fix(mesh): relay foreign packets whose channel hash collides with a local channel (#11544)

* fix(mesh): relay foreign packets whose channel hash collides with a local channel

The channel hash is one byte, so a foreign channel's name/PSK can fold to the
same hash as a local channel (~1/256 per local channel held). Since d6b12ea3f,
perhapsDecode returns DECODE_FAILURE whenever any local channel matched the
hash, and passesRoutingAuthGate turned that into REJECT - silently blackholing
legitimate foreign traffic that master and 2.7.x relay. A node with a wrong PSK
for a channel name stopped relaying the real channel entirely.

Channel crypto (AES-CTR) has no authentication tag, so "wrong key, foreign
channel" and "our channel, tampered payload" are indistinguishable at this
decision point. The strict drop bought nothing: an attacker picks a hash
matching no local channel and gets DECODE_OPAQUE relay anyway (test_C6), so
the rule only suppressed honest colliding traffic.

Return OPAQUE_RELAY_ONLY on DECODE_FAILURE unless the packet is addressed to
us or claims to be from us. isFromUs stays REJECT because OPAQUE_RELAY_ONLY
reaches perhapsGenerateImplicitAckForOwnOverheard, which matches pending sends
on header bytes alone - a forged sender with a colliding hash and matching id
could otherwise fake-ACK a DM and cancel its retransmissions. Other
DECODE_FAILURE sources are unaffected: legacy-DM rejection, pending-key
refusal, and failed PKI candidates are all isToUs, and the KNOWN_ONLY early
return is re-gated by relayOpaquePacket's own mode check. Opaque frames still
never touch PacketHistory, NodeDB, modules, MQTT, ACKs, or the phone.

test_C12's collision leg now expects OPAQUE_RELAY_ONLY (its tampered packet is
a broadcast - byte-identical to the foreign case); it still pins per-exact-byte
cache reevaluation. test_C9 renamed to match what it now verifies. New test_C17
covers the colliding-hash foreign broadcast and the spoofed-sender REJECT.

* style(mesh): trim collision-relay comment to two lines

src/mesh/Router.cpp

@@ -789,6 +789,12 @@ RoutingAuthVerdict passesRoutingAuthGate(meshtastic_MeshPacket *p)
         return RoutingAuthVerdict::REJECT;
     }
     if (state == DecodeState::DECODE_FAILURE) {
+        // One-byte hash collisions are indistinguishable from tampering, so relay opaquely
+        // instead of blackholing; isFromUs stays REJECT to keep forged senders off the ACK path.
+        if (!isToUs(p) && !isFromUs(p)) {
+            LOG_WARN("Decryptable packet failed decoding, relay opaquely");
+            return RoutingAuthVerdict::OPAQUE_RELAY_ONLY;
+        }
         LOG_WARN("Decryptable packet failed decoding, drop");
         return RoutingAuthVerdict::REJECT;
     }

test/test_packet_signing/test_main.cpp

@@ -1391,7 +1391,7 @@ void test_C8_trusted_local_decoded_delivery_is_not_filtered(void)
     packetPool.release(local);
 }
 
-void test_C9_known_channel_malformed_plaintext_is_not_relayed_as_opaque(void)
+void test_C9_known_channel_malformed_plaintext_has_no_pipeline_effects(void)
 {
     meshtastic_MeshPacket malformed = meshtastic_MeshPacket_init_zero;
     malformed.from = REMOTE_NODE;
@@ -1404,6 +1404,12 @@ void test_C9_known_channel_malformed_plaintext_is_not_relayed_as_opaque(void)
     malformed.encrypted.bytes[2] = 0xFF;
     malformed.channel = channels.setActiveByIndex(0);
     crypto->encryptPacket(malformed.from, malformed.id, malformed.encrypted.size, malformed.encrypted.bytes);
+
+    // Verdict is opaque-relay-eligible now (see test_C17); hop_limit 0 is what keeps this a no-op.
+    meshtastic_MeshPacket verdictCopy = malformed;
+    TEST_ASSERT_EQUAL(static_cast<int>(RoutingAuthVerdict::OPAQUE_RELAY_ONLY),
+                      static_cast<int>(passesRoutingAuthGate(&verdictCopy)));
+
     mockNodeDB->addNode(REMOTE_NODE);
     const uint32_t lastHeard = mockNodeDB->getMeshNode(REMOTE_NODE)->last_heard;
     runPipelineIngress(malformed);
@@ -1468,9 +1474,12 @@ void test_C12_exact_authenticated_replay_reuses_verdict_without_collision_bypass
     runPipelineIngress(valid);
     TEST_ASSERT_EQUAL_MESSAGE(2, routingAuthEvaluationCount(), "consumed verdict must not authenticate a later replay");
 
