Fix SSH cipher key length: dynamically determine based on negotiated algorithm

- Add cipher_key_len() helper function
- Store encryption_ctos/stoc in KexExchangeHandler
- Use algorithm name to determine key_len (aes256 → 32, aes128 → 16)
- Remove hardcoded cipher_key_len=32 TODO

All 229 tests pass.

markbase-core/src/ssh_server/kex_exchange.rs

@@ -10,9 +10,25 @@ use log::info;
 use sha2::Digest;
 use std::io::{Read, Write};
 
+/// Determine cipher key length from encryption algorithm name
+/// Reference: OpenSSH cipher.c, RFC 4253
+fn cipher_key_len(algorithm: &str) -> usize {
+    if algorithm.contains("aes256") || algorithm.contains("aes256-gcm") {
+        32
+    } else if algorithm.contains("aes128") || algorithm.contains("aes128-ctr") {
+        16
+    } else {
+        // Default to AES-256 for unknown algorithms
+        info!("Unknown encryption algorithm '{}', using default key_len=32", algorithm);
+        32
+    }
+}
+
 /// SSH密钥交换流程处理器（参考OpenSSH kex.c）
 pub struct KexExchangeHandler {
     kex_algorithm: String,
+    encryption_ctos: String,  // Phase fix: store encryption algorithm for key length
+    encryption_stoc: String,  // Phase fix: store encryption algorithm for key length
     server_kex: Option<Curve25519Kex>,
     host_key: Ed25519HostKey,
     shared_secret: Option<Vec<u8>>,
@@ -33,6 +49,8 @@ impl KexExchangeHandler {
 
         Ok(Self {
             kex_algorithm: kex_result.kex_algorithm,
+            encryption_ctos: kex_result.encryption_ctos,
+            encryption_stoc: kex_result.encryption_stoc,
             server_kex: None,
             host_key,
             shared_secret: None,
@@ -393,10 +411,9 @@ impl KexExchangeHandler {
         let client_public_key = self.client_public_key.as_ref().unwrap();
         let host_key_blob = self.build_ssh_host_key()?;
 
-        // ⭐ TODO: Get encryption algorithm from kex_result to determine cipher_key_len
-        // For now, hardcode 32 (AES-256) to maintain backward compatibility
-        let cipher_key_len = 32;
-        info!("compute_session_keys: cipher_key_len={}", cipher_key_len);
+        // Determine cipher key length from negotiated encryption algorithm
+        let cipher_key_len = cipher_key_len(&self.encryption_stoc);
+        info!("compute_session_keys: encryption_stoc={}, cipher_key_len={}", self.encryption_stoc, cipher_key_len);
         SessionKeys::derive(
             shared_secret,
             exchange_hash, // 使用保存的exchange hash（H参数）