Reduce key duplication by enabling hardware RNG (#8803)

* Reduce key duplication by enabling hardware RNG

* Apply suggestion from @Copilot

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Apply suggestion from @Copilot

Use micros() for worst case random seed for nrf52

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Minor cleanup, remove dead code and clarify comment

* trunk

* Add useRadioEntropy bool, default false.

---------

Co-authored-by: Ben Meadors <benmmeadors@gmail.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: Jonathan Bennett <jbennett@incomsystems.biz>

src/mesh/HardwareRNG.cpp

@@ -0,0 +1,159 @@
+#include "HardwareRNG.h"
+
+#include <algorithm>
+#include <cstring>
+#include <random>
+
+#include "configuration.h"
+
+#if HAS_RADIO
+#include "RadioLibInterface.h"
+#endif
+
+#if defined(ARCH_NRF52)
+#include <Adafruit_nRFCrypto.h>
+extern Adafruit_nRFCrypto nRFCrypto;
+#elif defined(ARCH_ESP32)
+#include <esp_system.h>
+#elif defined(ARCH_RP2040)
+#include <Arduino.h>
+#elif defined(ARCH_PORTDUINO)
+#include <random>
+#include <sys/random.h>
+#include <unistd.h>
+#endif
+
+namespace HardwareRNG
+{
+
+namespace
+{
+void fillWithRandomDevice(uint8_t *buffer, size_t length)
+{
+    std::random_device rd;
+    size_t offset = 0;
+    while (offset < length) {
+        uint32_t value = rd();
+        size_t toCopy = std::min(length - offset, sizeof(value));
+        memcpy(buffer + offset, &value, toCopy);
+        offset += toCopy;
+    }
+}
+
+#if HAS_RADIO
+bool mixWithLoRaEntropy(uint8_t *buffer, size_t length)
+{
+    // Only attempt to pull entropy from the modem if it is initialized and exposes the helper.
+    // When the radio stack is disabled or has not yet been configured, we simply skip this step
+    // and return false so callers know no extra mixing occurred.
+    RadioLibInterface *radio = RadioLibInterface::instance;
+    if (!radio) {
+        LOG_ERROR("No radio instance available to provide entropy");
+        return false;
+    }
+
+    constexpr size_t chunkSize = 16;
+    uint8_t scratch[chunkSize];
+    size_t offset = 0;
+    bool mixed = false;
+
+    while (offset < length) {
+        size_t toCopy = std::min(length - offset, chunkSize);
+
+        // randomBytes() returns false if the modem does not support it or is not ready
+        // (for instance, when the radio is powered down). We break immediately to avoid
+        // blocking or returning partially-filled entropy and simply report failure.
+        if (!radio->randomBytes(scratch, toCopy)) {
+            break;
+        }
+
+        for (size_t i = 0; i < toCopy; ++i) {
+            buffer[offset + i] ^= scratch[i];
+        }
+
+        mixed = true;
+        offset += toCopy;
+    }
+
+    // Avoid leaving the modem-sourced bytes sitting on the stack longer than needed.
+    if (mixed) {
+        memset(scratch, 0, sizeof(scratch));
+    }
+
+    return mixed;
+}
+#endif
+} // namespace
+
+bool fill(uint8_t *buffer, size_t length, bool useRadioEntropy)
+{
+    if (!buffer || length == 0) {
+        return false;
+    }
+
+    bool filled = false;
+
+#if defined(ARCH_NRF52)
+    // The Nordic SDK RNG provides cryptographic-quality randomness backed by hardware.
+    nRFCrypto.begin();
+    auto result = nRFCrypto.Random.generate(buffer, length);
+    nRFCrypto.end();
+    filled = result;
+#elif defined(ARCH_ESP32)
+    // ESP32 exposes a true RNG via esp_fill_random().
+    esp_fill_random(buffer, length);
+    filled = true;
+#elif defined(ARCH_RP2040)
+    // RP2040 has a hardware random number generator accessible through the Arduino core.
+    size_t offset = 0;
+    while (offset < length) {
+        uint32_t value = rp2040.hwrand32();
+        size_t toCopy = std::min(length - offset, sizeof(value));
+        memcpy(buffer + offset, &value, toCopy);
+        offset += toCopy;
+    }
+    filled = true;
+#elif defined(ARCH_PORTDUINO)
+    // Prefer the host OS RNG first when running under Portduino.
+    ssize_t generated = ::getrandom(buffer, length, 0);
+    if (generated == static_cast<ssize_t>(length)) {
+        filled = true;
+    }
+
+    if (!filled) {
+        fillWithRandomDevice(buffer, length);
+        filled = true;
+    }
+#endif
+
+    if (!filled) {
+        // As a last resort, fall back to std::random_device. This should only be reached
+        // if a platform-specific source was unavailable.
+        fillWithRandomDevice(buffer, length);
+        filled = true;
+    }
+
+#if HAS_RADIO
+    if (useRadioEntropy) {
+        // Best-effort: if the radio is active and can provide modem entropy, XOR it over the
+        // buffer to improve overall quality. We consider the filling a success if either a
+        // good platform RNG or the modem RNG provided data, so we return true as long as at
+        // least one of those steps succeeded.
+        filled = mixWithLoRaEntropy(buffer, length) || filled;
+    }
+#endif
+
+    return filled;
+}
+
+bool seed(uint32_t &seedOut)
+{
+    uint32_t candidate = 0;
+    if (!fill(reinterpret_cast<uint8_t *>(&candidate), sizeof(candidate), true)) {
+        return false;
+    }
+    seedOut = candidate;
+    return true;
+}
+
+} // namespace HardwareRNG