Budget the admin-key PKI decrypt fallback

src/mesh/Router.cpp

@@ -553,6 +553,54 @@ bool checkXeddsaReceivePolicy(meshtastic_MeshPacket *p)
 }
 #endif
 
+#if !(MESHTASTIC_EXCLUDE_PKI)
+// The fallback costs three X25519 ops before the AEAD tag is checked. Budget is global because p->from is
+// attacker-controlled; successful runs refund, and their key is then persisted for the fast path.
+#define ADMIN_KEY_FALLBACK_BURST 8
+#define ADMIN_KEY_FALLBACK_REFILL_MS 250
+
+static uint32_t adminKeyFallbackTokens = ADMIN_KEY_FALLBACK_BURST;
+static uint32_t adminKeyFallbackRefillMs = 0;
+
+static bool adminKeyFallbackAllowed()
+{
+    bool haveAdminKey = false;
+    for (int i = 0; i < 3; i++) {
+        if (config.security.admin_key[i].size == 32) {
+            haveAdminKey = true;
+            break;
+        }
+    }
+    if (!haveAdminKey)
+        return false; // nothing to try, so do not spend a token
+
+    uint32_t now = millis();
+    if (adminKeyFallbackRefillMs == 0)
+        adminKeyFallbackRefillMs = now;
+    uint32_t elapsed = now - adminKeyFallbackRefillMs;
+    if (elapsed >= ADMIN_KEY_FALLBACK_REFILL_MS) {
+        uint32_t refill = elapsed / ADMIN_KEY_FALLBACK_REFILL_MS;
+        adminKeyFallbackRefillMs += refill * ADMIN_KEY_FALLBACK_REFILL_MS;
+        if (refill >= ADMIN_KEY_FALLBACK_BURST - adminKeyFallbackTokens)
+            adminKeyFallbackTokens = ADMIN_KEY_FALLBACK_BURST;
+        else
+            adminKeyFallbackTokens += refill;
+    }
+
+    if (adminKeyFallbackTokens == 0)
+        return false;
+
+    adminKeyFallbackTokens--;
+    return true;
+}
+
+static void adminKeyFallbackRefund()
+{
+    if (adminKeyFallbackTokens < ADMIN_KEY_FALLBACK_BURST)
+        adminKeyFallbackTokens++;
+}
+#endif
+
 DecodeState perhapsDecode(meshtastic_MeshPacket *p)
 {
     concurrency::LockGuard g(cryptLock);
@@ -597,17 +645,21 @@ DecodeState perhapsDecode(meshtastic_MeshPacket *p)
             decrypted = true;
             viaPendingKey = havePendingKey;
         }
-        for (int i = 0; i < 3 && !decrypted; i++) {
-            if (config.security.admin_key[i].size != 32)
-                continue;
-            remotePublic.size = 32;
-            memcpy(remotePublic.bytes, config.security.admin_key[i].bytes, 32);
-
-            if (crypto->decryptCurve25519(p->from, remotePublic, p->id, rawSize, p->encrypted.bytes, bytes)) {
-                decrypted = true;
-                viaAdminKey = true;
-                break; // stop after first successful decryption
+        if (!decrypted && adminKeyFallbackAllowed()) {
+            for (int i = 0; i < 3 && !decrypted; i++) {
+                if (config.security.admin_key[i].size != 32)
+                    continue;
+                remotePublic.size = 32;
+                memcpy(remotePublic.bytes, config.security.admin_key[i].bytes, 32);
+
+                if (crypto->decryptCurve25519(p->from, remotePublic, p->id, rawSize, p->encrypted.bytes, bytes)) {
+                    decrypted = true;
+                    viaAdminKey = true;
+                    break; // stop after first successful decryption
+                }
             }
+            if (decrypted)
+                adminKeyFallbackRefund();
         }
         if (decrypted) {
             LOG_INFO("PKI Decryption worked!");

test/test_pki_admin_fallback/test_main.cpp

@@ -202,6 +202,35 @@ void test_wrong_admin_key_does_not_decode(void)
     TEST_ASSERT_NULL(mockNodeDB->getMeshNode(ADMIN_NODE));
 }
 
+// The fallback is budget-limited against flooding; see Router.cpp for why the budget is global.
+void test_admin_key_fallback_is_rate_limited(void)
+{
+    // Start from a full bucket regardless of what earlier tests consumed (8 tokens, one per 250ms).
+    delay(2500);
+
+    uint8_t otherPub[32], otherPriv[32];
+    crypto->generateKeyPair(otherPub, otherPriv);
+    crypto->setDHPrivateKey(ourPriv);
+    setAdminKey(0, otherPub); // wrong key, so every attempt below fails and keeps its token spent
+
+    // Drain the burst with undecryptable packets, as a flooding attacker would.
+    for (int i = 0; i < 8; i++) {
+        meshtastic_MeshPacket junk = makePkiPacket(ADMIN_NODE, meshtastic_PortNum_PRIVATE_APP, 16, adminPriv);
+        TEST_ASSERT_NOT_EQUAL(DECODE_SUCCESS, perhapsDecode(&junk));
+    }
+
+    // Budget exhausted: the fallback is skipped, so even a correct admin key does not decrypt.
+    setAdminKey(0, adminPub);
+    meshtastic_MeshPacket blocked = makePkiPacket(ADMIN_NODE, meshtastic_PortNum_PRIVATE_APP, 16, adminPriv);
+    TEST_ASSERT_NOT_EQUAL_MESSAGE(DECODE_SUCCESS, perhapsDecode(&blocked), "fallback should be budget-limited");
+
+    // The budget refills, so the throttle is not a permanent lockout.
+    delay(600);
+    meshtastic_MeshPacket allowed = makePkiPacket(ADMIN_NODE, meshtastic_PortNum_PRIVATE_APP, 16, adminPriv);
+    TEST_ASSERT_EQUAL_MESSAGE(DECODE_SUCCESS, perhapsDecode(&allowed), "budget should refill over time");
+    assertDecodedAndLearned(&allowed, adminPub);
+}
+
 // A pending key is an unverified identity claim from whoever opened a key-verification handshake, so it
 // must decrypt only the exchange itself. Otherwise they could send DMs that look PKI-authenticated as a
 // node they never proved they are.
@@ -243,6 +272,7 @@ void setup()
     RUN_TEST(test_admin_key_slot2_only_decrypts);
     RUN_TEST(test_no_admin_key_unknown_sender_not_decoded);
     RUN_TEST(test_wrong_admin_key_does_not_decode);
+    RUN_TEST(test_admin_key_fallback_is_rate_limited);
     RUN_TEST(test_pending_key_decrypts_only_key_verification);
 #endif
     exit(UNITY_END());