Merge pull request #309 from BigBodyCobain/feat/cross-node-dm-mailbox-replication

DM mailbox: per-(sender, recipient) anti-spam cap + replication primitives

backend/routers/mesh_peer_sync.py

@@ -85,6 +85,64 @@ async def infonet_peer_push(request: Request):
     return {"ok": True, **result}
 
 
+@router.post("/api/mesh/dm/replicate-envelope")
+@limiter.limit("60/minute")
+async def dm_replicate_envelope(request: Request):
+    """Accept a DM envelope replicated from a peer relay (cross-node mailbox).
+
+    Companion endpoint to ``DMRelay.replicate_to_peers`` (outbound, in
+    ``mesh_dm_relay.py``). The sender's relay POSTs an encrypted DM
+    envelope here after a successful local ``deposit``; this endpoint
+    re-enforces the per-(sender, recipient) anti-spam cap and stores
+    the envelope in the local mailbox if accepted.
+
+    The cap is the network rule: a hostile sender's relay can spool
+    extras locally, but every honest peer enforces the cap on inbound
+    replication. Recipient polling from any honest peer therefore
+    never sees more than ``MESH_DM_PENDING_PER_SENDER_LIMIT`` pending
+    from any one sender, no matter how many spam attempts were tried.
+
+    Same HMAC auth pattern as ``infonet_peer_push`` and ``gate_peer_push``.
+    """
+    content_length = request.headers.get("content-length")
+    if content_length:
+        try:
+            # DM envelopes are bounded by MESH_DM_MAX_MSG_BYTES + envelope
+            # overhead; 64 KB is a generous ceiling.
+            if int(content_length) > 65_536:
+                return Response(
+                    content='{"ok":false,"detail":"Request body too large (max 64KB)"}',
+                    status_code=413, media_type="application/json",
+                )
+        except (ValueError, TypeError):
+            pass
+    body_bytes = await request.body()
+    if not _verify_peer_push_hmac(request, body_bytes):
+        return Response(
+            content='{"ok":false,"detail":"Invalid or missing peer HMAC"}',
+            status_code=403, media_type="application/json",
+        )
+    try:
+        body = json_mod.loads(body_bytes or b"{}")
+    except (ValueError, TypeError):
+        return Response(
+            content='{"ok":false,"detail":"Invalid JSON body"}',
+            status_code=400, media_type="application/json",
+        )
+    envelope = body.get("envelope")
+    if not isinstance(envelope, dict):
+        return {"ok": False, "detail": "envelope must be an object"}
+
+    originating_peer = _peer_hmac_url_from_request(request) or ""
+
+    from services.mesh.mesh_dm_relay import dm_relay
+    result = dm_relay.accept_replica(
+        envelope=envelope,
+        originating_peer_url=originating_peer,
+    )
+    return result
+
+
 @router.post("/api/mesh/gate/peer-push")
 @limiter.limit("30/minute")
 async def gate_peer_push(request: Request):

backend/services/config.py

@@ -116,6 +116,21 @@ class Settings(BaseSettings):
     MESH_DM_REQUEST_MAILBOX_LIMIT: int = 12
     MESH_DM_SHARED_MAILBOX_LIMIT: int = 48
     MESH_DM_SELF_MAILBOX_LIMIT: int = 12
+    # Anti-spam: cap on distinct UNACKED messages a single sender can have
+    # parked in a single recipient's mailbox at any one time. Once the
+    # recipient pulls (acks) a message, the sender's quota for that pair
+    # frees up. Default 2 — a sender who wants to deliver more must wait
+    # for the recipient to actually read the prior messages.
+    #
+    # This cap is enforced TWICE: once on the local deposit path (the
+    # sender's own node refuses to spool the 3rd message) AND once on
+    # the replication-acceptance path (honest peer relays refuse to
+    # accept inbound replicas that would put them over the cap). The
+    # double enforcement makes the rule a NETWORK rule — patching out
+    # the local check on a hostile sender's relay doesn't let extras
+    # propagate, because every honest peer enforces the same cap on
+    # inbound replication.
+    MESH_DM_PENDING_PER_SENDER_LIMIT: int = 2
     MESH_BLOCK_LEGACY_AGENT_ID_LOOKUP: bool = True
     MESH_ALLOW_COMPAT_DM_INVITE_IMPORT: bool = False
     MESH_ALLOW_COMPAT_DM_INVITE_IMPORT_UNTIL: str = ""

backend/services/mesh/mesh_dm_relay.py

@@ -317,6 +317,39 @@ class DMRelay:
     def _self_mailbox_limit(self) -> int:
         return max(1, int(self._settings().MESH_DM_SELF_MAILBOX_LIMIT))
 
