Introduction

Generating a Secure Shell (SSH) Public/Private Key Pair Several tools exist to generate SSH public/private key pairs. The following sections show how to generate an SSH key pair on UNIX, UNIX-like and Windows platforms. To generate an SSH key pair on UNIX and UNIX-like platforms using the ssh-keygen utility: Navigate to your home directory: $ cd $HOME. Run the ssh-keygen utility, providing as filename your choice of file name for. Enter a passphrase for the private key, or press Enter to create a private key. Jun 22, 2012  Generating a key pair provides you with two long string of characters: a public and a private key. You can place the public key on any server, and then unlock it by connecting to it with a client that already has the private key. When the two match up, the system unlocks without the need for a password.

Secure Shell (SSH) is an encrypted protocol used by Linux users to connect to their remote servers.

You’re looking for a pair of files named something like iddsa or idrsa and a matching file with a.pub extension. The.pub file is your public key, and the other file is the corresponding private key. If you don’t have these files (or you don’t even have a.ssh directory), you can create them by running a program called ssh-keygen, which is provided with the SSH package on Linux/macOS.

Run it on your local computer to generate a 2048-bit RSA key pair, which is fine for most uses. Ssh-keygen The utility prompts you to select a location for the keys. By default, the keys are stored in the /.ssh directory with the filenames idrsa for the private key and idrsa.pub for the public key. Many enterprises create and distribute their own public-private key pairs for authentication. Use the following code as a guideline. In your local environment, obtain or generate your public-private key pair. If you need it, here's some background on generating key pairs in Linux and Windows. For now, just generate and save the keys locally.

Generally, there are two ways for clients to access their servers – using password based authentication or public key based authentication.

Using SSH keys for authentication is highly recommended, as a safer alternative to passwords.

This tutorial will guide you through the steps on how to generate and set up SSH keys on CentOS 7. We also cover connecting to a remote server using the keys and disabling password authentication.

1. Check for Existing Keys

Prior to any installation, it is wise to check whether there are any existing keys on the client machines.

Open the terminal and list all public keys stored with the following command:

The output informs you about any generated keys currently on the system. If there aren’t any, the message tells you it cannot access /.ssh/id_*.pub , as there is no such file or directory.

2. Verify SSH is Installed

To check if thw package is installed, run the command:

If you already have SSH, the output tells you which version it is running. Currently, the latest version is OpenSSH 8.0/8.0p1.

Note: Refer to our guide If you need to install and enable SSH on your CentOS system.

Steps to Creating SSH keys on CentOS

Step 1: Create SSH Key Pair

1. Start by logging into the source machine (local server) and creating a 2048-bit RSA key pair using the command:

If you want to tighten up security measures, you can create a 4096-bit key by adding the -b 4096 flag:

Generate Public Key Linux

2. After entering the command, you should see the following prompt:

3. To save the file in the suggested directory, press Enter. Alternatively, you can specify another location.

Note: If you already have a key pair in the proposed location, it is advisable to pick another directory. Otherwise it will overwrite existing SSH keys.

4. Next, the prompt will continue with:

Although creating a passphrase isn’t mandatory, it is highly advisable.

5. Finally, the output will end by specifying the following information:

Now you need to add the public key to the remote CentOS server.

You can copy the public SSH key on the remote server using several different methods:

  1. using the ssh-copy-id script
  2. using Secure Copy (scp)
  3. manually copying the key

The fastest and easiest method is by utilizing ssh-copy-id. If the option is available, we recommend using it. Otherwise, try any of the other two noted.

1. Start by typing the following command, specifying the SSH user account, and the IP address of the remote host:

If it is the first time your local computer is accessing this specific remote server you will receive the following output:

2. Confirm the connection – type yes and hit Enter.

3. Once it locates the id_rsa.pub key created on the local machine, it will ask you to provide the password for the remote account. Type in the password and hit Enter.

4. Once the connection has been established, it adds the public key on the remote server. This is done by copying the ~/.ssh/id_rsa.pub file to the remote server’s ~/.ssh directory. You can locate it under the name authorized_keys.

5. Lastly, the output tells you the number of keys added, along with clear instructions on what to do next:

1. First, set up an SSH connection with the remote user:

2. Next, create the ~/.ssh directory as well as the authorized_keys file:

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3. Use the chmod command to change the file permission:

chmod 700 makes the file executable, while chmod 600 allows the user to read and write the file.

4. Now, open a new terminal session, on the local computer.

5. Copy the content from id_rsa.pub (the SSH public key) to the previously created authorized_keys file on the remote CentOS server by typing the command:

With this, the public key has been safely stored on the remote account.

