kx: drop the advertisement echo

signed_version settles the server's version inside the key exchange, so
an echo of it on the messages after has nothing left to confirm. With it
go RendezvousMessage.kx_advertised, the check the transport ran on each
decrypted frame, and the tests of both.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019aokqJuhjvB3kijXtAg5Ns

protos/rendezvous.proto

@@ -299,7 +299,4 @@ message RendezvousMessage {
     HttpProxyResponse http_proxy_response = 28;
     IceCandidate ice_candidate = 29;
   }
-  // From the server, on every message after a key exchange: the version it advertised.
-  // Absent from servers that predate versions.
-  uint32 kx_advertised = 30;
 }

src/stream.rs

@@ -100,18 +100,6 @@ impl Stream {
         }
     }
 
-    /// Refuse the stream if an encrypted message from the server says it advertised a key
-    /// exchange version other than `seen`, the one this side read from the key exchange.
-    #[inline]
-    pub fn check_kx_advertised(&mut self, seen: u32) {
-        match self {
-            #[cfg(feature = "webrtc")]
-            Stream::WebRTC(_) => {}
-            Stream::WebSocket(s) => s.check_kx_advertised(seen),
-            Stream::Tcp(s) => s.check_kx_advertised(seen),
-        }
-    }
-
     /// An opaque token that changes whenever bytes have arrived from the peer, a fragment of a
     /// message that is still incomplete included. Compare successive samples; the value itself
     /// means nothing. `None` where the transport reports no such thing.

src/tcp.rs

@@ -54,10 +54,9 @@ pub struct KxTranscript<'a> {
     pub picked: u32,
 }
 
-/// The sending key, the send and receive counters, the receiving key where it differs, and
+/// The sending key, the send and receive counters, and the receiving key where it differs.
-/// the server's advertised version as this side saw it, while its echo is still awaited.
 #[derive(Clone)]
-pub struct Encrypt(pub Key, pub u64, pub u64, Option<Key>, Option<u32>);
+pub struct Encrypt(pub Key, pub u64, pub u64, Option<Key>);
 
 pub struct FramedStream(
     pub Framed<DynTcpStream, BytesCodec>,
@@ -242,12 +241,6 @@ impl FramedStream {
         Ok(())
     }
 
-    pub fn check_kx_advertised(&mut self, seen: u32) {
-        if let Some(enc) = self.2.as_mut() {
-            enc.check_kx_advertised(seen);
-        }
-    }
-
     fn get_nonce(seqnum: u64) -> Nonce {
         let mut nonce = Nonce([0u8; secretbox::NONCEBYTES]);
         nonce.0[..std::mem::size_of_val(&seqnum)].copy_from_slice(&seqnum.to_le_bytes());
@@ -343,35 +336,7 @@ impl<R: AsyncRead + AsyncWrite + Unpin> TcpStreamTrait for R {}
 
 impl Encrypt {
     pub fn new(key: Key) -> Self {
-        Self(key, 0, 0, None, None)
+        Self(key, 0, 0, None)
-    }
-
-    /// Arm the check of the server's advertisement echo, `seen` being the version this side
-    /// read from the key exchange. Call once the key is set and before the first frame is read.
-    pub fn check_kx_advertised(&mut self, seen: u32) {
-        self.4 = Some(seen);
-    }
-
-    /// The server repeats what it advertised inside its encrypted messages. Returns whether the
-    /// echo settled the question: a frame that carries none says nothing either way, and leaves
-    /// the check armed for the frames after it.
-    fn check_kx_advertised_echo(plain: &[u8], seen: u32) -> Result<bool, Error> {
-        let echoed = crate::rendezvous_proto::RendezvousMessage::parse_from_bytes(plain)
-            .map(|m| m.kx_advertised)
-            .unwrap_or(0);
-        if echoed == 0 {
-            return Ok(false);
-        }
-        if echoed != seen {
-            return Err(Error::new(
-                ErrorKind::Other,
-                format!(
-                    "key exchange advertised {} but {} arrived: altered in transit",
-                    echoed, seen
-                ),
-            ));
-        }
-        Ok(true)
     }
 
