = _session(tmp_path, "attacker-user")
-    attacker._do_file_upload({
-        "filename": "important.mp4",
-        "chunk_index": 0,
-        "total_chunks": 1,
-        "data": base64.b64encode(b"attacker content").decode(),
-    })
+    attacker._do_file_upload(sealed_upload(
+        attacker, filename="important.mp4", data=b"attacker content"))
 
     assert original.read_bytes() == b"operator's original content", (
         "an upload replaced an existing file (C5a)")
@@ -203,12 +203,15 @@ def test_upload_cannot_overwrite_another_members_file(tmp_path):
 def test_upload_second_attempt_cannot_replace_own_completed_file(tmp_path):
     """C5a: even the original uploader does not get to overwrite."""
     session = _session(tmp_path, "user-1")
-    payload = {"filename": "movie.mp4", "chunk_index": 0, "total_chunks": 1,
-               "data": base64.b64encode(b"first").decode()}
-    session._do_file_upload(dict(payload))
-    session.sent.clear()
+    def _send_it():
+        # Sealed afresh each time: a nonce is drawn per message, so re-sending
+        # the same dict would be a replay rather than a second upload.
+        session._do_file_upload(sealed_upload(
+            session, filename="movie.mp4", data=b"first"))
 
-    session._do_file_upload(dict(payload))
+    _send_it()
+    session.sent.clear()
+    _send_it()
     uploads = _uploads_dir(session)
     assert (uploads / "movie.mp4").read_bytes() == b"first", (
         "the first upload was replaced")
@@ -252,11 +255,8 @@ def test_the_client_names_a_folder_and_never_a_filesystem_path(tmp_path):
     for bad in ("../../etc", "/etc", "shared/../..", "shared/../../etc",
                 "nope", "shared/missing"):
         session.sent.clear()
-        session._do_file_upload({
-            "filename": "note.txt", "dir": bad,
-            "chunk_index": 0, "total_chunks": 1,
-            "data": base64.b64encode(b"x").decode(),
-        })
+        session._do_file_upload(sealed_upload(
+            session, filename="note.txt", data=b"x", dir=bad))
         refusal = [m for m in session.sent if m.get("type") == "error"]
         assert refusal, f"{bad!r} was accepted"
         assert refusal[0].get("code") in ("no_such_root", "no_such_directory"), bad
@@ -274,11 +274,8 @@ def test_an_upload_lands_in_the_folder_it_names(tmp_path):
     root = session._ctx["roots"].roots[0]
     (root.path / "Albums").mkdir()
 
-    session._do_file_upload({
-        "filename": "note.txt", "dir": f"{root.name}/Albums",
-        "chunk_index": 0, "total_chunks": 1,
-        "data": base64.b64encode(b"x").decode(),
-    })
+    session._do_file_upload(sealed_upload(
+        session, filename="note.txt", data=b"x", dir=f"{root.name}/Albums"))
 
     assert (root.path / "Albums" / "note.txt").read_bytes() == b"x"
     assert not (root.path / "Albums" / "uploads").exists(), (
@@ -304,11 +301,8 @@ def test_an_upload_goes_to_the_root_it_names(tmp_path):
         {"path": str(incoming), "writable": True},
     ])
 
-    session._do_file_upload({
-        "filename": "note.txt", "dir": "Incoming",
-        "chunk_index": 0, "total_chunks": 1,
-        "data": base64.b64encode(b"x").decode(),
-    })
+    session._do_file_upload(sealed_upload(
+        session, filename="note.txt", data=b"x", dir="Incoming"))
 
     assert (incoming / "note.txt").read_bytes() == b"x"
     assert not (media / "note.txt").exists(), "it went to the first root instead"
@@ -327,11 +321,8 @@ def test_a_read_only_root_refuses_an_upload(tmp_path):
     session._ctx["roots"] = RootSet.build([{"path": str(published)}])
     session._is_node_admin = lambda: True
 
