From a1460b0a2861d5bb34ba25d409b234468f269e4e Mon Sep 17 00:00:00 2001
From: tv <tv@krebsco.de>
Date: Sat, 24 Oct 2026 03:19:28 +0200
Subject: use nonce in handshake

To prevent stale hello replies to cause joins and thus stale sessions,
each run gets a random ID, which used as nonce in the handshake.
---
 src/swarm/peer.rs | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

(limited to 'src/swarm/peer.rs')

diff --git a/src/swarm/peer.rs b/src/swarm/peer.rs
index c8c86ac..13a980c 100644
--- a/src/swarm/peer.rs
+++ b/src/swarm/peer.rs
@@ -271,6 +271,7 @@ async fn transmit_hello(state: &AppState, peer: &str) -> Result<()> {
                 epoch: state.local_epoch(),
                 auth,
                 ready: state.is_ready().await,
+                run_id: state.run_id,
             }),
         )
         .await;
@@ -298,7 +299,8 @@ async fn ensure_handshake(state: &AppState, peer: &str) {
     }
 }
 
-async fn send_hello_reply(state: &AppState, peer: &str) -> Result<()> {
+/// Answers a HELLO that carried `run_id`.
+async fn send_hello_reply(state: &AppState, peer: &str, run_id: transport::RunId) -> Result<()> {
     let swarm_id = state
         .swarm_id()
         .await
@@ -335,6 +337,7 @@ async fn send_hello_reply(state: &AppState, peer: &str) -> Result<()> {
         epoch: state.local_epoch(),
         auth,
         ready,
+        run_id,
     };
 
     debug!(to = %peer, ready, "Sending HELLO_REPLY");
@@ -661,6 +664,15 @@ pub(crate) async fn handle_message(
         return Ok(());
     }
 
+    // A reply to a HELLO from an earlier run of ours (late, or queued in Mycelium while
+    // we were down) says nothing about now; the peer answers our current HELLO.
+    if let transport::ParsedMessage::HelloReply(reply) = &message {
+        if reply.run_id != state.run_id {
+            debug!(peer = %peer, "Ignoring a HELLO_REPLY to a HELLO from an earlier run");
+            return Ok(());
+        }
+    }
+
     if !admit(state, &peer, &message).await {
         return Ok(());
     }
@@ -700,7 +712,7 @@ pub(crate) async fn handle_message(
             }
             establish_peer(state, &peer, swarm_id, true, hello.ready).await?;
 
-            if let Err(error) = send_hello_reply(state, &peer).await {
+            if let Err(error) = send_hello_reply(state, &peer, hello.run_id).await {
                 // The peer retries its HELLO.
                 warn!(peer = %peer, error = %format!("{error:#}"), "HELLO_REPLY failed");
             }
-- 
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