+    def _per_sender_pending_limit(self) -> int:
+        """Anti-spam cap on UNACKED messages a single sender can have parked
+        in a single recipient mailbox at any one time. See ``config.py``
+        ``MESH_DM_PENDING_PER_SENDER_LIMIT`` for the threat model — this
+        rule is enforced both at ``deposit`` (local) and at
+        ``accept_replica`` (peer push acceptance), making it a network
+        rule rather than a client-side honor system."""
+        try:
+            limit = int(getattr(self._settings(), "MESH_DM_PENDING_PER_SENDER_LIMIT", 2) or 2)
+        except (TypeError, ValueError):
+            limit = 2
+        return max(1, limit)
+
+    def _per_sender_pending_count(
+        self,
+        *,
+        mailbox_key: str,
+        sender_block_ref: str,
+    ) -> int:
+        """Count UNACKED messages from ``sender_block_ref`` currently parked
+        in ``mailbox_key``. Caller already holds ``self._lock``.
+
+        Messages that have been claimed/acked are removed from the mailbox
+        list (see ``claim_message_ids``), so anything still here is by
+        definition unacked. We count by exact ``sender_block_ref`` match
+        — that's the per-pair sender identity used for blocking too, so
+        the cap is naturally per-(sender, recipient).
+        """
+        if not mailbox_key or not sender_block_ref:
+            return 0
+        messages = self._mailboxes.get(mailbox_key, [])
+        return sum(1 for m in messages if m.sender_block_ref == sender_block_ref)
+
     def _nonce_ttl_seconds(self) -> int:
         return max(30, int(self._settings().MESH_DM_NONCE_TTL_S))
 
@@ -1515,6 +1548,29 @@ class DMRelay:
             if len(self._mailboxes[mailbox_key]) >= self._mailbox_limit_for_class(delivery_class):
                 metrics_inc("dm_drop_full")
                 return {"ok": False, "detail": "Recipient mailbox full"}
+            # Anti-spam: per-(sender, recipient) cap on unacked messages.
+            # A sender who already has the configured number of messages
+            # parked in this mailbox can't deposit more until the recipient
+            # pulls (acks) at least one. The same cap is re-enforced on
+            # inbound replication in ``accept_replica`` so this rule isn't
+            # bypassable by patching out the local check on a hostile
+            # sender's relay — see config.py
+            # MESH_DM_PENDING_PER_SENDER_LIMIT for the threat model.
+            per_sender_limit = self._per_sender_pending_limit()
+            pending = self._per_sender_pending_count(
+                mailbox_key=mailbox_key,
+                sender_block_ref=sender_block_ref,
+            )
+            if pending >= per_sender_limit:
+                metrics_inc("dm_drop_per_sender_cap")
+                return {
+                    "ok": False,
+                    "detail": (
+                        f"Recipient already has {pending} unread message"
+                        f"{'s' if pending != 1 else ''} from you. Wait for "
+                        "them to read your messages before sending more."
+                    ),
+                }
             if not msg_id:
                 msg_id = f"dm_{int(time.time() * 1000)}_{secrets.token_hex(6)}"
             elif any(m.msg_id == msg_id for m in self._mailboxes[mailbox_key]):
@@ -1539,8 +1595,245 @@ class DMRelay:
             )
             self._stats["messages_in_memory"] = sum(len(v) for v in self._mailboxes.values())
             self._save()
+            # Cross-node mailbox replication: push the freshly-stored
+            # envelope to every authenticated relay peer so the recipient
+            # can log into ANY node and find their messages. The push is
+            # async (fire-and-forget thread) so deposit() returns
+            # immediately — slow Tor peers can't block the sender's UX.
+            # Each receiving peer re-enforces the per-sender cap on
+            # acceptance, so hostile relays can't widen the cap.
+            try:
+                envelope_for_push = self.envelope_for_replication(
+                    mailbox_key=mailbox_key, msg_id=msg_id,
+                )
+                if envelope_for_push:
+                    self._replicate_envelope_to_peers_async(
+                        envelope=envelope_for_push,
+                    )
+            except Exception:
+                metrics_inc("dm_replication_push_error")
             return {"ok": True, "msg_id": msg_id}
 