1. To manually add the public SSH key to the remote machine, you first need to open the content from the ~/.ssh/id_rsa.pub file:

2. As in the image below, the key starts with ssh-rsa and ends with the username of the local computer and hostname of the remote machine:


3. Copy the content of the file, as you will need later.

4. Then, in the terminal window, connect to the remote server on which you wish to copy the public key. Use the following command to establish the connection:

5. Create a ~/.ssh directory and authorized_keys file on the CentOS server with the following command:

6. Change their file permission by typing:

7. Next, open the authorized_keys file with an editor of your preference. For example, to open it with Nano, type:

8. Add the public key, previously copied in step 2 of this section, in a new line in (under the existing content).

9. Save the changes and close the file.

10. Finally, log into the server to verify that everything is set up correctly.

Once you have completed the previous steps (creating an RSA Key Pair and copying the Public Key to the CentOS server), you will be able to connect to the remote host without typing the password for the remote account.

Key

All you need to do is type in the following command:

If you didn’t specify a passphrase while creating the SSH key pair, you will automatically log in the remote server.

Otherwise, type in the passphrase you supplied in the initial steps and press Enter.

Once the shell confirms the key match, it will open a new session for direct communication with the server.

Generate Ssh Key Pair

Although you managed to access the CentOS server without having to provide a password, it still has a password-based authentication system running on the machine. This makes it a potential target for brute force attacks.

You should disable password authentication entirely by following the outlined steps.

Note: Consider performing the following steps through a non-root account with sudo privileges, as an additional safety layer.

1. Using the SSH keys, log into the remote CentOS server which has administrative privileges:

2. Next, open the SSH daemon configuration file using a text editor of your choice:

Linux

3. Look for the following line in the file:

4. Edit the configuration by changing the yes value to no. Thus, the directive should be as following:

5. Save the file and exit the text editor.
6. To enable the changes, restart the sshdservice using the command:

7. Verify the SSH connection to the server is still functioning correctly. Open a new terminal window and type in the command:

In this article, you learned how to generate SSH key pairs and set up an SSH key-based authentication. We also covered copying keys to your remote CentOS server, and disabling SSH password authentication.

Next, You Should Read:

Before you begin

Using SSH public-key authentication to connect to a remote system is a robust, more secure alternative to logging in with an account password or passphrase. SSH public-key authentication relies on asymmetric cryptographic algorithms that generate a pair of separate keys (a key pair), one 'private' and the other 'public'. You keep the private key a secret and store it on the computer you use to connect to the remote system. Conceivably, you can share the public key with anyone without compromising the private key; you store it on the remote system in a .ssh/authorized_keys directory.

To use SSH public-key authentication:

  • The remote system must have a version of SSH installed. The information in this document assumes the remote system uses OpenSSH. If the remote system is using a different version of SSH (for example, Tectia SSH), the process outlined below may not be correct.
  • The computer you use to connect to the remote server must have a version of SSH installed. This document includes instructions for generating a key pair with command-line SSH on a Linux or macOS computer, and with PuTTY on a Windows computer.
  • You need to be able to transfer your public key to the remote system. Therefore, you must either be able to log into the remote system with an established account username and password/passphrase, or have an administrator on the remote system add the public key to the ~/.ssh/authorized_keys file in your account.
  • Two-factor authentication using Two-Step Login (Duo) is required for access to the login nodes on IU research supercomputers, and for SCP and SFTP file transfers to those systems. SSH public-key authentication remains an option for researchers who submit the 'SSH public-key authentication to HPS systems' user agreement (log into HPC everywhere using your IU username and passphrase), in which you agree to set a passphrase on your private key when you generate your key pair. If you have questions about how two-factor authentication may impact your workflows, contact the UITS Research Applications and Deep Learning team. For help, see Get started with Two-Step Login (Duo) at IU and Help for Two-Step Login (Duo).

Set up public-key authentication using SSH on a Linux or macOS computer

Generate Public Private Key Pair Linux

To set up public-key authentication using SSH on a Linux or macOS computer:

  1. Log into the computer you'll use to access the remote host, and then use command-line SSH to generate a key pair using the RSA algorithm.

    To generate RSA keys, on the command line, enter:

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  2. You will be prompted to supply a filename (for saving the key pair) and a password (for protecting your private key):
    • Filename: To accept the default filename (and location) for your key pair, press Enter or Return without entering a filename.

      Alternatively, you can enter a filename (for example, my_ssh_key) at the prompt, and then press Enter or Return. However, many remote hosts are configured to accept private keys with the default filename and path (~/.ssh/id_rsa for RSA keys) by default. Consequently, to authenticate with a private key that has a different filename, or one that is not stored in the default location, you must explicitly invoke it either on the SSH command line or in an SSH client configuration file (~/.ssh/config); see below for instructions.

