Openssh Public Key

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Table of Contents

Introduction

SSH can handle authentication using a traditional username and password combination or by using a public and private key pair. The SSH key pair establishes trust between the client and server, thereby removing the need for a password during authentication. While not required, the SSH private key can be encrypted with a passphrase for added security.

If you add your public key to the server, you should be able to log in without typing the password all the time. OpenSSH provides a handy tool call called ssh-copy-id for copying ssh public keys to remote systems. It even creates required directories and files. Deploying the public key To use the user key that was created above, the public key needs to be placed on the server into a text file called authorizedkeys under users username.ssh. The OpenSSH tools include scp, which is a secure file-transfer utility, to help with this. With SSH, public key authentication improves security considerably as it frees the users from remembering complicated passwords (or worse yet, writing them down). In addition to security public key authentication also offers usability benefits - it allows users to implement single sign-on across the SSH servers they connect to. Key pairs refer to the public and private key files that are used by certain authentication protocols. SSH public-key authentication uses asymmetric cryptographic algorithms to generate two key files – one 'private' and the other 'public'. The private key files are the equivalent of a password, and should protected under all circumstances. If someone acquires your private key, they can log in as yo.

The PuTTY SSH client for Microsoft Windows does not share the same key format as the OpenSSH client. Therefore, it is necessary to create a new SSH public and private key using the PuTTYgen tool or convert an existing OpenSSH private key.

Requirements

  • PuTTY SSH client for Microsoft Windows
  • Remote server accessible over OpenSSH

Install PuTTY and PuTTYgen

Both PuTTY and PuTTYgen are required to convert OpenSSH keys and to connect to the server over SSH. These two tools can be downloaded individually or, preferably, as a Windows installer from the PuTTY Download Page.

Once the PuTTY Windows installer is downloaded, double-click the executable in the Download folder and follow the installation wizard. The default settings are suitable for most installations. Both PuTTY and PuTTYgen should now be accessible from the Windows Programs list.

Use Existing Public and Private Keys

If you have an existing OpenSSH public and private key, copy the id_rsa key to your Windows desktop. This can be done by copying and pasting the contents of the file or using an SCP client such as PSCP which is supplied with the PuTTY install or FileZilla.

Next launch PuTTYgen from the Windows Programs list.

  1. Click Conversions from the PuTTY Key Generator menu and select Import key.
  2. Navigate to the OpenSSH private key and click Open.
  3. Under Actions / Save the generated key, select Save private key.
  4. Choose an optional passphrase to protect the private key.
  5. Save the private key to the desktop as id_rsa.ppk.

If the public key is already appended to the authorized_keys file on the remote SSH server, then proceed to Connect to Server with Private Key.

Otherwise, proceed to Copy Public Key to Server.

Create New Public and Private Keys

Launch PuTTYgen from the Windows Programs list and proceed with the following steps.

  1. Under Parameters, increase the Number of bits in a generated key: to a minimum value of 2048.
  2. Under Actions / Generate a public/private key pair, click Generate.
  3. You will be instructed to move the mouse cursor around within the PuTTY Key Generator window as a randomizer to generate the private key.
  4. Once the key information appears, click Save private key under Actions / Save the generated key.
  5. Save the private key to the desktop as id_rsa.ppk.
  6. The box under Key / Public key for pasting into OpenSSH authorized_keys file: contains the public key.

Copy Public Key to Server

The OpenSSH public key is located in the box under Key / Public key for pasting info OpenSSH authorized_keys file:. The public key begins with ssh-rsa followed by a string of characters.

  1. Highlight entire public key within the PuTTY Key Generator and copy the text.
  2. Launch PuTTY and log into the remote server with your existing user credentials.

  3. Use your preferred text editor to create and/or open the authorized_keys file:

  4. Paste the public key into the authorized_keys file.

  5. Save the file and close the text editor.

  6. Adjust the permissions of the authorized_keys file so that the file does not allow group writable permissions.

  7. Logout of the remote server.

Connect to Server with Private Key

Now it is time to test SSH key authentication. The PuTTYgen tool can be closed and PuTTY launched again.

  1. Enter the remote server Host Name or IP address under Session.
  2. Navigate to Connection > SSH > Auth.
  3. Click Browse... under Authentication parameters / Private key file for authentication.
  4. Locate the id_rsa.ppk private key and click Open.
  5. Finally, click Open again to log into the remote server with key pair authentication.

Lets say you have a private/public key pair that you use to login to your server via SSH and you lose the public key, either it was deleted or corrupt and you don’t want to have to regenerate a new pair what options do you have? In this post I will demonstrate how to regenerate a public key from the corresponding private key that you still have.

Generate public key and store into a file

Openssh

It is a simple one liner command to generate a public key from a private key, so lets say our private key is named ‘user@myserver.key’ and we want to generate the public key and name it ‘authorized_keys’. Below is the command to do this.

Generator Openssh Public Key