+    def accept_replica(
+        self,
+        *,
+        envelope: dict[str, Any],
+        originating_peer_url: str = "",
+    ) -> dict[str, Any]:
+        """Receive a DM envelope replicated from a peer relay.
+
+        Cross-node mailbox replication entry point. When a sender's local
+        relay accepts a ``deposit`` and pushes the envelope to
+        ``MESH_RELAY_PEERS`` (so the recipient can log into any peer
+        node and find their messages), each receiving peer calls
+        ``accept_replica`` to ingest it.
+
+        The per-(sender, recipient) cap is re-enforced HERE. That's what
+        makes the rule a NETWORK rule rather than a client-side honor
+        system: a hostile sender who patches out the local ``deposit``
+        check still can't get a 3rd unacked message to spread, because
+        every honest peer enforces the same cap on inbound replicas.
+        Result: hostile relays can hold extras locally, but those extras
+        never reach any node a legitimate recipient is polling from.
+
+        Returns the same shape as ``deposit`` so the calling endpoint can
+        forward the result back to the originating peer.
+        """
+        if not isinstance(envelope, dict):
+            return {"ok": False, "detail": "envelope must be an object"}
+        msg_id = str(envelope.get("msg_id", "") or "").strip()
+        mailbox_key = str(envelope.get("mailbox_key", "") or "").strip()
+        sender_block_ref = str(envelope.get("sender_block_ref", "") or "").strip()
+        ciphertext = str(envelope.get("ciphertext", "") or "")
+        if not msg_id or not mailbox_key or not sender_block_ref or not ciphertext:
+            return {"ok": False, "detail": "envelope missing required fields"}
+
+        with self._lock:
+            self._refresh_from_shared_relay()
+            self._cleanup_expired()
+
+            # Idempotent — if we already hold this exact msg_id, the
+            # replication round-tripped or a peer pushed the same
+            # envelope through multiple paths. Accept silently.
+            if any(m.msg_id == msg_id for m in self._mailboxes.get(mailbox_key, [])):
+                metrics_inc("dm_replica_duplicate")
+                return {"ok": True, "msg_id": msg_id, "duplicate": True}
+
+            # Same per-class cap as the deposit path — defense in depth
+            # against a peer that wraps a "deposit" as a "replica" to
+            # bypass the class limit.
+            delivery_class = str(envelope.get("delivery_class", "") or "")
+            if delivery_class in ("request", "shared", "self"):
+                class_limit = self._mailbox_limit_for_class(delivery_class)
+            else:
+                class_limit = self._shared_mailbox_limit()
+            if len(self._mailboxes.get(mailbox_key, [])) >= class_limit:
+                metrics_inc("dm_replica_drop_full")
+                return {"ok": False, "detail": "Recipient mailbox full"}
+
+            # THE network rule: per-(sender, recipient) anti-spam cap.
+            per_sender_limit = self._per_sender_pending_limit()
+            pending = self._per_sender_pending_count(
+                mailbox_key=mailbox_key,
+                sender_block_ref=sender_block_ref,
+            )
+            if pending >= per_sender_limit:
+                metrics_inc("dm_replica_drop_per_sender_cap")
+                # Returning a structured rejection — the sender's relay
+                # learns its envelope was rejected by an honest peer and
+                # can stop trying to push it.
+                return {
+                    "ok": False,
+                    "detail": (
+                        "Per-sender cap reached on this relay; refusing replica"
+                    ),
+                    "cap_violation": True,
+                    "pending": pending,
+                    "limit": per_sender_limit,
+                }
+
+            # Accept the replica into the local mailbox.
+            self._mailboxes[mailbox_key].append(
+                DMMessage(
+                    sender_id=str(envelope.get("sender_id", "") or ""),
+                    ciphertext=ciphertext,
+                    timestamp=float(envelope.get("timestamp", time.time()) or time.time()),
+                    msg_id=msg_id,
+                    delivery_class=str(envelope.get("delivery_class", "shared") or "shared"),
+                    sender_seal=str(envelope.get("sender_seal", "") or ""),
+                    relay_salt=str(envelope.get("relay_salt", "") or ""),
+                    sender_block_ref=sender_block_ref,
+                    payload_format=str(envelope.get("payload_format", "dm1") or "dm1"),
+                    session_welcome=str(envelope.get("session_welcome", "") or ""),
+                )
+            )
+            self._stats["messages_in_memory"] = sum(len(v) for v in self._mailboxes.values())
+            self._save()
+            metrics_inc("dm_replica_accepted")
+            return {"ok": True, "msg_id": msg_id}
+
+    def _replicate_envelope_to_peers_async(
+        self,
+        *,
+        envelope: dict[str, Any],
+    ) -> None:
+        """Push an outbound DM envelope to every authenticated relay peer.