     /// Version 1: the initiator, the side that sent the sealed key, sends under one subkey and
@@ -380,9 +345,9 @@ impl Encrypt {
         let initiator = Self::derive_subkey(&key, KX_SPLIT_INITIATOR, t)?;
         let responder = Self::derive_subkey(&key, KX_SPLIT_RESPONDER, t)?;
         Ok(if is_initiator {
-            Self(initiator, 0, 0, Some(responder), None)
+            Self(initiator, 0, 0, Some(responder))
         } else {
-            Self(responder, 0, 0, Some(initiator), None)
+            Self(responder, 0, 0, Some(initiator))
         })
     }
 
@@ -418,11 +383,6 @@ impl Encrypt {
         let nonce = FramedStream::get_nonce(self.2);
         match secretbox::open(bytes, &nonce, self.3.as_ref().unwrap_or(&self.0)) {
             Ok(res) => {
-                if let Some(seen) = self.4 {
-                    if Self::check_kx_advertised_echo(&res, seen)? {
-                        self.4 = None;
-                    }
-                }
                 bytes.clear();
                 bytes.put_slice(&res);
                 Ok(())
@@ -551,59 +511,4 @@ mod tests {
         };
         assert!(Encrypt::new_split(secretbox::gen_key(), true, &t).is_err());
     }
-
-    fn rendezvous_frame(kx_advertised: u32) -> Vec<u8> {
-        let mut msg = crate::rendezvous_proto::RendezvousMessage::new();
-        msg.set_hc(Default::default());
-        msg.kx_advertised = kx_advertised;
-        msg.write_to_bytes().unwrap()
-    }
-
-    fn first_frame_after(seen: u32, echoed: u32) -> Result<(), Error> {
-        let key = secretbox::gen_key();
-        let mut server = Encrypt::new(key.clone());
-        let mut client = Encrypt::new(key);
-        client.check_kx_advertised(seen);
-        // An empty heartbeat is not a frame, and must not use up the check.
-        let mut empty = BytesMut::new();
-        client.dec(&mut empty).unwrap();
-        let mut buf = BytesMut::from(&server.enc(&rendezvous_frame(echoed))[..]);
-        client.dec(&mut buf)
-    }
-
-    #[test]
-    fn test_kx_advertised_echo_must_match_what_was_seen() {
-        assert!(first_frame_after(KX_VERSION_LATEST, KX_VERSION_LATEST).is_ok());
-        // A server from before versions echoes nothing, and advertised nothing.
-        assert!(first_frame_after(0, 0).is_ok());
-        // The echo differs from what this side saw.
-        assert!(first_frame_after(0, KX_VERSION_LATEST).is_err());
-        assert!(first_frame_after(KX_VERSION_LATEST, KX_VERSION_LATEST + 1).is_err());
-    }
-
-    #[test]
-    fn test_kx_advertised_check_survives_an_untagged_frame() {
-        let key = secretbox::gen_key();
-        let mut server = Encrypt::new(key.clone());
-        let mut client = Encrypt::new(key);
-        client.check_kx_advertised(KX_VERSION_LATEST);
-        // A message the server does not tag says nothing about the advertisement.
-        let mut untagged = BytesMut::from(&server.enc(&rendezvous_frame(0))[..]);
-        client.dec(&mut untagged).unwrap();
-        // So the frame that does carry an echo is still held to it.
-        let mut tagged = BytesMut::from(&server.enc(&rendezvous_frame(KX_VERSION_LATEST + 1))[..]);
-        assert!(client.dec(&mut tagged).is_err());
-    }
-
-    #[test]
-    fn test_kx_advertised_check_is_one_shot() {
-        let key = secretbox::gen_key();
-        let mut server = Encrypt::new(key.clone());
-        let mut client = Encrypt::new(key);
-        client.check_kx_advertised(KX_VERSION_LATEST);
-        let mut first = BytesMut::from(&server.enc(&rendezvous_frame(KX_VERSION_LATEST))[..]);
-        client.dec(&mut first).unwrap();
-        let mut later = BytesMut::from(&server.enc(&rendezvous_frame(KX_VERSION_LATEST + 9))[..]);
-        client.dec(&mut later).unwrap();
-    }
 }

src/websocket.rs

@@ -268,13 +268,6 @@ impl WsFramedStream {
         Ok(())
     }
 
-    #[inline]
-    pub fn check_kx_advertised(&mut self, seen: u32) {
-        if let Some(enc) = self.encrypt.as_mut() {
-            enc.check_kx_advertised(seen);
-        }
-    }
-
     #[inline]
     pub fn is_secured(&self) -> bool {
         self.encrypt.is_some()