H3C Technologies H3C SecBlade LB Cards - Manual (Page 168)

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Stages Description
Algorithm negotiation
SSH supports multiple algorithms. Based on the local algorithms, the two parties
determine the key exchange algorithm for generating session keys, the
encryption algorithm for encrypting data, public key algorithm for digital
signature and authentication, and the HMAC algorithm for protecting data
integrity.
Key exchange
The two parties use the Diffie-Hellman (DH) exchange algorithm to dynamically
generate the session key for protecting data transfer and the session ID for
identifying the SSH connection. In this stage, the client authenticates the server
as well.
Authentication
The SSH server authenticates the client in response to the client's authentication
request.
Session request
After passing authentication, the client sends a session request to the server to
request the establishment of a session (Stelnet, SFTP, or SCP).
Interaction
After the server grants the request, the client and the server start to communicate
with each other in the session.
In the interaction stage, you can execute commands from the client by pasting
the commands in text format (the text must be within 2000 bytes). The
commands must be available in the same view. Otherwise, the server might not
be able to execute the commands correctly.
If you want to execute commands of more than 2000 bytes, you can save the
commands in a configuration file, upload it to the server through SFTP, and use
it to restart the server.
SSH authentication
When the device acts as an SSH server, it supports the following authentication methods:
• Password authentication—The SSH server uses AAA for authentication of the client. During
password authentication, the SSH client encrypts its username and password, encapsulates them
into an authentication request, and sends the request to the server. After receiving the request, the
SSH server decrypts the request to get the username and password in plain text, checks the validity
of the username and password locally or by a remote AAA server, and then informs the client of the
authentication result.
In a password authentication process, if the remote AAA server requires the user for a secondary
password authentication, it sends the SSH server an authentication response with a prompt. The
prompt is transparently transmitted to the client, and displayed on the client to notify the user to
enter a specific password. After the user enters the correct password and passes validity check by
the remote AAA server, the device returns an authentication success message to the client.
NOTE:
Only clients that run SSH2 or a later version support secondary password authentication that is initiated
by the AAA server.
• Publickey authentication—The server authenticates the client by the digital signature. During
publickey authentication, the client sends the server a publickey authentication request that contains
its username, public key, and publickey algorithm information (or the digital certificate that carries
the public key information). The server examines whether the public key is valid. If the public key is
invalid, the authentication fails. Otherwise, the server authenticates the client by the digital