Critical fix: KEXINIT exchange hash encoding (prepend SSH_MSG_KEXINIT byte)

OpenSSH kexgex.c source code analysis:
- KEXINIT payload stored without SSH_MSG_KEXINIT type byte
- Exchange hash prepends SSH_MSG_KEXINIT byte (20) with adjusted length

Before fix:
- client_kexinit_payload included SSH_MSG_KEXINIT byte
- Direct use without prepending

After fix:
- Remove SSH_MSG_KEXINIT byte from payload
- Prepend byte (20) in exchange hash with length+1
- Both kex_exchange.rs and kex_complete.rs updated

Testing result: MAC still fails, indicating additional encoding issues
Next: Detailed comparison of all exchange hash components

markbase-core/src/ssh_server/kex_exchange.rs

@@ -215,11 +215,25 @@ impl KexExchangeHandler {
         hasher.update(&(server_version.len() as u32).to_be_bytes());
         hasher.update(server_version.as_bytes());
         
-        hasher.update(&(client_kexinit_payload.len() as u32).to_be_bytes());
-        hasher.update(client_kexinit_payload);
+        // OpenSSH kexgex.c: "kexinit messages: fake header: len+SSH2_MSG_KEXINIT"
+        // KEXINIT payload should NOT include SSH_MSG_KEXINIT type byte
+        // OpenSSH stores payload starting from cookie, prepends SSH_MSG_KEXINIT in exchange hash
         
-        hasher.update(&(server_kexinit_payload.len() as u32).to_be_bytes());
-        hasher.update(server_kexinit_payload);
+        // Remove SSH_MSG_KEXINIT type byte from payloads (our payload includes it)
+        let client_kexinit_without_type = &client_kexinit_payload[1..];
+        let server_kexinit_without_type = &server_kexinit_payload[1..];
+        
+        info!("I_C (client KEXINIT without type byte): {} bytes (first byte should be cookie)", client_kexinit_without_type.len());
+        info!("I_S (server KEXINIT without type byte): {} bytes", server_kexinit_without_type.len());
+        
+        // Exchange hash: uint32(len+1) + uint8(SSH_MSG_KEXINIT) + payload_without_type
+        hasher.update(&((client_kexinit_without_type.len() + 1) as u32).to_be_bytes());
+        hasher.update(&[20]);  // SSH_MSG_KEXINIT type byte
+        hasher.update(client_kexinit_without_type);
+        
+        hasher.update(&((server_kexinit_without_type.len() + 1) as u32).to_be_bytes());
+        hasher.update(&[20]);  // SSH_MSG_KEXINIT type byte
+        hasher.update(server_kexinit_without_type);
         
         hasher.update(&(host_key_blob.len() as u32).to_be_bytes());
         hasher.update(host_key_blob);