ZyXEL Communications ES-2024 Series, User Manual, Page: 219 / 282

Chapter 29 Access Control
ES-2024 Series User’s Guide
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29.4 SSH Overview
Unlike Telnet or FTP, which transmit data in clear text, SSH (Secure Shell) is a secure
communication protocol that combines authentication and data encryption to provide secure
encrypted communication between two hosts over an unsecured network.
Figure 131 SSH Communication Example
29.5 How SSH works
The following table summarizes how a secure connection is established between two remote
hosts.
Figure 132 How SSH Works
1 Host Identification
The SSH client sends a connection request to the SSH server. The server identifies itself
with a host key. The client encrypts a randomly generated session key with the host key
and server key and sends the result back to the server.
The client automatically saves any new server public keys. In subsequent connections,
the server public key is checked against the saved version on the client computer.