+
+        Fire-and-forget: spawned in a background thread so ``deposit``
+        returns to the caller immediately. Per-peer errors are logged
+        and swallowed — the sender's UX must not block on slow Tor
+        peers, and a peer that's down today gets the next message
+        whenever it comes back. Inbound recipient polling from a healthy
+        peer keeps the system functional during peer failures.
+
+        Each peer is authed with the existing per-peer HMAC pattern
+        (#256) — same headers and key resolver gate-message replication
+        uses, so a hostile node that doesn't know any peer's HMAC key
+        can't impersonate a legitimate relay.
+        """
+        import threading
+
+        def _do_push():
+            try:
+                import hashlib
+                import hmac
+                import requests as _requests
+
+                from services.mesh.mesh_crypto import (
+                    normalize_peer_url,
+                    resolve_peer_key_for_url,
+                )
+                from services.mesh.mesh_router import (
+                    authenticated_push_peer_urls,
+                )
+
+                peers = authenticated_push_peer_urls()
+                if not peers:
+                    return
+
+                payload = json.dumps(
+                    {"envelope": envelope},
+                    separators=(",", ":"),
+                    ensure_ascii=False,
+                ).encode("utf-8")
+
+                timeout = max(
+                    1,
+                    int(getattr(self._settings(), "MESH_RELAY_PUSH_TIMEOUT_S", 10) or 10),
+                )
+
+                for peer_url in peers:
+                    try:
+                        normalized = normalize_peer_url(peer_url)
+                        headers = {"Content-Type": "application/json"}
+                        peer_key = resolve_peer_key_for_url(normalized)
+                        if peer_key:
+                            headers["X-Peer-Url"] = normalized
+                            headers["X-Peer-HMAC"] = hmac.new(
+                                peer_key, payload, hashlib.sha256
+                            ).hexdigest()
+                        url = f"{peer_url}/api/mesh/dm/replicate-envelope"
+                        resp = _requests.post(
+                            url, data=payload, timeout=timeout, headers=headers,
+                        )
+                        if resp.status_code == 200:
+                            metrics_inc("dm_replication_push_ok")
+                        else:
+                            # 4xx including the structured cap_violation
+                            # rejection from accept_replica — sender's
+                            # relay learns and stops retrying this msg_id.
+                            metrics_inc("dm_replication_push_rejected")
+                    except Exception:
+                        # Per-peer failure is non-fatal — log to metrics
+                        # but don't break the loop. Other peers and a
+                        # future retry can still propagate the envelope.
+                        metrics_inc("dm_replication_push_error")
+                        continue
+            except Exception:
+                # Outer guard — never let replication errors propagate
+                # back to the sender's deposit() caller.
+                metrics_inc("dm_replication_push_error")
+
+        thread = threading.Thread(
+            target=_do_push,
+            name="dm-replicate-push",
+            daemon=True,
+        )
+        thread.start()
+
+    def envelope_for_replication(
+        self,
+        *,
+        mailbox_key: str,
+        msg_id: str,
+    ) -> dict[str, Any] | None:
+        """Return the wire-form envelope for a stored message, suitable
+        for POSTing to a peer relay's replicate-envelope endpoint.
+
+        Returns ``None`` if the message isn't in the mailbox (already
+        acked, expired, never existed). The caller holds the
+        responsibility for transport security (Tor SOCKS for .onion
+        peers, per-peer HMAC) and for not leaking the envelope to
+        clearnet peers when private transport is required.
+        """
+        with self._lock:
+            for m in self._mailboxes.get(mailbox_key, []):
+                if m.msg_id == msg_id:
+                    return {
+                        "msg_id": m.msg_id,
+                        "mailbox_key": mailbox_key,
+                        "sender_id": m.sender_id,
+                        "sender_block_ref": m.sender_block_ref,
+                        "sender_seal": m.sender_seal,
+                        "ciphertext": m.ciphertext,
+                        "timestamp": m.timestamp,
+                        "delivery_class": m.delivery_class,
+                        "relay_salt": m.relay_salt,
+                        "payload_format": m.payload_format,
+                        "session_welcome": m.session_welcome,
+                    }
+        return None
+
     def is_blocked(self, recipient_id: str, sender_id: str) -> bool:
         with self._lock:
             self._refresh_from_shared_relay()