+    // Broadcast, so isToUs() is false like any colliding-hash foreign broadcast (see test_C17);
+    // this still guards that the cache is reevaluated per exact bytes, not reused for a same-ID replay.
     meshtastic_MeshPacket collision = valid;
     collision.encrypted.bytes[0] ^= 0x80;
-    TEST_ASSERT_EQUAL(static_cast<int>(RoutingAuthVerdict::REJECT), static_cast<int>(passesRoutingAuthGate(&collision)));
+    TEST_ASSERT_EQUAL(static_cast<int>(RoutingAuthVerdict::OPAQUE_RELAY_ONLY),
+                      static_cast<int>(passesRoutingAuthGate(&collision)));
     TEST_ASSERT_EQUAL_MESSAGE(3, routingAuthEvaluationCount(), "same packet ID with different bytes must be reevaluated");
 }
 
@@ -1573,6 +1582,33 @@ void test_C16_reliable_broadcast_keeps_three_total_attempts(void)
     TEST_ASSERT_EQUAL_UINT8(3, pipelineRouter->pendingTotalAttempts(LOCAL_NODE, p.id));
 }
 
+void test_C17_colliding_channel_hash_foreign_broadcast_is_relay_only(void)
+{
+    // Foreign channel whose PSK collides with our channel 0's one-byte hash (see test_C9/test_C12
+    // for the paired tradeoff): indistinguishable from tampering, so it must relay opaquely.
+    setPolicy(meshtastic_Config_SecurityConfig_PacketSignaturePolicy_PACKET_SIGNATURE_POLICY_STRICT);
+    meshtastic_MeshPacket foreign = meshtastic_MeshPacket_init_zero;
+    foreign.from = REMOTE_NODE;
+    foreign.to = NODENUM_BROADCAST;
+    foreign.id = 0xC1700017;
+    foreign.hop_limit = 1;
+    foreign.hop_start = 2;
+    foreign.which_payload_variant = meshtastic_MeshPacket_encrypted_tag;
+    const int16_t hash = channels.setActiveByIndex(0);
+    TEST_ASSERT_GREATER_OR_EQUAL_MESSAGE(0, hash, "no usable primary channel");
+    foreign.channel = (uint8_t)hash; // collides with our channel 0, but the ciphertext below is not ours
+    foreign.encrypted.size = 16;
+    memset(foreign.encrypted.bytes, 0xA5, foreign.encrypted.size);
+
+    TEST_ASSERT_EQUAL(static_cast<int>(RoutingAuthVerdict::OPAQUE_RELAY_ONLY), static_cast<int>(passesRoutingAuthGate(&foreign)));
+
+    // Same undecodable frame claiming to be from us must still be dropped: OPAQUE_RELAY_ONLY would
+    // reach perhapsGenerateImplicitAckForOwnOverheard, which acts on header bytes alone.
+    meshtastic_MeshPacket spoofed = foreign;
+    spoofed.from = LOCAL_NODE;
+    TEST_ASSERT_EQUAL(static_cast<int>(RoutingAuthVerdict::REJECT), static_cast<int>(passesRoutingAuthGate(&spoofed)));
+}
+
 // C5: the packet survives (C4) but the identity claim inside it must not land - the pubkey guard
 // can't tell a signer from an impersonator replaying its (public) key. Only the write is refused.
 void test_N5_unsigned_unicast_nodeinfo_from_signer_does_not_change_name(void)
@@ -2142,7 +2178,7 @@ void setup()
     RUN_TEST(test_C6_opaque_unknown_channel_is_relay_only);
     RUN_TEST(test_C7_strict_rejects_unsigned_decoded_simradio_ingress);
     RUN_TEST(test_C8_trusted_local_decoded_delivery_is_not_filtered);
-    RUN_TEST(test_C9_known_channel_malformed_plaintext_is_not_relayed_as_opaque);
+    RUN_TEST(test_C9_known_channel_malformed_plaintext_has_no_pipeline_effects);
     RUN_TEST(test_C10_legacy_channel_dm_failure_has_no_pipeline_effects);
     RUN_TEST(test_C11_malformed_pki_plaintext_has_no_pipeline_effects);
     RUN_TEST(test_C12_exact_authenticated_replay_reuses_verdict_without_collision_bypass);
@@ -2150,6 +2186,7 @@ void setup()
     RUN_TEST(test_C14_duty_cycle_limited_reliable_send_remains_pending);
     RUN_TEST(test_C15_reliable_unicast_tracks_five_total_attempts);
     RUN_TEST(test_C16_reliable_broadcast_keeps_three_total_attempts);
+    RUN_TEST(test_C17_colliding_channel_hash_foreign_broadcast_is_relay_only);
     printf("\n=== Group N: NodeInfoModule authentication ===\n");
     RUN_TEST(test_N1_unsigned_nodeinfo_from_signer_dropped);
     RUN_TEST(test_N2_signed_nodeinfo_from_signer_not_dropped);