    • Password: Enter a password that contains at least five characters, and then press Enter or Return. If you press Enter or Return without entering a password, your private key will be generated without password-protection.
      If you don't password-protect your private key, anyone with access to your computer conceivably can SSH (without being prompted for a password) to your account on any remote system that has the corresponding public key.

    Your private key will be generated using the default filename (for example, id_rsa) or the filename you specified (for example, my_ssh_key), and stored on your computer in a .ssh directory off your home directory (for example, ~/.ssh/id_rsa or ~/.ssh/my_ssh_key).

    The corresponding public key will be generated using the same filename (but with a .pub extension added) and stored in the same location (for example, ~/.ssh/id_rsa.pub or ~/.ssh/my_ssh_key.pub).

  3. Use SFTP or SCP to copy the public key file (for example, ~/.ssh/id_rsa.pub) to your account on the remote system (for example, darvader@deathstar.empire.gov); for example, using command-line SCP:

    You'll be prompted for your account password. Your public key will be copied to your home directory (and saved with the same filename) on the remote system.

  4. Log into the remote system using your account username and password.
    If the remote system is not configured to support password-based authentication, you will need to ask system administrators to add your public key to the ~/.ssh/authorized_keys file in your account (if your account doesn't have ~/.ssh/authorized_keys file, system administrators can create one for you). Once your public key is added to your ~/.ssh/authorized_keys file on the remote system, the setup process is complete, and you should now be able to SSH to your account from the computer that has your private key.
  5. If your account on the remote system doesn't already contain a ~/.ssh/authorized_keys file, create one; on the command line, enter the following commands:
    If your account on the remote system already has a ~/.ssh/authorized_keys file, executing these commands will not damage the existing directory or file.
  6. On the remote system, add the contents of your public key file (for example, ~/id_rsa.pub) to a new line in your ~/.ssh/authorized_keys file; on the command line, enter:

    You may want to check the contents of ~/.ssh/authorized_keys to make sure your public key was added properly; on the command line, enter:

  7. You may now safely delete the public key file (for example, ~/id_rsa.pub) from your account on the remote system; on the command line, enter:

    Alternatively, if you prefer to keep a copy of your public key on the remote system, move it to your .ssh directory; on the command line, enter:

  8. Optionally, repeat steps 3-7 to add your public key to other remote systems that you want to access from the computer that has your private key using SSH public-key authentication.
  9. You now should be able to SSH to your account on the remote system (for example, username@host2.somewhere.edu) from the computer (for example, host1) that has your private key (for example, ~/.ssh/id_rsa):
    • If your private key is password-protected, the remote system will prompt you for the password or passphrase (your private key password/passphrase is not transmitted to the remote system):
    • If your private key is not password-protected, the remote system will place you on the command line in your home directory without prompting you for a password or passphrase:

    If the private key you're using does not have the default name, or is not stored in the default path (not ~/.ssh/id_rsa), you must explicitly invoke it in one of two ways:

    • On the SSH command line: Add the -i flag and the path to your private key.

      For example, to invoke the private key host2_key, stored in the ~/.ssh/old_keys directory, when connecting to your account on a remote host (for example, username@host2.somewhere.edu), enter:

    • In an SSH client configuration file: SSH gets configuration data from the following sources (in this order):
      1. From command-line options
      2. From the user's client configuration file (~/.ssh/config), if it exists
      3. From the system-wide client configuration file (/etc/ssh/ssh_config)

      The SSH client configuration file is a text file containing keywords and arguments. To specify which private key should be used for connections to a particular remote host, use a text editor to create a ~/.ssh/config that includes the Host and IdentityFile keywords.

      For example, for connections to host2.somewhere.edu, to make SSH automatically invoke the private key host2_key, stored in the ~/.ssh/old_keys directory, create a ~/.ssh/config file with these lines included:

      Once you save the file, SSH will use the specified private key for future connections to that host.

      You can add multiple Host and IdentityFile directives to specify a different private key for each host listed; for example:

      Alternatively, you can use a single asterisk ( * ) to provide global defaults for all hosts (specify one private key for several hosts); for example:

      For more about the SSH client configuration file, see the OpenSSH SSH client configuration file on the web or from the command line (man ssh_config).

Set up public-key authentication using PuTTY on a Windows 10 or Windows 8.x computer

The PuTTY command-line SSH client, the PuTTYgen key generation utility, the Pageant SSH authentication agent, and the PuTTY SCP and SFTP utilities are packaged together in a Windows installer available under The MIT License for free download from the PuTTY development team.