-    session._do_file_upload({
-        "filename": "note.txt", "dir": "Published",
-        "chunk_index": 0, "total_chunks": 1,
-        "data": base64.b64encode(b"x").decode(),
-    })
+    session._do_file_upload(sealed_upload(
+        session, filename="note.txt", data=b"x", dir="Published"))
 
     refusal = [m for m in session.sent if m.get("type") == "error"]
     assert refusal and refusal[0].get("code") == "root_read_only"
@@ -349,11 +340,8 @@ def test_a_fully_read_only_group_refuses_an_unaddressed_upload(tmp_path):
     session = _session(tmp_path, "user-1")
     session._ctx["roots"] = RootSet.build([{"path": str(published)}])
 
-    session._do_file_upload({
-        "filename": "note.txt",
-        "chunk_index": 0, "total_chunks": 1,
-        "data": base64.b64encode(b"x").decode(),
-    })
+    session._do_file_upload(sealed_upload(
+        session, filename="note.txt", data=b"x"))
 
     refusal = [m for m in session.sent if m.get("type") == "error"]
     assert refusal and refusal[0].get("code") == "no_writable_root"
@@ -375,11 +363,8 @@ def test_an_ejected_root_refuses_an_upload(tmp_path):
     roots.roots[0].available = False
     session._ctx["roots"] = roots
 
-    session._do_file_upload({
-        "filename": "note.txt", "dir": "USB",
-        "chunk_index": 0, "total_chunks": 1,
-        "data": base64.b64encode(b"x").decode(),
-    })
+    session._do_file_upload(sealed_upload(
+        session, filename="note.txt", data=b"x", dir="USB"))
 
     refusal = [m for m in session.sent if m.get("type") == "error"]
     assert refusal and refusal[0].get("code") == "root_unavailable"
@@ -392,22 +377,22 @@ def test_two_members_can_send_the_same_filename(tmp_path):
     IMG_1234.jpg. The second gets a free name; neither replaces the other.
     """
     first = _session(tmp_path, "user-1")
-    first._do_file_upload({
-        "filename": "IMG_1234.jpg", "chunk_index": 0, "total_chunks": 1,
-        "data": base64.b64encode(b"first").decode(),
-    })
+    first._do_file_upload(sealed_upload(
+        first, filename="IMG_1234.jpg", data=b"first"))
     second = _session(tmp_path, "user-2")
-    second._do_file_upload({
-        "filename": "IMG_1234.jpg", "chunk_index": 0, "total_chunks": 1,
-        "data": base64.b64encode(b"second").decode(),
-    })
+    # Same group, so the same key: `_session` builds one per call, and two
+    # members of one group do not have two.
+    second._ctx["gek"] = first._ctx["gek"]
+    second._do_file_upload(sealed_upload(
+        second, filename="IMG_1234.jpg", data=b"second"))
 
     uploads = _uploads_dir(first)
     assert (uploads / "IMG_1234.jpg").read_bytes() == b"first"
     assert (uploads / "IMG_1234 (2).jpg").read_bytes() == b"second"
 
     ack = [m for m in second.sent if m.get("type") == "file_upload_ack"][-1]
-    assert ack["stored_as"] == "IMG_1234 (2).jpg", (
+    assert "stored_as" not in ack, "the name the node chose must be sealed"
+    assert opened_ack(second, ack)["stored_as"] == "IMG_1234 (2).jpg", (
         "the sender must be told the name that was used, or a chat attachment "
         "points at someone else's file")
 
