Secure Shell(SSH) is a cryptographic network protocol used for operating remote services securely. It is used for remote operation of devices on secure channels using a client-server architecture that generally operates on Port 22. SSH is the successor of Telnet. SSH uses public and private keys to validate and authenticate users. ssh-keygen is used to generate these key pairs.
You can learn more about SSH and Telnet here
How does SSH work?
- SSH protocol needs to have 2 pairs. A Public Key and a Private Key
- The Public Key is added to the remote server (or) device into a special folder $HOME/.ssh/authorized_keys.
- When the server sends any response encrypted using the public key, as only the client has the private key, it can only decrypt the response.
- After successful authentication, a shell session is created or the requested command is executed on the remote server.
How SSH works
ssh-keygen is the utility used to generate, manage, and convert authentication keys for SSH. ssh-keygen comes installed with SSH in most of the operating systems. ssh-keygen is able to generate a key using one of three different digital signature algorithms.
Files generated by ssh-keygen
- $HOME/.ssh/identity: File containing the RSA private key when using SSH protocol version 1.
- $HOME/.ssh/identity.pub: File containing the RSA public key for authentication when you are using the SSH protocol version
- $HOME/.ssh/id_dsa: File containing the protocol version 2 DSA authentication identity of the user.
- $HOME/.ssh/id_dsa.pub: File containing the DSA public key for authentication when you are using the SSH protocol version.
- $HOME/.ssh/id_rsa: File containing the protocol version 2 RSA authentication identity of the user. This file should not be readable by anyone but the user.
- $HOME/.ssh/id_rsa.pub: File containing the protocol version 2 RSA public key for authentication.
“.pub” files should be copied to the $HOME/.ssh/authorized_keys file of the remote system where a user wants to log in using SSH authentication.
Generating key pairs using ssh-keygen
Almost all Unix and Linux Distro’s come pre-installed with SSH and ssh-keygen, so we will have no need to install. We will get started directly. This process is almost similar to almost all Linux Distro’s
Open your terminal and type ssh-keygen
It asks for the names of the ssh key pairs. If you wish to enter the passphrase, go on and ssh-keygen will automatically create your keys.
Generating public/private rsa key pair.
// enter the name for ssh key pairs
Enter file in which to save the key (/home/kushwanth/.ssh/id_rsa): gfg
// enter passpharse for security
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
// ssh keys generated
Your identification has been saved in gfg
Your public key has been saved in gfg.pub
ssh key gen created
A public key looks like the one below.
Sample public key
This is the key you need to copy into your remote device to get successful SSH authentication.
Copying Public key to the remote server
After the key pair is created, now we need to copy the public key into the server. There are 2 ways to do this, using ssh-copy-id (or) manually copying it into the server.
Use the ssh-copy-id command to copy your public key file (e.g., $HOME/.ssh/id_rsa.pub) to your user account on the remote server.
ssh-copy-id -i $HOME/.ssh/id_rsa.pub <user>@<your-remote-host>
Manually copying the public key
Login to your remote server using the password and create a directory at $HOME/.ssh. You can use the command below.
ssh <user>@<your-remote-host> “umask 077; test -d .ssh || mkdir .ssh”
- ssh <user>@<host> allows you to login into your remote host server
- If the .ssh directory is already present, it will set the permissions of the directory to 077 so that it allows read, write, and execute permission for the file’s owner, but prohibits reading, writing, and execute permission for everyone else.
- If the directory is not present, then it will create a new one.
Now send your public key to the remote server,
cat $HOME/.ssh/id_rsa.pub | ssh <user>@<your-remote-host> “cat >> .ssh/authorized_keys”
- cat allows you to print the contents of the file in the terminal.
- The output from the cat is piped into SSH to append the public key to a remote server.
Now you can logout and test whether you can connect to the remote server using the SSH protocol.
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