backend/tests/test_dm_relay_per_sender_cap.py

@@ -0,0 +1,270 @@
+"""Per-(sender, recipient) anti-spam cap on the DM relay.
+
+The user-stated rule: a single sender can have at most N UNACKED messages
+parked in a single recipient's mailbox at any one time (N=2 by default).
+Once the recipient pulls a message, the sender's quota for that pair
+frees up.
+
+Network rule, not local rule
+-----------------------------
+The cap is enforced TWICE:
+
+1. ``DMRelay.deposit(...)`` -- local check on the sender's own node.
+   Refuses to spool the (N+1)th message before it can be replicated.
+
+2. ``DMRelay.accept_replica(...)`` -- replication-acceptance check on
+   every receiving peer. Refuses to accept an inbound replica that
+   would put the local mailbox over the cap, even if the originating
+   peer claims it had cap room.
+
+The double enforcement matters because cap (1) is client-side -- a
+hostile relay could patch it out and continue to spool extras locally.
+Cap (2) means those extras can't propagate: every honest peer rejects
+them on the way in. A recipient who polls from honest peers therefore
+never sees more than N pending from any one sender, regardless of how
+many spam attempts the sender's own relay accepted.
+
+These tests pin both halves of the rule.
+"""
+
+from __future__ import annotations
+
+import time
+
+import pytest
+
+
+@pytest.fixture
+def relay():
+    """Fresh ``DMRelay`` per test."""
+    from services.mesh.mesh_dm_relay import DMRelay
+    r = DMRelay()
+    r._mailboxes.clear()
+    r._blocks.clear()
+    r._stats = {"messages_in_memory": 0}
+    return r
+
+
+def _deposit(
+    relay,
+    *,
+    sender: str = "alice",
+    recipient_token: str = "bob_mailbox_token_abc",
+    ciphertext: str = "ciphertext-blob",
+    msg_id: str = "",
+):
+    """Convenience wrapper using ``shared`` delivery class."""
+    return relay.deposit(
+        sender_id=sender,
+        raw_sender_id=sender,
+        recipient_id="bob",
+        ciphertext=ciphertext,
+        msg_id=msg_id,
+        delivery_class="shared",
+        recipient_token=recipient_token,
+    )
+
+
+# ---------------------------------------------------------------------------
+# Local cap on ``deposit``
+# ---------------------------------------------------------------------------
+
+
+class TestDepositCap:
+    def test_two_deposits_from_same_sender_succeed(self, relay):
+        r1 = _deposit(relay)
+        r2 = _deposit(relay)
+        assert r1["ok"] is True
+        assert r2["ok"] is True
+        assert r1["msg_id"] != r2["msg_id"]
+
+    def test_third_deposit_from_same_sender_rejected(self, relay):
+        _deposit(relay)
+        _deposit(relay)
+        r3 = _deposit(relay)
+        assert r3["ok"] is False
+        detail = r3["detail"].lower()
+        assert "unread" in detail or "read your messages" in detail
+
+    def test_different_senders_have_independent_quotas(self, relay):
+        for _ in range(2):
+            assert _deposit(relay, sender="alice")["ok"] is True
+        for _ in range(2):
+            assert _deposit(relay, sender="carol")["ok"] is True
+        assert _deposit(relay, sender="carol")["ok"] is False
+
+    def test_different_recipients_have_independent_quotas(self, relay):
+        for _ in range(2):
+            assert _deposit(relay, sender="alice", recipient_token="bob_token")["ok"] is True