diff --git a/packages/meshbay-node/tests/test_upload_sealed.py b/packages/meshbay-node/tests/test_upload_sealed.py
new file mode 100644
index 0000000..7c1be96
--- /dev/null
+++ b/packages/meshbay-node/tests/test_upload_sealed.py
@@ -0,0 +1,285 @@
+"""
+The write path, sealed under the group key (MNP 2.0).
+
+Downloads have been encrypted under a GEK-derived key since the beginning:
+`file_chunk` and `stream_data` both go through `chunk_ciphertext`. Uploads did
+not. `file_upload` carried the filename and the raw bytes in plain msgpack and
+`file_upload_ack` carried the name the node stored them under, so the same file
+was ciphertext leaving a node and plaintext arriving at one — an asymmetry with
+no threat model behind it.
+
+What sealing buys is what `groupbox.py` says and no more: nothing against a
+network observer (DTLS covers that), nothing against the hub (never on this
+channel), nothing against a member (they hold the GEK). It buys defence in
+depth against our own next handshake bug, of a class already shipped twice —
+C1, the unauthenticated node HTTP API, and C6, the transport that took a bare
+JWT. Both were "a peer that had not finished the handshake was served data".
+Sealed, the equivalent bug on this path leaks ciphertext instead of the
+operator's filenames.
+"""
+
+from pathlib import Path
+
+import pytest
+from cryptography.hazmat.primitives.asymmetric.ed25519 import Ed25519PrivateKey
+from meshbay_common.crypto import generate_gek
+from meshbay_common.protocol import MNP, file_upload_wire
+from meshbay_node.indexer.group_index import GroupIndex
+from meshbay_node.transport.webrtc_server import WebRTCPeerSession
+
+from conftest import one_root, opened_ack, sealed_upload
+
+GROUP = "g" * 32
+
+
+def _session(tmp_path: Path, *, gek: bytes | None = None) -> WebRTCPeerSession:
+    shared_root = tmp_path / "shared"
+    shared_root.mkdir(exist_ok=True)
+    index = GroupIndex(group_id=GROUP, sk_node=Ed25519PrivateKey.generate())
+    session = WebRTCPeerSession.__new__(WebRTCPeerSession)
+    session._ctx = {"roots": one_root(shared_root), "index": index,
+                    "sk_node": index.sk_node, "gek": gek or generate_gek()}
+    session._group_id = GROUP
+    session._user_id = "user-1"
+    session._pk_user = ""
+    session._uploads = {}
+    session.sent = []
+    session._send = session.sent.append
+    session._audit = lambda *a, **k: None
+    return session
+
+
+def _root(session):
+    return session._ctx["roots"].roots[0]
+
+
+def _errors(session):
+    return [m for m in session.sent if m.get("type") == "error"]
+
+
+def _wrote_anything(tmp_path) -> bool:
+    return any(p.is_file() for p in tmp_path.rglob("*"))
+
+
+# ── The message itself ───────────────────────────────────────────────────────
+
+def test_the_wire_message_carries_no_filename_and_no_plaintext(tmp_path):
+    """
+    The point of the exercise. `upload_id` and `chunk_index` are outside the
+    seal because the node routes and orders on them before it can decrypt;
+    everything that names or is the operator's content is inside it.
+    """
+    session = _session(tmp_path)
+    msg = sealed_upload(session, filename="holiday.jpg", data=b"JPEGDATA",
+                        dir=f"{_root(session).name}")
+
+    assert set(msg) == {"type", "v", "upload_id", "chunk_index",
+                        "total_chunks", "nonce", "ct"}
+    blob = repr(msg).encode() + msg["ct"]
+    assert b"holiday.jpg" not in blob, "the filename is on the wire in clear"
+    assert b"JPEGDATA" not in blob, "the file content is on the wire in clear"
+
+
+def test_the_ack_carries_no_stored_name(tmp_path):
+    """
+    `stored_as` is the name the node settled on — it finds a free one rather
+    than replacing anything — and naming it in clear would hand back exactly
+    what the request took the trouble to hide.
+    """
+    session = _session(tmp_path)
+    session._do_file_upload(sealed_upload(
+        session, filename="holiday.jpg", data=b"x", dir=_root(session).name))
+
+    ack = [m for m in session.sent if m.get("type") == MNP.FILE_UPLOAD_ACK][-1]
