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diff --git a/packages/meshbay-common/tests/test_groupbox.py b/packages/meshbay-common/tests/test_groupbox.py
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+"""
+The group-key envelope: round trip, and every refusal it owes.
+
+`groupbox.seal`/`unseal` is what puts `index_sync`, `index_delta` and the
+`handshake_ack` configuration under a key the hub does not hold. The structural
+tests here matter less than `test_index_no_cleartext.py`, which asserts the
+property on a real frame; these pin the primitive.
+"""
+
+import msgpack
+import pytest
+from cryptography.exceptions import InvalidTag
+from meshbay_common.crypto import generate_gek
+from meshbay_common.groupbox import (
+    PURPOSE_ACK,
+    PURPOSE_INDEX,
+    group_key,
+    seal,
+    unseal,
+)
+
+PAYLOAD = {"version": 7, "entries": [{"name": "a.mkv", "size": 12}], "dirs": ["root"]}
+
+
+@pytest.fixture
+def gek():
+    return generate_gek()
+
+
+def test_round_trip(gek):
+    for purpose in (PURPOSE_INDEX, PURPOSE_ACK):
+        sealed = seal(gek, purpose, "index_sync", "g1", PAYLOAD)
+        assert set(sealed) == {"nonce", "ct"}
+        assert unseal(gek, purpose, "index_sync", "g1", sealed) == PAYLOAD
+
+
+def test_a_wrong_key_does_not_open(gek):
+    sealed = seal(gek, PURPOSE_INDEX, "index_sync", "g1", PAYLOAD)
+    with pytest.raises(InvalidTag):
+        unseal(generate_gek(), PURPOSE_INDEX, "index_sync", "g1", sealed)
+
+
+def test_purposes_are_separate_key_spaces(gek):
+    """
+    The reason there are two info strings rather than one key reused.
+
+    An ack sealed under the index subkey would otherwise be openable by anything
+    holding the index subkey, which is the confusion `GroupIndex.serialize()`'s
+    "the index as chunk 0 of a virtual index file" creates for chunk keys.
+    """
+    assert group_key(gek, PURPOSE_INDEX) != group_key(gek, PURPOSE_ACK)
+    sealed = seal(gek, PURPOSE_INDEX, "index_sync", "g1", PAYLOAD)
+    with pytest.raises(InvalidTag):
+        unseal(gek, PURPOSE_ACK, "index_sync", "g1", sealed)
+
+
+def test_a_body_cannot_be_replayed_as_another_message_type(gek):
+    """The AAD's first half: an index_sync body is not an index_delta."""
+    sealed = seal(gek, PURPOSE_INDEX, "index_sync", "g1", PAYLOAD)
+    with pytest.raises(InvalidTag):
+        unseal(gek, PURPOSE_INDEX, "index_delta", "g1", sealed)
+
+
+def test_a_body_cannot_be_moved_between_groups(gek):
+    """
+    The AAD's second half. Two groups on one node share a GEK-holding process but
+    not a GEK; this closes the case where they do share one (a rotation in flight,
+    a test fixture, an operator reusing a key) as well.
+    """
+    sealed = seal(gek, PURPOSE_INDEX, "index_sync", "group-a", PAYLOAD)
+    with pytest.raises(InvalidTag):
+        unseal(gek, PURPOSE_INDEX, "index_sync", "group-b", sealed)
+
+
+def test_a_tampered_ciphertext_does_not_open(gek):
+    sealed = seal(gek, PURPOSE_INDEX, "index_sync", "g1", PAYLOAD)
+    sealed["ct"] = bytes([sealed["ct"][0] ^ 1]) + sealed["ct"][1:]
+    with pytest.raises(InvalidTag):
+        unseal(gek, PURPOSE_INDEX, "index_sync", "g1", sealed)
+
+
+def test_a_message_that_is_not_sealed_is_refused_as_such(gek):
+    """
+    Not an empty payload, and not a crash on a missing key — the two shapes a
+    caller might otherwise paper over.
+    """
+    with pytest.raises(ValueError):
+        unseal(gek, PURPOSE_INDEX, "index_sync", "g1", {"entries": []})
+
+
+def test_a_fresh_nonce_per_message(gek):
+    """
+    Never derived from the payload: two identical payloads under one long-lived
+    subkey would then reuse a nonce, which for GCM is a total break.
+    """
+    nonces = {seal(gek, PURPOSE_INDEX, "index_sync", "g1", PAYLOAD)["nonce"]
+              for _ in range(50)}
+    assert len(nonces) == 50
+
+
+def test_no_key_is_an_error_not_a_plaintext_fallback(gek):
+    with pytest.raises(ValueError):
+        seal(None, PURPOSE_INDEX, "index_sync", "g1", PAYLOAD)
+
+
+def test_an_unknown_purpose_is_refused(gek):
+    with pytest.raises(ValueError):
+        group_key(gek, "chat")
+
+
+def test_the_envelope_carries_no_readable_payload(gek):
+    """
+    The property, at the level of the primitive: what `seal` returns holds nothing
+    of what went in. `test_index_no_cleartext.py` asserts the same thing on the
+    real frames.
+    """
+    payload = {"video_root": "holidays-2019-invoices", "entries": ["ledger.pdf"]}
+    frame = msgpack.packb(seal(gek, PURPOSE_ACK, "handshake_ack", "g1", payload))
+    for word in (b"holidays", b"invoices", b"ledger", b"video_root"):
+        assert word not in frame