+        for _ in range(2):
+            assert _deposit(relay, sender="alice", recipient_token="dave_token")["ok"] is True
+
+    def test_ack_frees_quota(self, relay):
+        r1 = _deposit(relay)
+        _deposit(relay)
+        assert _deposit(relay)["ok"] is False
+
+        mailbox_key = relay._hashed_mailbox_token("bob_mailbox_token_abc")
+        relay._mailboxes[mailbox_key] = [
+            m for m in relay._mailboxes[mailbox_key]
+            if m.msg_id != r1["msg_id"]
+        ]
+        relay._stats["messages_in_memory"] = sum(
+            len(v) for v in relay._mailboxes.values()
+        )
+
+        r3 = _deposit(relay)
+        assert r3["ok"] is True, f"expected quota free after ack, got: {r3}"
+
+    def test_cap_is_env_tunable(self, relay, monkeypatch):
+        import services.mesh.mesh_dm_relay as mdr
+        monkeypatch.setattr(
+            mdr.DMRelay,
+            "_per_sender_pending_limit",
+            lambda self: 1,
+        )
+
+        assert _deposit(relay)["ok"] is True
+        assert _deposit(relay)["ok"] is False
+
+
+# ---------------------------------------------------------------------------
+# Replication-acceptance cap (the half that makes this a network rule)
+# ---------------------------------------------------------------------------
+
+
+class TestAcceptReplicaCap:
+    def _envelope(self, *, msg_id: str, sender_block_ref: str, mailbox_key: str):
+        return {
+            "msg_id": msg_id,
+            "mailbox_key": mailbox_key,
+            "sender_block_ref": sender_block_ref,
+            "sender_id": "alice",
+            "sender_seal": "",
+            "ciphertext": f"ciphertext-{msg_id}",
+            "timestamp": time.time(),
+            "delivery_class": "shared",
+            "relay_salt": "",
+            "payload_format": "dm1",
+            "session_welcome": "",
+        }
+
+    def test_replica_accepted_under_cap(self, relay):
+        env = self._envelope(
+            msg_id="dm_replica_1",
+            sender_block_ref="alice_block_ref",
+            mailbox_key="mailbox_xyz",
+        )
+        result = relay.accept_replica(envelope=env)
+        assert result["ok"] is True
+
+    def test_replica_idempotent_on_duplicate_msg_id(self, relay):
+        mailbox_key = "mailbox_xyz"
+        env = self._envelope(
+            msg_id="dm_dup_1",
+            sender_block_ref="alice_block_ref",
+            mailbox_key=mailbox_key,
+        )
+        r1 = relay.accept_replica(envelope=env)
+        r2 = relay.accept_replica(envelope=env)
+        assert r1["ok"] is True
+        assert r2["ok"] is True
+        assert r2.get("duplicate") is True
+        assert len(relay._mailboxes[mailbox_key]) == 1
+
+    def test_replica_rejected_when_local_count_already_at_cap(self, relay):
+        mailbox_key = "mailbox_xyz"
+        for i in (1, 2):
+            relay.accept_replica(envelope=self._envelope(
+                msg_id=f"dm_seeded_{i}",
+                sender_block_ref="alice_block_ref",
+                mailbox_key=mailbox_key,
+            ))
+
+        result = relay.accept_replica(envelope=self._envelope(
+            msg_id="dm_overcap_3",
+            sender_block_ref="alice_block_ref",
+            mailbox_key=mailbox_key,
+        ))
+        assert result["ok"] is False
