Ubuntu Generate Public Key From Private

| 14-05-2021
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I am a new Ubuntu 18.04 LTS user and I would like to setup ssh public key authentication. How do I set up ssh keys based authentication on Ubuntu Linux 18.04 LTS server? How do I set up SSH keys on an Ubuntu Linux 18.04 LTS server? In Ubuntu 18.04 LTS, how do I set up public key authentication?
Introduction

Nov 10, 2011  How to Generate A Public/Private SSH Key Linux By Damien – Posted on Nov 10, 2011 Nov 18, 2011 in Linux If you are using SSH frequently to connect to a remote host, one of the way to secure the connection is to use a public/private SSH key so no password is transmitted over the network and it can prevent against brute force attack. Dec 18, 2019  In this tutorial, we will walk through how to generate SSH keys on Ubuntu 18.04 machines. We will also show you how to set up an SSH key-based authentication and connect to your remote Linux servers without entering a password.

: OpenSSH is a free and open source client/server technology for secure remote login. It is an implementation of the SSH protocol. OpenSSH divided into sshd (server) and various client tools such as sftp, scp, ssh and more. One can do remote login with OpenSSH either using password or combination of private and public keys named as public key based authentication. It is an alternative security method for user passwords. This method is recommended on a VPS, cloud, dedicated or even home-based server or laptop. This page shows how to set up SSH keys on Ubuntu 18.04 LTS server.
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Ubuntu 18.04 Setup SSH Public Key Authentication

The procedure to set up secure ssh keys on Ubuntu 18.04:

Ubuntu Generate Rsa Key

  1. Create the key pair using ssh-keygen command.
  2. Copy and install the public key using ssh-copy-id command.
  3. Add yourself to sudo admin account on Ubuntu 18.04 server.
  4. Disable the password login for root account on Ubuntu 18.04.

Sample set up for SSH Keys on Ubuntu 18.04


Where,

  • 202.54.1.55 – You store your public key on the remote hosts and you have an accounts on this Ubuntu Linux 18.04 LTS server.
  • Linux/macbook laptop – Your private key stays on the desktop/laptop/computer (or local server) you use to connect to 202.54.1.55 server. Do not share or give your private file to anyone.

In public key based method you can log into remote hosts and server, and transfer files to them, without using your account passwords. Feel free to replace 202.54.1.55 and client names with your actual setup. Enough talk, let’s set up public key authentication on Ubuntu Linux 18.04 LTS.

How to create the RSA/ed25519 key pair on your local desktop/laptop

Open the Terminal and type following commands if .ssh directory does not exists:
$ mkdir -p $HOME/.ssh
$ chmod 0700 $HOME/.ssh
Next generate a key pair for the protocol, run:
$ ssh-keygen
OR
$ ssh-keygen -t rsa 4096 -C 'My key for Linode server'
These days ED25519 keys are favored over RSA keys when backward compatibility is not needed:
$ ssh-keygen -t ed25519 -C 'My key for Linux server # 42'

How to install the public key in Ubuntu 18.04 remote server

The syntax is as follows:
ssh-copy-id your-user-name@your-ubuntu-server-name
ssh-copy-id -i ~/.ssh/file.pub your-user-name@your-ubuntu-server-name
For example:
## for RSA KEY ##
ssh-copy-id -i $HOME/.ssh/id_rsa.pub user@202.54.1.55
## for ED25519 KEY ##
ssh-copy-id -i $HOME/.ssh/id_ed25519.pub user@202.54.1.55
## install SSH KEY for root user ##
ssh-copy-id -i $HOME/.ssh/id_ed25519.pub root@202.54.1.55

I am going to install ssh key for a user named vivek (type command on your laptop/desktop where you generated RSA/ed25519 keys):
$ ssh-copy-id -i ~/.ssh/id_ed25519.pub vivek@202.54.1.55

Test it

Now try logging into the Ubuntu 18.04 LTS server, with ssh command from your client computer/laptop using ssh keys:
$ ssh your-user@your-server-name-here
$ ssh vivek@202.54.1.55

What are ssh-agent and ssh-add, and how do I use them on Ubuntu 18.04?

To get rid of a passphrase for the current session, add a passphrase to ssh-agent (see ssh-agent command for more info) and you will not be prompted for it when using ssh or scp/sftp/rsync to connect to hosts with your public key. The syntax is as follows:
$ eval $(ssh-agent)
Type the ssh-add command to prompt the user for a private key passphrase and adds it to the list maintained by ssh-agent command:
$ ssh-add
Enter your private key passphrase. Now try again to log into vivek@202.54.1.55 and you will NOT be prompted for a password:
$ ssh vivek@202.54.1.55

How to disable the password based login on a Ubuntu 18.04 server

Login to your server, type:
## client commands ##
$ eval $(ssh-agent)
$ ssh-add
$ ssh vivek@202.54.1.55
Now login as root user:
$ sudo -i
OR
$ su -i
Edit sshd_config file:
# vim /etc/ssh/sshd_config
OR
# nano /etc/ssh/sshd_config
Find PermitRootLogin and set it as follows:
PermitRootLogin no
Save and close the file. I am going to add a user named vivek to sudoers group on Ubuntu 18.04 server so that we can run sysadmin tasks:
# adduser vivek sudo
Restart/reload the sshd service:
# systemctl reload ssh
You can exit from all session and test it as follows:
$ ssh vivek@202.54.1.55
## become root on server for sysadmin task ##
$ sudo -i

How do I add or replace a passphrase for an existing private key?