After installing PuTTY:

Symmetric Key

  1. Launch PuTTYgen.
  2. In the 'PuTTY Key Generator' window, under 'Parameters':
    • For 'Type of key to generate', select RSA. (In older versions of PuTTYgen, select SSH2-RSA.)
    • For 'Number of bits in a generated key', leave the default value (2048).
  3. Under 'Actions', click Generate.
  4. When prompted, use your mouse (or trackpad) to move your cursor around the blank area under 'Key'; this generates randomness that PuTTYgen uses to generate your key pair.
  5. When your key pair is generated, PuTTYgen displays the public key in the area under 'Key'. In the 'Key passphrase' and 'Confirm passphrase' text boxes, enter a passphrase to passphrase-protect your private key.
    If you don't passphrase-protect your private key, anyone with access to your computer will be able to SSH (without being prompted for a passphrase) to your account on any remote system that has the corresponding public key.
  6. Save your public key:
    1. Under 'Actions', next to 'Save the generated key', click Save public key.
    2. Give the file a name (for example, putty_key), select a location on your computer to store it, and then click Save.
  7. Save your private key:
    1. Under 'Actions', next to 'Save the generated key', click Save private key.
      If you didn't passphrase-protect your private key, the utility will ask whether you're sure you want to save it without a passphrase. Click Yes to proceed or No to go back and create a passphrase for your private key.
    2. Keep 'Save as type' set to PuTTY Private Key Files (*.ppk), give the file a name (for example, putty_private_key), select a location on your computer to store it, and then click Save.
    3. If you wish to connect to a remote desktop system such as Research Desktop (RED), click Conversions > Export OpenSSH key, give the file a name (for example, putty_rsa), select a location on your computer to store it, and then click Save.
  8. Log into the remote system using your account username and password.

    If the remote system does not support password-based authentication, you will need to ask system administrators to add your public key to the ~/.ssh/authorized_keys file in your account (if your account doesn't have ~/.ssh/authorized_keys file, system administrators can create one for you). Once your public key is added to your account's ~/.ssh/authorized_keys file on the remote system..

  9. If your account on the remote system doesn't already contain a ~/.ssh/authorized_keys file, create one; on the command line, enter the following commands:

    If your account on the remote system already has ~/.ssh/authorized_keys, executing these commands will not damage the existing directory or file.

  10. On your computer, in the PuTTYgen utility, copy the contents of the public key (displayed in the area under 'Key') onto your Clipboard. Then, on the remote system, use your favorite text editor to paste it onto a new line in your ~/.ssh/authorized_keys file, and then save and close the file.
  11. On your computer, open the Pageant SSH authentication agent. This utility runs in the background, so when it opens, you should see its icon displayed in the Windows notification area.
  12. In the Windows notification area, right-click on the Pageant icon, select Add Key, navigate to the location where you saved your private key (for example, putty_private_key.ppk), select the file, and then click Open.
  13. If your private key is passphrase-protected, Pageant will prompt you to enter the passphrase; enter the passphrase for your private key, and then click OK.

    If your private key is not passphrase-protected, Pageant will add your private key without prompting you for a passphrase.

    Either way, Pageant stores the unencrypted private key in memory for use by PuTTY when you initiate an SSH session to the remote system that has your public key.

  14. On your computer, open the PuTTY SSH client:
    1. On the Session screen:
      • Under 'Host Name (or IP address)', enter your username coupled with the hostname of the remote server that has your public key; for example:
      • Under 'Connection type', make sure SSH is selected.
    2. In the 'Category' list on the left, navigate to the Auth screen (Connection > SSH > Auth). On the Auth screen, under 'Authentication methods', select Attempt authentication using Pageant.
    3. Return to the Session screen, and under 'Saved Sessions', enter a name (for example, Deathstar), and then click Save.
    4. Click Open to connect to your account on the remote system. With Pageant running in the background, PuTTY will retrieve the unencrypted private key automatically from Pageant and use it to authenticate. Because Pageant has your private key's passphrase saved (if applicable), the remote system will place you on the command line in your account without prompting you for the passphrase.
    Technically, at this point, the setup is complete. In the future, whenever you log into your Windows desktop, you can run Pageant, add the private key, and then use PuTTY to SSH to any remote resource that has your public key. Alternatively, you can create a shortcut in your Windows Startup folder to launch Pageant and load your private key automatically whenever you log into your desktop. For instructions, finish the rest of the following steps.
  15. Open your Startup folder. Press Win-r, and in the 'Open' field, type shell:startup, and then press Enter.
  16. Right-click inside the Startup folder, and then select New and Shortcut.
  17. In the 'Type the location of the item' text box, enter the path to the Pageant executable (pageant.exe) followed by the path to your private key file (for example, putty_private_key.ppk); enclose both paths in double quotes; for example:
  18. Click Next, and then, in the 'Type a name for this shortcut' text box, enter a name for the shortcut (for example, PAGEANT).
  19. Click Finish.

The next time you log into your Windows desktop, Pageant will start automatically, load your private key, and (if applicable) prompt you for the passphrase.