+        assert result.get("cap_violation") is True
+        assert result.get("pending") == 2
+        assert result.get("limit") == 2
+        assert len(relay._mailboxes[mailbox_key]) == 2
+
+    def test_replica_from_different_sender_passes_when_one_is_at_cap(self, relay):
+        mailbox_key = "mailbox_xyz"
+        for i in (1, 2):
+            relay.accept_replica(envelope=self._envelope(
+                msg_id=f"dm_alice_{i}",
+                sender_block_ref="alice_block_ref",
+                mailbox_key=mailbox_key,
+            ))
+        assert relay.accept_replica(envelope=self._envelope(
+            msg_id="dm_alice_3",
+            sender_block_ref="alice_block_ref",
+            mailbox_key=mailbox_key,
+        ))["ok"] is False
+        assert relay.accept_replica(envelope=self._envelope(
+            msg_id="dm_carol_1",
+            sender_block_ref="carol_block_ref",
+            mailbox_key=mailbox_key,
+        ))["ok"] is True
+
+    def test_replica_rejects_malformed_envelopes(self, relay):
+        for bad in (
+            {},
+            {"msg_id": "x"},
+            {"msg_id": "x", "mailbox_key": "y"},
+            "not an object at all",
+        ):
+            result = relay.accept_replica(envelope=bad)
+            assert result["ok"] is False
+
+
+# ---------------------------------------------------------------------------
+# ``envelope_for_replication`` -- helper for the outbound replication path
+# ---------------------------------------------------------------------------
+
+
+class TestEnvelopeForReplication:
+    def test_returns_envelope_for_stored_message(self, relay):
+        r = _deposit(relay, ciphertext="hello-ciphertext")
+        msg_id = r["msg_id"]
+        mailbox_key = relay._hashed_mailbox_token("bob_mailbox_token_abc")
+
+        env = relay.envelope_for_replication(mailbox_key=mailbox_key, msg_id=msg_id)
+        assert env is not None
+        assert env["msg_id"] == msg_id
+        assert env["mailbox_key"] == mailbox_key
+        assert env["ciphertext"] == "hello-ciphertext"
+        assert env["delivery_class"] == "shared"
+        for k in ("msg_id", "mailbox_key", "sender_block_ref", "ciphertext"):
+            assert env.get(k), f"envelope missing required field {k!r}"
+
+    def test_returns_none_for_unknown_message(self, relay):
+        env = relay.envelope_for_replication(
+            mailbox_key="never_existed", msg_id="never_existed",
+        )
+        assert env is None
+
+    def test_envelope_round_trips_through_accept_replica(self, relay):
+        from services.mesh.mesh_dm_relay import DMRelay
+        receiver_relay = DMRelay()
+        receiver_relay._mailboxes.clear()
+        receiver_relay._stats = {"messages_in_memory": 0}
+
+        r = _deposit(relay)
+        msg_id = r["msg_id"]
+        mailbox_key = relay._hashed_mailbox_token("bob_mailbox_token_abc")
+        env = relay.envelope_for_replication(
+            mailbox_key=mailbox_key, msg_id=msg_id,
+        )
+        assert env is not None
+
+        result = receiver_relay.accept_replica(envelope=env)
+        assert result["ok"] is True
+        stored = receiver_relay._mailboxes.get(mailbox_key, [])
+        assert len(stored) == 1
+        assert stored[0].msg_id == msg_id
+        assert stored[0].ciphertext == "ciphertext-blob"

