Telnet SSH is a common remote management protocol, both of which are TCP protocols. TCP three-way handshake is needed before establishing a session, so it is a reliable transmission protocol with acknowledgement mechanism. The port numbers are 23 and 22 respectively.

Telnet is transmitted in clear text, and the transmission process is easy to be stolen. In addition to management, it can also be used for port scanning and testing whether the target port is accessible.

SSH only uses SSL encryption for transmission. Creating a session will produce an asymmetric key pair (RSA, DSA, DH algorithm). The server has a private key, and the client will prompt to receive/save the public key. Only the private key can decrypt data encrypted by the public key.

Then negotiate and exchange a symmetric key to encrypt the data between them (DES, 3DES, AES processing speed is fast) to prevent information leakage.

SSH can not only be used for remote management, but also extend SFTP, SCP, login and so on.

As for SSHv 1 and SSHv2 (also called v 1.5 and v 1.9), the former is slightly defective, and v2 carries out algorithm repair.