From de2633b8bca88d877c815271c9860855dd35cd64 Mon Sep 17 00:00:00 2001
From: R0ck <random@email.com>
Date: Wed, 9 Sep 2026 08:06:40 +0100
Subject: [PATCH] bridge: each node draws its own receiver noise

The chip model's noise seed was never set, so every native node ran the
same receiver-noise stream from the same counter. That stream is what a node
seeds its RNG from - fast_rng.begin(radio.getRngSeed()) reads it - so every
node's CSMA backoff and advert jitter were correlated across the whole mesh,
and any result that depends on two nodes not choosing the same slot was
optimistic or pessimistic in a way nothing measured.

The chip's noise seed now defaults to the node's identity seed, which the
simulator already makes distinct per node, XORed so it is not literally the
number the keypair is also derived from. --noise-seed overrides it for a
caller that wants to set it directly. This is the native half of the per-node
noise the emulated backend already had (MeshBench/meshbench#556).

MeshBench/meshbench#720

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Q9HbD44EKWWTRYgxbFGxf6
---
 bridge/main.cpp | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/bridge/main.cpp b/bridge/main.cpp
index 2545af2..df0902f 100644
--- a/bridge/main.cpp
+++ b/bridge/main.cpp
@@ -53,6 +53,15 @@ void loop();
 // Defined by the host variant.
 extern uint32_t g_sim_millis;
 int g_stuck_irq_ms = 0;
+// The receiver-noise stream the chip model draws from. Distinct per node so
+// two nodes do not read the same "random" bytes at the same instants - which
+// is what seeds each node's RNG, its CSMA backoff and its advert jitter. Left
+// at 0 the whole mesh drew one stream and every node's timing was correlated.
+// Defaults to the identity seed, which the simulator already makes distinct
+// per node, so a node differs without the simulator having to send anything;
+// XORed so it is not literally the number the keypair is also derived from.
+uint64_t g_noise_seed = 0;
+bool g_noise_seed_set = false;
 extern int g_sf;
 extern float g_bwKHz;
 extern int g_cr;
@@ -181,6 +190,7 @@ int main(int argc, char** argv) {
     // identity. The simulator sends a value that fits in 32 bits either way,
     // so a seed it sends means the same thing on every platform.
     else if (!strcmp(argv[i], "--seed")) g_identity_seed = strtoull(next(), nullptr, 10);
+    else if (!strcmp(argv[i], "--noise-seed")) { g_noise_seed = strtoull(next(), nullptr, 10); g_noise_seed_set = true; }
     else if (!strcmp(argv[i], "--sf")) g_sf = atoi(next());
     else if (!strcmp(argv[i], "--bw-khz")) g_bwKHz = (float)atof(next());
     else if (!strcmp(argv[i], "--cr")) g_cr = atoi(next());
@@ -217,6 +227,10 @@ int main(int argc, char** argv) {
   // Only if asked for on the command line. A build compiled as a faulty
   // variant already has its value, and overwriting it with the flag default
   // silently turned that whole variant back into a well-behaved one.
+  // Per-node receiver noise, so the mesh does not run on one shared stream.
+  sim_hal.chip().setNoiseSeed(g_noise_seed_set ? g_noise_seed
+                                               : (g_identity_seed ^ 0x9E3779B97F4A7C15ULL));
+
   if (g_stuck_irq_ms > 0) sim_hal.chip().setStuckIrqMs((uint32_t)g_stuck_irq_ms);
   setup();
   drainTx();