+    assert set(ack) == {"type", "v", "upload_id", "chunk_index", "nonce", "ct"}
+    assert b"holiday.jpg" not in repr(ack).encode() + ack["ct"]
+    assert opened_ack(session, ack) == {
+        "filename": "holiday.jpg", "stored_as": "holiday.jpg",
+        "dir": _root(session).name}
+
+
+def test_the_ack_names_the_upload_so_one_refusal_fails_one_upload(tmp_path):
+    """
+    `filename` used to be the correlation key on both sides. It cannot be one
+    any more, and `upload_id` replaces it — a client-chosen label, opaque to
+    the node, never an authorization input. Without it a client running several
+    uploads could only match replies by arrival order, which is how a refusal
+    for one file used to fail every upload in flight.
+    """
+    session = _session(tmp_path)
+    session._do_file_upload(sealed_upload(
+        session, filename="a.txt", data=b"x", dir=_root(session).name,
+        upload_id="upload-A"))
+    session._do_file_upload(sealed_upload(
+        session, filename="../evil", data=b"x", dir=_root(session).name,
+        upload_id="upload-B"))
+
+    ack = [m for m in session.sent if m.get("type") == MNP.FILE_UPLOAD_ACK][-1]
+    assert ack["upload_id"] == "upload-A"
+    assert _errors(session)[0]["upload_id"] == "upload-B"
+
+
+# ── What is refused ──────────────────────────────────────────────────────────
+
+def test_a_plaintext_upload_is_refused(tmp_path):
+    """
+    The MNP 1.x shape, which is what an un-updated client sends. Refused with a
+    code and a message saying which side is old — never accepted "just this
+    once", because a path that still takes plaintext is not a sealed path.
+    """
+    session = _session(tmp_path)
+    session._do_file_upload({
+        "filename": "note.txt", "dir": _root(session).name,
+        "chunk_index": 0, "total_chunks": 1, "data": b"x",
+    })
+
+    assert _errors(session)[0]["code"] == "upload_not_sealed"
+    assert not _wrote_anything(tmp_path)
+
+
+def test_a_tampered_chunk_is_refused(tmp_path):
+    """
+    AES-GCM's tag, asserted where it matters: a flipped bit in the ciphertext
+    must stop the upload, not produce a corrupt file with a plausible name.
+    """
+    session = _session(tmp_path)
+    msg = sealed_upload(session, filename="note.txt", data=b"x" * 64,
+                        dir=_root(session).name)
+    msg["ct"] = bytes([msg["ct"][0] ^ 0x01]) + msg["ct"][1:]
+    session._do_file_upload(msg)
+
+    assert _errors(session)[0]["code"] == "upload_not_sealed"
+    assert not _wrote_anything(tmp_path)
+
+
+def test_an_upload_sealed_for_another_group_is_refused(tmp_path):
+    """
+    The group is the AAD, so a node hosting two groups cannot have a chunk
+    moved between them — and a member of one cannot write into the other by
+    reaching a session that is on it (finding H1's shape, on the write path).
+    """
+    session = _session(tmp_path)
+    msg = file_upload_wire(
+        session._ctx["gek"], "some-other-group",
+        upload_id="u1", chunk_index=0, total_chunks=1,
+        filename="note.txt", data=b"x", dir=_root(session).name)
+    session._do_file_upload(msg)
+
+    assert _errors(session)[0]["code"] == "upload_not_sealed"
+    assert not _wrote_anything(tmp_path)
+
+
+def test_an_upload_under_another_key_is_refused(tmp_path):
+    """A peer past the handshake with the wrong GEK still writes nothing."""
+    session = _session(tmp_path)
+    msg = file_upload_wire(
+        generate_gek(), GROUP,
+        upload_id="u1", chunk_index=0, total_chunks=1,
+        filename="note.txt", data=b"x", dir=_root(session).name)
+    session._do_file_upload(msg)
+
+    assert _errors(session)[0]["code"] == "upload_not_sealed"
+    assert not _wrote_anything(tmp_path)
+
+
+def test_an_ack_replayed_as_a_request_does_not_open(tmp_path):
+    """
+    The message type is in the AAD, so the two halves of an upload cannot be
+    confused for each other. Cheap, and it closes a class that is tedious to
+    reason about after the fact.
+    """
+    from cryptography.exceptions import InvalidTag
