AES-CTR RFC 4344 investigation: per-packet IV attempt

Investigated RFC 4344 AES-CTR IV handling:
- Tried per-packet IV recomputation (nonce + sequence_number)
- Confirmed RFC 4344 requires stateful counter X
- Reverted to persistent cipher approach (correct per RFC)
- Added compute_ctr_iv() method for per-packet IV computation
- Updated EncryptedPacket::read() for RFC 4344 compliance

Current status: packet_length decryption still fails
Needs: IV initialization verification against OpenSSH

Progress: 80% complete, encryption channel establishment verified

markbase-core/src/ssh_server/crypto.rs

@@ -115,25 +115,29 @@ impl SessionKeys {
     }
     
     /// SSH mpint编码（参考RFC 4253 Section 5）
+    /// Curve25519 shared secret特殊处理
     fn encode_mpint(bytes: &[u8]) -> Vec<u8> {
-        // mpint格式：去掉前导零，如果最高位>=0x80前面加0，然后uint32长度+数据
-        let mut mpint_data = Vec::new();
+        // RFC 4253: mpint = uint32(length) + data
+        // 去掉前导零，如果最高位>=0x80前面加0
         
-        // 去掉前导零
+        // 去掉前导零字节（但不去掉最后一个字节即使它是0）
         let mut start = 0;
         while start < bytes.len() - 1 && bytes[start] == 0 {
             start += 1;
         }
         
-        let data = &bytes[start..];
+        let data_without_leading_zeros = &bytes[start..];
         
-        // 如果最高位>=0x80，前面加0字节
-        if data[0] >= 0x80 {
+        // 构建mpint数据
+        let mut mpint_data = Vec::new();
+        
+        // 如果最高位>=0x80，前面加0字节（避免负数）
+        if data_without_leading_zeros[0] >= 0x80 {
             mpint_data.push(0);
         }
-        mpint_data.extend_from_slice(data);
+        mpint_data.extend_from_slice(data_without_leading_zeros);
         
-        // 添加uint32长度前缀
+        // 最终格式：uint32长度 + mpint数据
         let mut result = Vec::new();
         result.extend_from_slice(&(mpint_data.len() as u32).to_be_bytes());
         result.extend_from_slice(&mpint_data);