feat: send psk_ke_modes ext in first flight

bindings/rust/bench/src/harness.rs

@@ -506,8 +506,7 @@ mod tests {
             .try_init()
             .unwrap();
         session_resumption::<S2NConnection, S2NConnection>();
-        // https://github.com/aws/s2n-tls/issues/4124
-        // session_resumption::<S2NConnection, RustlsConnection>();
+        session_resumption::<S2NConnection, RustlsConnection>();
         session_resumption::<S2NConnection, OpenSslConnection>();
 
         session_resumption::<RustlsConnection, RustlsConnection>();

tests/unit/s2n_psk_key_exchange_modes_extension_test.c

@@ -221,6 +221,48 @@ int main(int argc, char **argv)
         };
     };
 
+    /* Test: s2n_psk_key_exchange_modes_should_send */
+    {
+        /* When neither resumption nor PSKs are enabled, the extension should not be sent. */
+        {
+            DEFER_CLEANUP(struct s2n_config *no_resumption_config = s2n_config_new(), s2n_config_ptr_free);
+            EXPECT_NOT_NULL(no_resumption_config);
+            EXPECT_SUCCESS(s2n_config_set_session_tickets_onoff(no_resumption_config, false));
+
+            DEFER_CLEANUP(struct s2n_connection *conn = s2n_connection_new(S2N_CLIENT), s2n_connection_ptr_free);
+            EXPECT_SUCCESS(s2n_connection_set_config(conn, no_resumption_config));
+
+            EXPECT_FALSE(s2n_psk_key_exchange_modes_extension.should_send(conn));
+        };
+
+        /* When session resumption is enabled, the extension should be sent. */
+        {
+            DEFER_CLEANUP(struct s2n_config *resumption_config = s2n_config_new(), s2n_config_ptr_free);
+            EXPECT_NOT_NULL(resumption_config);
+            EXPECT_SUCCESS(s2n_config_set_session_tickets_onoff(resumption_config, true));
+
+            DEFER_CLEANUP(struct s2n_connection *conn = s2n_connection_new(S2N_CLIENT), s2n_connection_ptr_free);
+            EXPECT_SUCCESS(s2n_connection_set_config(conn, resumption_config));
+
+            EXPECT_TRUE(s2n_psk_key_exchange_modes_extension.should_send(conn));
+        };
+
+        /* When a client is using out-of-band PSKs, the extension should be sent. */
+        {
+            DEFER_CLEANUP(struct s2n_config *psk_config = s2n_config_new(), s2n_config_ptr_free);
+            EXPECT_NOT_NULL(psk_config);
+            EXPECT_SUCCESS(s2n_config_set_session_tickets_onoff(psk_config, false));
+
+            DEFER_CLEANUP(struct s2n_connection *conn = s2n_connection_new(S2N_CLIENT), s2n_connection_ptr_free);
+            EXPECT_SUCCESS(s2n_connection_set_config(conn, psk_config));
+
+            DEFER_CLEANUP(struct s2n_psk *psk = s2n_test_psk_new(conn), s2n_psk_free);
+            EXPECT_SUCCESS(s2n_connection_append_psk(conn, psk));
+
+            EXPECT_TRUE(s2n_psk_key_exchange_modes_extension.should_send(conn));
+        };
+    };
+
     /* Functional test */
     {
         struct s2n_stuffer out = { 0 };

tls/extensions/s2n_psk_key_exchange_modes.c

@@ -38,8 +38,20 @@ const s2n_extension_type s2n_psk_key_exchange_modes_extension = {
 
 static bool s2n_psk_key_exchange_modes_should_send(struct s2n_connection *conn)
 {
-    /* Only send a psk_key_exchange_modes extension if psks available */
-    return conn->psk_params.psk_list.len > 0;
+    /**
+     *= https://tools.ietf.org/rfc/rfc8446#section-4.2.9
+     *# Servers MUST NOT select a key exchange mode that is not listed by the
+     *# client.  This extension also restricts the modes for use with PSK
+     *# resumption.
+     *
+     * The RFC is ambiguous about whether the psk_kx_modes extension should be
+     * sent in the first flight of messages, but we choose to do so for
+     * interoperability with other TLS implementations.
+     * https://github.com/aws/s2n-tls/issues/4124
+     * The final check for psk_list.len > 0 is necessary because external
+     * PSKs are used without setting `use_tickets` to true.
+     */
+    return conn->config->use_tickets || conn->psk_params.psk_list.len > 0;
 }
 
 static int s2n_psk_key_exchange_modes_send(struct s2n_connection *conn, struct s2n_stuffer *out)