SSH (secure shell) :used for making encrypted connection between 2 computers by using a secure Channel between these computers for (remote access, copying files, etc.. ).

SSH is based on client server model:
  • server (SSHD)
  • clients (scp,sftp,ssh)

Why SSH: 
SSH with it's sub-system (scp , sftp ) is instead of (telnet , ftp , rcp ), and protocols that run these program are unsecured.
Telnet for example use unsecured channel to connect with another computer , so , man in the middle attack can intercept telnet messages and can get sensitive information such as (user name and password) .
you can try it , use telnet to connect to another computer in your network then use a sniffer program such as (wire shark or TCP dump)you will see every message between computers.
So , we using ssh to securing connections and can copy files and sending commands between networks securely .
SSH uses asymmetric key mechanism to authenticate and securing the connection .
asymmetric key or public-key cryptography mechanism:
It's consist of two pairs of keys , public key and private key.

  • sender side:
public key use to encrypt message to send it.

  • receiver side:
private key use to decrypt encrypted message.

This mechanism is uses in SSH

  • SSH generates public and private keys
by applying a command ssh-keygen
  • keys located in ~/.ssh/
id_rsa ---> private key (must keep secured).
id_rsa.pub ---> public key and can copy to another computer to connect with SSH server (SSHD).

  • public key can be copied to another computer by using command ssh-copy-id
known_hosts ---> file contains public keys which this computer use it to connect with another ssh server (SSHD) and not a public key.

SSH also contains sub-systems (scp , sftp )
  • scp use to copy a file in a secure channel (secured by ssh).
  • sftp use instead of ftp to transfare file between 2 computers securely.

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