Gate the PKI-decrypt key on signer-proven, not the opportunistic cache (#11116)

Author: caveman99

src/mesh/NodeDB.cpp

@@ -3783,6 +3783,22 @@ bool NodeDB::copyPublicKey(NodeNum n, meshtastic_NodeInfoLite_public_key_t &out)
     return false;
 }
 
+bool NodeDB::copyPublicKeyForDecrypt(NodeNum n, meshtastic_NodeInfoLite_public_key_t &out)
+{
+    if (copyPublicKeyAuthoritative(n, out))
+        return true;
+#if HAS_TRAFFIC_MANAGEMENT
+    // A cold-tier cache key backs an authenticated decrypt only when signer-proven; unverified TOFU
+    // cache keys must not. Outbound encryption still uses the opportunistic copyPublicKey().
+    bool signerProven = false;
+    if (trafficManagementModule && trafficManagementModule->copyPublicKey(n, out.bytes, &signerProven) && signerProven) {
+        out.size = 32;
+        return true;
+    }
+#endif
+    return false;
+}
+
 bool NodeDB::isVerifiedSignerForKey(NodeNum n, const uint8_t *key32)
 {
     if (!key32)

src/mesh/NodeDB.h

@@ -364,6 +364,10 @@ class NodeDB
     /// opportunistic caches) - the pin reference for caches that mirror NodeDB's key hygiene.
     bool copyPublicKeyAuthoritative(NodeNum n, meshtastic_NodeInfoLite_public_key_t &out);
 
+    /// Key for the inbound-decrypt path: authoritative (hot/warm), or a cold-tier cache key only when
+    /// it is signer-proven. Keeps unverified TOFU cache keys from backing pki_encrypted attribution.
+    bool copyPublicKeyForDecrypt(NodeNum n, meshtastic_NodeInfoLite_public_key_t &out);
+
     /// True if n is a known XEdDSA signer for exactly `key32` (hot signed bitfield or warm
     /// signer bit); the key match stops a rotated key inheriting a stale signer verdict.
     bool isVerifiedSignerForKey(NodeNum n, const uint8_t *key32);

src/mesh/Router.cpp

@@ -815,13 +815,13 @@ DecodeState perhapsDecode(meshtastic_MeshPacket *p)
         (ourNode = nodeDB->getMeshNode(p->to)) != nullptr && ourNode->public_key.size > 0) {
         pkiAttempted = true;
         LOG_DEBUG("Attempt PKI decryption");
-        // Resolve the sender's public key only for actual PKI-decrypt candidates: prefer NodeDB
-        // (hot store or warm tier), else a not-yet-committed key held during an in-progress
-        // key-verification handshake. On a full NodeDB miss, copyPublicKey() falls through to a
-        // linear scan of TrafficManagement's large NodeInfo cache, so it must not run for every
-        // encrypted channel packet from an unknown sender - only for packets we might decrypt.
+        // Resolve the sender's key only for actual PKI-decrypt candidates, not every encrypted channel
+        // packet: copyPublicKeyForDecrypt() can fall through to a linear scan of TrafficManagement's large
+        // NodeInfo cache. It returns authoritative keys (hot/warm), or a cold-tier cache key only when it is
+        // signer-proven - an unverified TOFU cache key must not back authenticated (pki_encrypted, p->from)
+        // DM attribution.
         meshtastic_NodeInfoLite_public_key_t remotePublic = {0, {0}};
-        bool haveRemoteKey = nodeDB->copyPublicKey(p->from, remotePublic);
+        bool haveRemoteKey = nodeDB->copyPublicKeyForDecrypt(p->from, remotePublic);
         // A pending key is an unverified identity claim supplied by whoever opened the handshake, so it is
         // accepted only for the exchange itself (checked after decode). perhapsEncode applies the same rule.
         bool havePendingKey = false;