+    from meshbay_common.protocol import file_upload_ack_wire, file_upload_payload
+
+    session = _session(tmp_path)
+    ack = file_upload_ack_wire(
+        session._ctx["gek"], GROUP, upload_id="u1", chunk_index=0,
+        filename="note.txt", stored_as="note.txt", dir="shared")
+    with pytest.raises(InvalidTag):
+        file_upload_payload(session._ctx["gek"], GROUP, ack)
+
+
+def test_a_group_with_no_key_refuses_rather_than_falling_back(tmp_path):
+    """
+    A node whose group has no GEK yet cannot open anything. It must say so, not
+    read the message as though it were the old plaintext shape.
+    """
+    session = _session(tmp_path)
+    msg = sealed_upload(session, filename="note.txt", data=b"x",
+                        dir=_root(session).name)
+    session._ctx["gek"] = b""
+    session._do_file_upload(msg)
+
+    assert _errors(session)[0]["code"] == "no_group_key"
+    assert not _wrote_anything(tmp_path)
+
+
+# ── What must still work ─────────────────────────────────────────────────────
+
+def test_a_multi_chunk_upload_reassembles(tmp_path):
+    """
+    Every chunk is sealed under its own nonce, and the node appends in order.
+    Nothing about the seal may change what lands on disk.
+    """
+    session = _session(tmp_path)
+    body = bytes(range(256)) * 40
+    parts = [body[i:i + 1024] for i in range(0, len(body), 1024)]
+    for i, part in enumerate(parts):
+        session._do_file_upload(sealed_upload(
+            session, filename="blob.bin", data=part,
+            chunk_index=i, total_chunks=len(parts), dir=_root(session).name))
+
+    assert (_root(session).path / "blob.bin").read_bytes() == body
+    assert not list(_root(session).path.glob("*.part")), "a temp file was left"
+    acks = [m for m in session.sent if m.get("type") == MNP.FILE_UPLOAD_ACK]
+    assert [m["chunk_index"] for m in acks] == list(range(len(parts)))
+
+
+def test_two_identical_chunks_do_not_reuse_a_nonce(tmp_path):
+    """
+    A file of repeated bytes is ordinary, so the nonce must come from the RNG
+    and never from the payload. Cheap to assert and expensive to discover.
+    """
+    session = _session(tmp_path)
+    a = sealed_upload(session, filename="f", data=b"same", chunk_index=0)
+    b = sealed_upload(session, filename="f", data=b"same", chunk_index=0)
+    assert a["nonce"] != b["nonce"]
+    assert a["ct"] != b["ct"]
+
+
+def test_a_sealed_payload_is_authenticated_not_validated(tmp_path):
+    """
+    Opening a payload proves a member wrote it, not that they wrote something
+    sensible. A member can seal anything, so the fields still need their types
+    checked — `SAFE_UPLOAD_NAME.match(123)` raises where a refusal was meant,
+    and the dispatcher's catch-all would turn that into "Request failed".
+    """
+    from meshbay_common.groupbox import PURPOSE_UPLOAD, seal
+
+    session = _session(tmp_path)
+    for payload in ({"filename": 123, "data": b"x"},
+                    {"filename": "note.txt", "data": "not bytes"},
+                    {"filename": "note.txt"}):
+        session.sent.clear()
+        session._do_file_upload({
+            "type": MNP.FILE_UPLOAD, "v": "2.0", "upload_id": "u1",
+            "chunk_index": 0, "total_chunks": 1,
+            **seal(session._ctx["gek"], PURPOSE_UPLOAD, MNP.FILE_UPLOAD,
+                   GROUP, payload),
+        })
+        errs = _errors(session)
+        assert errs, f"{payload!r} was accepted"
+        assert errs[0]["code"] in ("upload_incomplete", "bad_chunk_encoding")
+    assert not _wrote_anything(tmp_path)
+
+
+def test_a_peer_controlled_chunk_index_cannot_crash_the_handler(tmp_path):
+    """`chunk_index` is outside the seal by necessity, so it is unchecked input."""
+    session = _session(tmp_path)
+    msg = sealed_upload(session, filename="note.txt", data=b"x",
+                        dir=_root(session).name)
+    msg["chunk_index"] = "zero"
+    session._do_file_upload(msg)
+
+    assert _errors(session)[0]["code"] == "bad_chunk_index"
+    assert not _wrote_anything(tmp_path)