fix: handshake padding wire format (breaking fix)

The previous commit added random padding after Noise handshake payloads
but the receiver passed the entire raw buffer (including padding) to
snow::read_handshake(), which cannot handle trailing bytes.

New wire format:
  [session_id:4][noise_len:2][noise_payload:N][random_padding:32-128]

Changes:
- wrap_datagram_handshake: puts noise_len (u16 BE) at bytes [4..6]
  before the Noise payload, followed by 32-128 random padding bytes
- handle_inbound: reads noise_len from [4..6], passes only
  raw_vec[6..6+noise_len] to snow, ignoring trailing padding
- obfuscation: handshake mask extended from 4 to 6 bytes to also
  cover the noise_len field (prevents DPI from seeing constant u16)
- dispatcher: key-trial loop updated to deobfuscate 6-byte header

Both client and server now produce/consume the same padded format.

ostp-core/src/crypto/obfuscation.rs

@@ -23,14 +23,15 @@ pub fn derive_psk(access_key: &[u8]) -> [u8; 32] {
     psk
 }
 
-/// Derives a unique 4-byte session_id mask using HMAC-SHA256(key, nonce).
-/// Used strictly for handshake phase obfuscation.
-fn derive_session_mask(key: &[u8; 8], nonce: u64) -> [u8; 4] {
+/// Derives a 6-byte handshake mask using HMAC-SHA256(key, nonce).
+/// Covers session_id (4 bytes) + noise_len (2 bytes) to prevent
+/// DPI from seeing a constant length field in the handshake header.
+fn derive_handshake_mask(key: &[u8; 8], nonce: u64) -> [u8; 6] {
     let mut mac = HmacSha256::new_from_slice(key).expect("HMAC accepts any key length");
     mac.update(&nonce.to_be_bytes());
     let result = mac.finalize().into_bytes();
-    let mut mask = [0u8; 4];
-    mask.copy_from_slice(&result[..4]);
+    let mut mask = [0u8; 6];
+    mask.copy_from_slice(&result[..6]);
     mask
 }
 
@@ -61,10 +62,10 @@ pub fn obfuscate_packet_inplace(raw: &mut [u8], key: &[u8; 8], is_handshake: boo
                 raw[i] ^= mask_result[i];
             }
         }
-    } else if raw.len() >= 4 {
-        // Handshake packets: mask session_id with a fixed handshake-phase mask
-        let mask = derive_session_mask(key, u64::MAX);
-        for i in 0..4 {
+    } else if raw.len() >= 6 {
+        // Handshake: mask session_id (4 bytes) + noise_len (2 bytes)
+        let mask = derive_handshake_mask(key, u64::MAX);
+        for i in 0..6 {
             raw[i] ^= mask[i];
         }
     }
@@ -90,8 +91,8 @@ pub fn deobfuscate_header_inplace(
             header[i] ^= mask_result[i];
         }
     } else {
-        let mask = derive_session_mask(key, u64::MAX);
-        for i in 0..4 {
+        let mask = derive_handshake_mask(key, u64::MAX);
+        for i in 0..header.len().min(6) {
             header[i] ^= mask[i];
         }
     }
@@ -104,9 +105,9 @@ pub fn deobfuscate_packet_inplace(raw: &mut [u8], key: &[u8; 8], is_handshake: b
         header.copy_from_slice(header_slice);
         deobfuscate_header_inplace(&mut header, ciphertext, key, is_handshake);
         header_slice.copy_from_slice(&header);
-    } else if raw.len() >= 4 {
-        let mask = derive_session_mask(key, u64::MAX);
-        for i in 0..4 {
+    } else if raw.len() >= 6 {
+        let mask = derive_handshake_mask(key, u64::MAX);
+        for i in 0..6 {
             raw[i] ^= mask[i];
         }
     }

ostp-core/src/protocol.rs

@@ -214,8 +214,20 @@ impl ProtocolMachine {
         }
 
         if self.state == OstpState::Handshaking {
+            // Wire format: [session_id:4][noise_len:2][noise_payload:N][random_padding:*]
+            // Extract noise_len to pass exactly the right bytes to snow
+            if raw_vec.len() < 6 {
+                return Err(ProtocolError::Framing("handshake too short for length prefix".to_string()));
+            }
+            let noise_len = u16::from_be_bytes([raw_vec[4], raw_vec[5]]) as usize;
+            if raw_vec.len() < 6 + noise_len {
+                return Err(ProtocolError::Framing(format!(
+                    "handshake truncated: expected {} noise bytes, got {}",
+                    noise_len, raw_vec.len() - 6
+                )));
+            }
             let mut read_out = vec![0_u8; 1024];
-            let n = self.noise.read_handshake(&raw_vec[4..], &mut read_out)?;
+            let n = self.noise.read_handshake(&raw_vec[6..6 + noise_len], &mut read_out)?;
 
             let response = match self.role {
                 NoiseRole::Responder => {
@@ -380,15 +392,17 @@ impl ProtocolMachine {
         // Anti-DPI: add random padding after the Noise payload to prevent
         // size fingerprinting. Without this, every handshake is exactly 52 bytes
         // which is trivially detectable by TSPU/DPI systems.
+        //
+        // Wire format: [session_id:4][noise_len:2][noise_payload:N][random_padding:32-128]
         let pad_len: usize = rand::thread_rng().gen_range(32..=128);
         let mut pad = vec![0u8; pad_len];
         rand::thread_rng().fill(&mut pad[..]);
 
-        let mut out = Vec::with_capacity(4 + noise_payload.len() + 2 + pad_len);
+        let noise_len = noise_payload.len() as u16;
+        let mut out = Vec::with_capacity(4 + 2 + noise_payload.len() + pad_len);
         out.extend_from_slice(&self.session_id.to_be_bytes());
+        out.extend_from_slice(&noise_len.to_be_bytes());
         out.extend_from_slice(noise_payload);
-        // 2-byte padding length prefix so receiver can strip it
-        out.extend_from_slice(&(pad_len as u16).to_be_bytes());
         out.extend_from_slice(&pad);
         crate::crypto::obfuscate_packet_inplace(&mut out, &self.obfuscation_key, true);
         Ok(Bytes::from(out))

ostp-server/src/dispatcher.rs

@@ -158,10 +158,12 @@ impl Dispatcher {
             let psk = ostp_core::crypto::derive_psk(candidate_key.as_bytes());
 
             // Decode the session_id using this key's obfuscation
-            let mut header = [0u8; 4];
-            header.copy_from_slice(&packet[0..4]);
+            // Handshake header is 6 bytes: [session_id:4][noise_len:2]
+            let mut header = [0u8; 6];
+            if packet.len() < 6 { continue; }
+            header.copy_from_slice(&packet[0..6]);
             ostp_core::crypto::deobfuscate_packet_inplace(&mut header, &obf_key, true);
-            let candidate_session_id = u32::from_be_bytes(header);
+            let candidate_session_id = u32::from_be_bytes([header[0], header[1], header[2], header[3]]);
 
             let mut cfg = self.machine_config.clone();
             cfg.session_id = candidate_session_id;