Implement SSH Compression Phase 3: Actual packet compression

- EncryptedPacket::new(): compress payload before encryption
- EncryptedPacket::read(): decompress payload after decryption
- Apply to AES-GCM, ChaCha20-Poly1305, and AES-CTR modes
- Compression order: compress → encrypt (write)
- Decompression order: decrypt → decompress (read)

All 179 tests pass.

markbase-core/src/ssh_server/cipher.rs

@@ -278,6 +278,7 @@ pub struct EncryptedPacket {
 impl EncryptedPacket {
     /// 创建加密packet（参考OpenSSH cipher.c）
     /// Phase 1: 支持 AES-CTR (MtE) 和 AES-GCM (AEAD) 两种模式
+    /// Phase 3: 支持压缩（压缩发生在加密之前）
     pub fn new(
         plaintext_payload: &[u8],
         encryption_ctx: &mut EncryptionContext,
@@ -285,8 +286,23 @@ impl EncryptedPacket {
     ) -> Result<Self> {
         let block_size = 16;
         let min_padding = 4;
-
-        let payload_length = plaintext_payload.len();
+        
+        // Phase 3: 压缩 payload（如果启用）
+        // 压缩顺序：压缩 → 加密（参考 RFC 4253 §6.2）
+        let compressed_payload = if is_server_to_client {
+            // Server → Client: 使用 compression_stoc
+            if encryption_ctx.compression_stoc.is_enabled() {
+                info!("Compressing payload (server→client, {} bytes)", plaintext_payload.len());
+                encryption_ctx.compression_stoc.compress(plaintext_payload)?
+            } else {
+                plaintext_payload.to_vec()
+            }
+        } else {
+            // Client → Server: 使用 compression_ctos（server 解压缩，这里不压缩）
+            plaintext_payload.to_vec()
+        };
+        
+        let payload_length = compressed_payload.len();
 
         // Padding calculation:
         // AES-GCM: RFC 4253 body (padding_length + payload + padding = packet_length) must be % 16 == 0
@@ -321,7 +337,7 @@ impl EncryptedPacket {
             let total_plaintext_size = 1 + payload_length + padding_length as usize;
             let mut plaintext_payload_buffer = SshBuf::with_capacity(total_plaintext_size);
             plaintext_payload_buffer.put(&[padding_length])?;
-            plaintext_payload_buffer.put(plaintext_payload)?;
+            plaintext_payload_buffer.put(&compressed_payload)?;
             
             let mut random_padding = vec![0u8; padding_length as usize];
             use rand::RngCore;
@@ -421,7 +437,7 @@ impl EncryptedPacket {
             let total_plaintext_size = 1 + payload_length + padding_length as usize;
             let mut plaintext_payload_buffer = SshBuf::with_capacity(total_plaintext_size);
             plaintext_payload_buffer.put(&[padding_length])?;
-            plaintext_payload_buffer.put(plaintext_payload)?;
+            plaintext_payload_buffer.put(&compressed_payload)?;
             
             let mut random_padding = vec![0u8; padding_length as usize];
             use rand::RngCore;
@@ -505,7 +521,7 @@ impl EncryptedPacket {
             let mut plaintext_packet = SshBuf::with_capacity(total_packet_size);
             plaintext_packet.put(&(packet_length as u32).to_be_bytes())?;
             plaintext_packet.put(&[padding_length])?;
-            plaintext_packet.put(plaintext_payload)?;
+            plaintext_packet.put(&compressed_payload)?;
 
             let mut random_padding = vec![0u8; padding_length as usize];
             use rand::RngCore;
@@ -705,7 +721,23 @@ impl EncryptedPacket {
             
             info!("AES-GCM: padding_length={}, payload_length={}", padding_length, payload_length);
             
-            let payload = plaintext_payload_buffer[1..1 + payload_length].to_vec();
+            let compressed_payload = plaintext_payload_buffer[1..1 + payload_length].to_vec();
+            
+            // Phase 3: 解压缩 payload（如果启用）
+            // 解压缩顺序：解密 → 解压缩（参考 RFC 4253 §6.2）
+            let payload = if is_client_to_server {
+                // Client → Server: 使用 compression_ctos
+                if encryption_ctx.compression_ctos.is_enabled() {
+                    info!("Decompressing payload (client→server, {} bytes)", compressed_payload.len());
+                    encryption_ctx.compression_ctos.decompress(&compressed_payload)?
+                } else {
+                    compressed_payload
+                }
+            } else {
+                // Server → Client: 使用 compression_stoc（client 解压缩，这里不解压缩）
+                compressed_payload
+            };
+            
             let padding = Vec::new();  // AES-GCM: padding 不需要存储（write 时使用 payload 中的 ciphertext）
 
             // 9. 提取 GCM tag (last 16 bytes of ciphertext)
@@ -917,6 +949,23 @@ impl EncryptedPacket {
             let mut payload = Vec::with_capacity(payload_length);
             payload.extend_from_slice(payload_part1);
             payload.extend_from_slice(payload_part2);
+            
+            // Phase 3: 解压缩 payload（如果启用）
+            // 解压缩顺序：解密 → 解压缩（参考 RFC 4253 §6.2）
+            let decompressed_payload = if is_client_to_server {
+                // Client → Server: 使用 compression_ctos
+                if encryption_ctx.compression_ctos.is_enabled() {
+                    info!("Decompressing payload (client→server, {} bytes)", payload.len());
+                    encryption_ctx.compression_ctos.decompress(&payload)?
+                } else {
+                    payload
+                }
+            } else {
+                // Server → Client: 使用 compression_stoc（client 解压缩，这里不解压缩）
+                payload
+            };
+            
+            let payload = decompressed_payload;
 
             // 提取padding（从remaining_encrypted的末尾）
             let padding = remaining_encrypted[payload_part2_len..].to_vec();