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Budget the admin-key PKI decrypt fallback (#11100)

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This commit is contained in:
Thomas Göttgens
2026-07-21 09:48:09 +02:00
committed by GitHub
co-authored by GitHub
parent 6bff231d53
commit 7bdc2569e8
2 changed files with 91 additions and 9 deletions
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src/mesh/Router.cpp
+61 -9
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@@ -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 (!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 (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!");