1-2Figure 1-1 Encryption and decryptionKey-based algorithm is usually classified into symmetric key algorithm and asymmetric key algorithm.Asymmetric Key AlgorithmAsymmetric key algorithm means that a key pair exists at both ends. The key pair consists of a privatekey and a public key. The public key is effective for both ends, while the private key is effective only forthe 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 privatekey, thus ensuring data security.You can also use the asymmetric key algorithm for data signature. For example, user 1 adds hissignature to the data using the private key, and then sends the data to user 2. User 2 verifies thesignature using the public key of user 1. If the signature is correct, this means that the data originatesfrom user 1.Both Revest-Shamir-Adleman Algorithm (RSA) and Digital Signature Algorithm (DSA) are asymmetrickey algorithms. RSA is used for data encryption and signature, whereas DSA is used for addingsignature.Currently, SSH supports both RSA and DSA.SSH Operating ProcessThe session establishment between an SSH client and the SSH server involves the following fivestages:Table 1-1 Stages in establishing a session between the SSH client and serverStages DescriptionVersion negotiation SSH1 and SSH2 are supported. The two parties negotiate aversion to use.Key and algorithm negotiation SSH supports multiple algorithms. The two parties negotiate analgorithm for communication.Authentication The SSH server authenticates the client in response to the client’sauthentication 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.
