ZyXEL Communications GS-3012F Series - Manual (Page 180)

Chapter 25 Access Control
GS-3012/GS-3012F User’s Guide
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25.4 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 98 SSH Communication Example
25.5 How SSH works
The following table summarizes how a secure connection is established between two remote
hosts.
Figure 99 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.
2 Encryption Method
Once the identification is verified, both the client and server must agree on the type of
encryption method to use.
3 Authentication and Data Transmission
After the identification is verified and data encryption activated, a secure tunnel is
established between the client and the server. The client then sends its authentication
information (user name and password) to the server to log in to the server.