3Com Switch 4800G 24-Port Configuration Manual page 1108

1108 C 88: SSH C
HAPTER
n
SSH Operating Process
n
ONFIGURATION
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 digital 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.
Revest-Shamir-Adleman Algorithm (RSA) and Digital Signature Algorithm (DSA)
are both asymmetric key algorithms. RSA can be used for data encryption and
signature, whereas DSA is used for signatures only.
Currently, SSH2 supports both RSA and DSA.
The session establishment between an SSH client and the SSH server involves the
following five stages:
Table 88 Stages in establishing a session between the SSH client and the server
Stages
"Version negotiation" on page
1108
"Key and algorithm negotiation"
on page 1109
"Authentication" on page 1109
"Session request" on page 1110
"Interactive session" on page
1110
Version negotiation
The server opens port 22 to listen to connection requests from clients.
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The client sends a TCP connection request to the server. After the TCP
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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
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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
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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 proceed with key and
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algorithm negotiation; otherwise, the server breaks the TCP connection.
All the packets involved in the above steps are transferred in plain text.
Description
SSH1 and SSH2 are supported. The two parties negotiate
a version to use.
SSH supports multiple algorithms. The two parties
negotiate an algorithm for communication.
The SSH server authenticates the client in response to the
client's authentication request.
This client sends a session request to the server.
The client and the server start to communicate with each
other.