To to change your SSH passphrase type the following command:
$ ssh-keygen -p

How do I backup my existing private/public SSH keys

Just copy files to your backup server or external USB pen/hard drive:

How do I protect my ssh keys?

  1. Always use a strong passphrase.
  2. Do not share your private keys anywhere online or store in insecure cloud storage or gitlab/github servers.
  3. Restrict privileges of the account.

Tip: Create and setup an OpenSSH config file to create shortcuts for servers

Add public key ubuntu

See how to create and use an OpenSSH ssh_config file for more info.

How do I secure my OpenSSH server?

See “OpenSSH Server Best Security Practices” for more info.

Conclusion

You learned how to create and install ssh keys for SSH key-based authentication for Ubuntu Linux 18.04 LTS server. See OpenSSH server documents here and here for more info.

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On Windows, you can create SSH keys in many ways. This document explains how to use two SSH applications, PuTTY and Git Bash.

Joyent recommends RSA keys because the node-manta CLI programs work with RSA keys both locally and with the ssh agent. DSA keys will work only if the private key is on the same system as the CLI, and not password-protected.

PuTTY

PuTTY is an SSH client for Windows. You can use PuTTY to generate SSH keys. PuTTY is a free open-source terminal emulator that functions much like the Terminal application in macOS in a Windows environment. This section shows you how to manually generate and upload an SSH key when working with PuTTY in the Windows environment.

About PuTTY

PuTTY is an SSH client for Windows that you will use to generate your SSH keys. You can download PuTTY from www.chiark.greenend.org.uk.

When you install the PuTTY client, you also install the PuTTYgen utility. PuTTYgen is what you will use to generate your SSH key for a Windows VM.

This page gives you basic information about using PuTTY and PuTTYgen to log in to your provisioned machine. For more information on PuTTY, see the PuTTY documentation

Generating an SSH key

To generate an SSH key with PuTTYgen, follow these steps:

  1. Open the PuTTYgen program.
  2. For Type of key to generate, select SSH-2 RSA.
  3. Click the Generate button.
  4. Move your mouse in the area below the progress bar. When the progress bar is full, PuTTYgen generates your key pair.
  5. Type a passphrase in the Key passphrase field. Type the same passphrase in the Confirm passphrase field. You can use a key without a passphrase, but this is not recommended.
  6. Click the Save private key button to save the private key. You must save the private key. You will need it to connect to your machine.
  7. Right-click in the text field labeled Public key for pasting into OpenSSH authorized_keys file and choose Select All.
  8. Right-click again in the same text field and choose Copy.

Importing your SSH key

Now you must import the copied SSH key to the portal.

Ubuntu Generate Public Key From Private Key

  1. After you copy the SSH key to the clipboard, return to your account page.
  2. Choose to Import Public Key and paste your SSH key into the Public Key field.
  3. In the Key Name field, provide a name for the key. Note: although providing a key name is optional, it is a best practice for ease of managing multiple SSH keys.
  4. Add the key. It will now appear in your table of keys under SSH.

PuTTY and OpenSSH use different formats of public SSH keys. If the text you pasted in the SSH Key starts with —— BEGIN SSH2 PUBLIC KEY, it is in the wrong format. Be sure to follow the instructions carefully. Your key should start with ssh-rsa AAAA….

Once you upload your SSH key to the portal, you can connect to your virtual machine from Windows through a PuTTY session.

Git Bash

The Git installation package comes with SSH. Using Git Bash, which is the Git command line tool, you can generate SSH key pairs. Git Bash has an SSH client that enables you to connect to and interact with Triton containers on Windows.

To install Git:

  1. (Download and initiate the Git installer](https://git-scm.com/download/win).
  2. When prompted, accept the default components by clicking Next.
  3. Choose the default text editor. If you have Notepad++ installed, select Notepad++ and click Next.
  4. Select to Use Git from the Windows Command Prompt and click Next.
  5. Select to Use OpenSSL library and click Next.
  6. Select to Checkout Windows-style, commit Unix-style line endings and click Next.
  7. Select to Use MinTTY (The default terminal of mYSYS2) and click Next.
  8. Accept the default extra option configuration by clicking Install.
Add public key ubuntu

When the installation completes, you may need to restart Windows.

Launching GitBash

To open Git Bash, we recommend launching the application from the Windows command prompt:

  1. In Windows, press Start+R to launch the Run dialog.
  2. Type C:Program FilesGitbinbash.exe and press Enter.

Generating SSH keys

First, create the SSH directory and then generate the SSH key pair.

One assumption is that the Windows profile you are using is set up with administrative privileges. Given this, you will be creating the SSH directory at the root of your profile, for example:

  1. At the Git Bash command line, change into your root directory and type.

  1. Change into the .ssh directory C:Usersjoetest.ssh

  2. To create the keys, type:
  1. When prompted for a password, type apassword to complete the process. When finished, the output looks similar to:

Uploading an SSH key

To upload the public SSH key to your Triton account:

  1. Open Triton Service portal, select Account to open the Account Summary page.
  2. From the SSH section, select Import Public Key.
  3. Enter a Key Name. Although naming a key is optional, labels are a best practice for managing multiple SSH keys.
  4. Add your public SSH key.

When Triton finishes the adding or uploading process, the public SSH key appears in the list of SSH keys.

What are my next steps?

  • Adding SSH keys to agent.
  • Set up the Triton CLI and CloudAPI on Windows.
  • Set up the Triton CLI and CloudAPI.
  • Create an instance in the Triton Service Portal.
  • Set up the triton-docker command line tool.
  • Visit PuTTYgen to learn more about the PuTTYgen and to seethe complete installation and usage guide.