security: use OS RNG for session ID generation

lightway-core/src/connection.rs

@@ -1158,7 +1158,8 @@ impl<AppState: Send> Connection<AppState> {
                 ref mut pending_session_id,
                 ..
             } => {
-                let new_session_id = self.rng.lock().unwrap().random();
+                // Use OS RNG directly for cryptographic security
+                let new_session_id = wire::secure_random_session_id();
 
                 self.session.io_cb_mut().set_session_id(new_session_id);
 

lightway-core/src/lib.rs

@@ -58,7 +58,7 @@ pub use utils::{
     tcp_adjust_packet_checksum, udp_adjust_packet_checksum,
 };
 pub use version::Version;
-pub use wire::{AuthMethod, Header, SessionId};
+pub use wire::{secure_random_session_id, AuthMethod, Header, SessionId};
 
 /// Default MTU size for a packet on the outside path (on the wire)
 pub const MAX_OUTSIDE_MTU: usize = 1500;

lightway-core/src/wire.rs

@@ -42,6 +42,7 @@ use crate::borrowed_bytesmut::BorrowedBytesMut;
 use bytes::{Buf, BufMut, BytesMut};
 use more_asserts::*;
 use num_enum::{IntoPrimitive, TryFromPrimitive};
+use rand::rngs::OsRng;
 use rand::Rng;
 
 // A module for each frame type with a payload
@@ -160,6 +161,24 @@ impl rand::distr::Distribution<SessionId> for rand::distr::StandardUniform {
     }
 }
 
+/// Generate a cryptographically secure session ID using OS RNG.
+///
+/// This function uses the operating system's secure random number generator
+/// (typically /dev/urandom on Unix or CryptGenRandom on Windows) which is
+/// suitable for cryptographic purposes.
+pub fn secure_random_session_id() -> SessionId {
+    let mut bytes = [0u8; 8];
+    OsRng.fill_bytes(&mut bytes);
+    let session_id = SessionId(bytes);
+
+    // Ensure we don't return a reserved value
+    if session_id.is_reserved() {
+        secure_random_session_id()
+    } else {
+        session_id
+    }
+}
+
 impl std::fmt::Debug for SessionId {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         let n = u64::from_be_bytes(self.0);