1-2
Figure 1-1 Encryption and decryption
Key-based algorithm is usually classified into symmetric key algorithm and asymmetric key algorithm.
Asymmetric Key Algorithm
Asymmetric key algorithm means that a key pair exists at both ends. The key pair consists of a private
key and a public key. The public key is effective for both ends, 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.
Currently, SSH supports both RSA and DSA.
SSH Operating Process
The session establishment between an SSH client and the SSH server involves the following five
stages:
Table 1-1 Stages in establishing a session between the SSH client and server
Stages Description
Version negotiation
SSH1 and SSH2 are supported. 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.
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