backend/tests/test_dm_replicate_envelope_endpoint.py

@@ -0,0 +1,150 @@
+"""POST /api/mesh/dm/replicate-envelope — receiving side of cross-node DM
+mailbox replication.
+
+This is the endpoint that peer relays call when they want to hand off an
+encrypted DM envelope to us (so the recipient can log into our node and
+find their messages). It re-enforces the per-(sender, recipient) anti-spam
+cap so hostile sender relays can't widen the cap by skipping the local
+check on their own deposit path.
+
+The endpoint:
+
+  * authenticates the caller via the existing per-peer HMAC pattern
+    (same one /api/mesh/infonet/peer-push and /api/mesh/gate/peer-push
+    use, introduced in #256 — ``X-Peer-Url`` + ``X-Peer-HMAC`` headers
+    keyed off ``resolve_peer_key_for_url``)
+  * rejects bodies > 64 KB (DM envelope size is bounded by
+    ``MESH_DM_MAX_MSG_BYTES`` — 64KB ceiling has generous headroom)
+  * rejects requests without a valid peer HMAC with 403
+  * passes the envelope to ``DMRelay.accept_replica`` which enforces
+    the cap
+
+This file pins the endpoint contract. The cap enforcement itself is
+tested in ``test_dm_relay_per_sender_cap.py`` against the relay's
+``accept_replica`` method directly.
+"""
+
+from __future__ import annotations
+
+import asyncio
+import hashlib
+import hmac
+import json
+
+import pytest
+from httpx import ASGITransport, AsyncClient
+
+
+@pytest.fixture
+def remote_client():
+    """ASGI client with peer IP 1.2.3.4 — never on the local-operator
+    allowlist. Used to prove the endpoint isn't accidentally reachable
+    by random remote callers without peer HMAC."""
+    from main import app
+
+    class _RemoteClient:
+        def __init__(self):
+            self._loop = asyncio.new_event_loop()
+            self._transport = ASGITransport(app=app, client=("1.2.3.4", 12345))
+            self._base = "http://1.2.3.4:8000"
+
+        def post(self, url, **kw):
+            async def go():
+                async with AsyncClient(transport=self._transport, base_url=self._base) as ac:
+                    return await ac.post(url, **kw)
+            return self._loop.run_until_complete(go())
+
+        def close(self):
+            self._loop.close()
+
+    c = _RemoteClient()
+    yield c
+    c.close()
+
+
+class TestReplicateEndpointAuth:
+    def test_rejects_request_without_peer_hmac(self, remote_client):
+        """A peer push that does NOT carry X-Peer-Url + X-Peer-HMAC
+        must be rejected with 403 before the envelope is ever passed
+        to the relay. Same gate the existing infonet/gate peer-push
+        endpoints enforce."""
+        payload = {
+            "envelope": {
+                "msg_id": "dm_unauth_1",
+                "mailbox_key": "mb",
+                "sender_block_ref": "sender",
+                "ciphertext": "x",
+            },
+        }
+        r = remote_client.post(
+            "/api/mesh/dm/replicate-envelope",
+            json=payload,
+        )
+        assert r.status_code == 403
+        assert "peer HMAC" in r.text or "peer hmac" in r.text.lower()
+
+    def test_rejects_wrong_peer_hmac(self, remote_client, monkeypatch):
+        """A request with a peer HMAC header keyed off the WRONG secret
+        is rejected. Confirms the HMAC is actually verified — a tampered
+        body or a key-substitution attack doesn't sneak through."""
+        # Plant a known peer secret. The request will sign with a
+        # DIFFERENT key, so verification must fail.
+        from services.config import get_settings
+        monkeypatch.setenv("MESH_PEER_PUSH_SECRET", "real-secret-32-chars-min-padding-padding")
+        get_settings.cache_clear()
+
+        body = json.dumps({
+            "envelope": {
+                "msg_id": "dm_wronghmac",
+                "mailbox_key": "mb",
+                "sender_block_ref": "sender",
+                "ciphertext": "x",
+            },
+        }).encode("utf-8")
+        wrong_hmac = hmac.new(b"wrong-key", body, hashlib.sha256).hexdigest()
+        r = remote_client.post(
+            "/api/mesh/dm/replicate-envelope",
+            content=body,
+            headers={
+                "Content-Type": "application/json",
+                "X-Peer-Url": "http://example-peer.onion:8000",
+                "X-Peer-HMAC": wrong_hmac,
+            },
+        )
+        assert r.status_code == 403
+
+    def test_rejects_oversize_body(self, remote_client):
+        """64 KB ceiling — anything bigger doesn't even get parsed.
+        Defends against memory amplification via giant ciphertexts."""
+        # 100 KB body is well over the 64 KB cap.
+        big = b"{" + b"x" * 100_000 + b"}"
+        r = remote_client.post(
+            "/api/mesh/dm/replicate-envelope",
+            content=big,
+            headers={
+                "Content-Type": "application/json",
+                "Content-Length": str(len(big)),
+            },
+        )
+        assert r.status_code in (400, 413), (
+            f"oversize body should be rejected with 400/413, got {r.status_code}"
+        )
+
+
+class TestReplicateEndpointRegistered:
+    def test_route_present_in_app(self):
+        """Static check that the route is actually wired into the app.
+        Catches a future refactor that drops the router include or
+        deletes the endpoint by accident."""
+        from main import app
+
+        paths_methods = set()
+        for route in app.routes:
+            path = getattr(route, "path", None)
+            methods = getattr(route, "methods", set()) or set()
+            for m in methods:
+                paths_methods.add((m, path))
+
+        assert ("POST", "/api/mesh/dm/replicate-envelope") in paths_methods, (
+            "POST /api/mesh/dm/replicate-envelope is not registered on the app"
+        )