2 files changed, 15 insertions, 2 deletions
diff --git a/oxish-proto/src/key_exchange.rs b/oxish-proto/src/key_exchange.rs
index cbb8e2e..898fb46 100644
--- a/oxish-proto/src/key_exchange.rs
+++ b/oxish-proto/src/key_exchange.rs
@@ -614,8 +614,15 @@ impl KeyExchangeOutput {
         // Write the server's reply public value (`Q_S` / `S_REPLY`) to the exchange hash
         exchange.prefixed(&completed.public_key);
         let secret_bytes = completed.shared_secret.secret_bytes();
-        let mut shared_secret = Vec::with_capacity(secret_bytes.len() + 4);
-        secret_bytes.encode(&mut shared_secret);
+        let mut shared_secret = Vec::with_capacity(secret_bytes.len() + 5);
+        match negotiated.key_exchange {
+            // RFC 8731 section 3: `K` is the raw X25519 output encoded as an mpint
+            KeyExchangeAlgorithm::Curve25519Sha256 => {
+                encode_mpint(secret_bytes, &mut shared_secret)
+            }
+            // The PQ hybrid draft encodes `K` (a fixed-length hash output) as a string
+            _ => secret_bytes.encode(&mut shared_secret),
+        }
         exchange.update(&shared_secret);
 
         let exchange_hash = exchange.finish();
diff --git a/oxish-proto/src/named.rs b/oxish-proto/src/named.rs
index bc6dbea..4b24417 100644
--- a/oxish-proto/src/named.rs
+++ b/oxish-proto/src/named.rs
@@ -180,6 +180,10 @@ pub enum KeyExchangeAlgorithm<'a> {
     ///
     /// As defined in <https://datatracker.ietf.org/doc/draft-kampanakis-curdle-ssh-pq-ke/>.
     MlKem768X25519Sha256,
+    /// `curve25519-sha256` key exchange algorithm: ECDH using X25519
+    ///
+    /// As defined in <https://www.rfc-editor.org/rfc/rfc8731>.
+    Curve25519Sha256,
     /// A key exchange algorithm not known to this implementation
     Unknown(&'a str),
 }
@@ -188,6 +192,7 @@ impl<'a> Named<'a> for KeyExchangeAlgorithm<'a> {
     fn typed(name: &'a str) -> Self {
         match name {
             "mlkem768x25519-sha256" => Self::MlKem768X25519Sha256,
+            "curve25519-sha256" => Self::Curve25519Sha256,
             _ => Self::Unknown(name),
         }
     }
@@ -195,6 +200,7 @@ impl<'a> Named<'a> for KeyExchangeAlgorithm<'a> {
     fn name(&self) -> &str {
         match self {
             Self::MlKem768X25519Sha256 => "mlkem768x25519-sha256",
+            Self::Curve25519Sha256 => "curve25519-sha256",
             Self::Unknown(name) => name,
         }
     }