The server opens port 22 to listen to connection requests from clients.
The client sends a TCP connection request to the server. After the TCP connection is established,
the server sends the first packet to the client, which includes a version identification string in the
number>-<software version number>". The primary and secondary protocol version numbers
constitute the protocol version number, while the software version number is used for debugging.
The client receives and resolves the packet. If the protocol version of the server is lower but
supportable, the client uses the protocol version of the server; otherwise, the client uses its own
The client sends to the server a packet that contains the number of the protocol version it decides
to use. The server compares the version carried in the packet with that of its own. If the server
supports the version, the server and client will use the version. Otherwise, the negotiation fails.
If the negotiation is successful, the server and the client proceed with key and algorithm negotiation;
otherwise, the server breaks the TCP connection.
All the packets involved in the above steps are transferred in plain text.
Key and algorithm negotiation
The server and the client send key algorithm negotiation packets to each other, which include the
supported public key algorithm list, encryption algorithm list, Message Authentication Code (MAC)
algorithm list, and compression algorithm list.
Based on the received algorithm negotiation packets, the server and the client figure out the
algorithms to be used. If the negotiation of any type of algorithm fails, the algorithm negotiation fails
and the server tears down the connection with the client.
The server and the client use the DH key exchange algorithm and parameters such as the host key
pair to generate the session key and session ID and the client authenticates the identity of the
Through the above steps, the server and client get the same session key and session ID. The session
key will be used to encrypt and decrypt data exchanged between the server and client later, and the
session ID will be used to identify the session established between the server and client and will be
used in the authentication stage.
After the server grants the request, the client and server start to
communicate with each other.