3com 4210 PWR 9-Port 3CR17341-91-ME 用户手册

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 36: SSH C

ONFIGURATION

while the private key is effective only for the local end. Normally you cannot use 
the private key through the public key.

Asymmetric key algorithm encrypts data using the public key and decrypts the 
data using the private key, thus ensuring data security.

You can also use the asymmetric key algorithm for data signature. For example, 
user 1 adds his signature to the data using the private key, and then sends the 
data to user 2. User 2 verifies the signature using the public key of user 1. If the 
signature is correct, this means that the data originates from user 1.

Both Revest-Shamir-Adleman Algorithm (RSA) and Digital Signature Algorithm 
(DSA) are asymmetric key algorithms. RSA is used for data encryption and 
signature, whereas DSA is used for adding signature.

n 

The session establishment between an SSH client and the SSH server involves the 
following five stages:

■

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 format of "SSH-<primary protocol 
version number>.<secondary protocol version 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 protocol version.

■

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 to determine whether it can cooperate with the 
client.

■

If the negotiation is successful, the server and the client go on to the key and 
algorithm negotiation. If not, the server breaks the TCP connection.

Table 298   Stages in establishing a session between the SSH client and server

Version negotiation 

The two parties negotiate a version to use. 

Key and algorithm negotiation 

SSH supports multiple algorithms. The two parties 
negotiate an algorithm for communication. 

Authentication 

The SSH server authenticates the client in response to 
the client’s authentication request. 

Session request 

This client sends a session request to the server. 

Data exchange 

The client and the server start to communicate with 
each other.