Use SSH Public Key Authentication on Linux, macOS, and Windows

Updated , by Linode

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Password authentication is the default method most SSH (Secure Shell) clients use to authenticate with remote servers, but it suffers from potential security vulnerabilities like brute-force login attempts. An alternative to password authentication is using public key authentication with SSH, in which you generate and store on your computer a pair of cryptographic keys and then configure your server to recognize and accept your keys. Using key-based authentication offers a range of benefits:

  • Key-based login is not a major target for brute-force hacking attacks.

  • If a server that uses SSH keys is compromised by a hacker, no authorization credentials are at risk of being exposed.

  • Because a password isn’t required at login, you can log into servers from within scripts or automation tools that you need to run unattended. For example, you can set up periodic updates for your servers with a configuration management tool like Ansible, and you can run those updates without having to be physically present.

This guide explains how the SSH key login scheme works, how to generate an SSH key, and how to use those keys with your Linode.

Note
If you’re unfamiliar with SSH connections, review the Getting Started with Linode guide.

How SSH Public Keys Work

SSH keys are generated in pairs and stored in plain-text files. The key pair (or keypair) consists of two parts:

  • A private key, usually named id_rsa. The private key is stored on your local computer and should be kept secure, with permissions set so that no other users on your computer can read the file.

    Caution
    Do not share your private key with others.
  • A public key, usually named id_rsa.pub. The public key is placed on the server you intend to log in to. You can freely share your public key with others. If someone else adds your public key to their server, you will be able to log in to that server.

When a site or service asks for your SSH key, they are referring to your SSH public key (id_rsa.pub). For instance, services like GitHub and Gitlab allow you to place your SSH public key on their servers to streamline the process of pushing code changes to remote repositories.

How Does SSH Public Key Authentication Work?

In the previous section, we saw that we have a public key and a private key. We understand that they play an important role in enabling secure access. But how? The best way to understand them is to understand that the following components in this authentication system are mathematically related to each other:

  1. Public key
  2. Private key
  3. Authentication algorithm

If you use your Public key to encrypt something, then only your private key can decrypt it. Similarly, once you encrypt something using your private key, it can only be decrypted by your public key. And to enable secure access between servers/machines, we share our public key with the other machine to enable secure access.

But to carry this encryption and decryption, there is an algorithm that runs in the background and keeps SSH secure. Here’s how it works:

  1. Signed communication: Any message that goes out is signed using your private keys.
  2. Verification of communication: Your server has a public key from the sender stored. A signed message is verified by using this public key to decrypt the message.

When we sign a message, we allow others to decrypt the message as well. But when the receiver decrypts this message, they can safely and securely validate that the communication is in fact from you. To match these keys and validate, we use an algorithm like Diffie-Hellman.

The authorized_keys File

In order for your Linode to recognize and accept your key pair, you must upload your public key to your server. More specifically, you must upload your public key to the home directory of the user you would like to log in as. If you would like to log in to more than one user on the server using your key pair, you must add your public key to each of those users.

To set up SSH key authentication for one of your server’s users, add your public key to a new line inside the user’s authorized_keys file. This file is stored inside a directory named .ssh/ under the user’s home folder. A user’s authorized_keys file can store more than one public key, and each public key is listed on its own line. If your file contains more than one public key, then the owner of each key listed can log in as that user.

Granting Someone Else Access to Your Server

To give someone else access to your server’s user, simply add their public key on a new line in your authorized_keys file, just as you would add your own. To revoke access for that person, remove that same line and save the changes.

Challenge-Response

When logging in to a server using SSH, if that servers has a public key on file, the server creates a challenge. This challenge is crafted in such a way that only the holder of the private SSH key can decipher it.

This challenge-response action happens without any user interaction. If the person attempting to log in has the corresponding private key, then they can safely log in. If not, the login either fails or falls back to a password-based authentication scheme.

SSH Private Key Passphrases

You can optionally provide an additional level of security for your SSH private key by encrypting it locally with a passphrase at the time of creation. When you attempt to log in using an encrypted SSH key, you are prompted to enter its passphrase. This is not to be confused with a password, as this passphrase only decrypts the key file locally. A passphrase is not transferred over the Internet as a password might be.

If you’d like to set up your logins so that they require no user input, then creating a passphrase might not be desirable. Nevertheless, using a passphrase to protect your private key is strongly recommended.

Is it Safe to Share Public SSH Key?

Yes, it is safe to share your public SSH key with others. Public keys usually stored as id_rsa.pub are used to log into other servers. If anyone else has your public SSH keys on their server and they add them, you can log into their servers.

Caution
Do not confuse private SSH keys with public SSH keys. Private SSH keys should be kept safe and secure, unlike public SSH keys.

How Secure is SSH Key Authentication?

SSH key authentication is very secure. In addition to allowing secure remote authentication, it also brings its ability to withstand brute force attacks. Typically, passwords sent over any network can be vulnerable to these brute force attacks. With SSH key authentication, signed messages are exchanged using SSH keys that are up to 4096 bits in length, which is equivalent to a 20 character password.

SSH keys are machine-generated, and not human-generated. Human bias towards certain strings and numbers has proven to increase vulnerability in secure systems as opposed to machine-generated keys.

What makes SSH even more secure is the fact that you can easily add a passphrase on top of your SSH key authentication. This is also commonly referred to as multi-factor authentication or MFA.

Public Key Authentication on Linux And macOS

Generate an SSH Key Pair on Linux And macOS

Perform the steps in this section on your local machine.

  1. Create a new key pair.

    Caution

    This command will overwrite an existing RSA key pair, potentially locking you out of other systems.

    If you’ve already created a key pair, skip this step. To check for existing keys, run ls ~/.ssh/id_rsa*.

    If you accidentally lock yourself out of the SSH service on your Linode, you can still use the Lish console to login to your server. After you’ve logged in via Lish, update your authorized_keys file to use your new public key. This should re-establish normal SSH access.

    ssh-keygen -b 4096
    

    The -b flag instructs ssh-keygen to increase the number of bits used to generate the key pair, and is suggested for additional security.

  2. Press Enter to use the default names id_rsa and id_rsa.pub in the /home/your_username/.ssh directory before entering your passphrase.

      Generating public/private rsa key pair.
    Enter file in which to save the key (/home/your_username/.ssh/id_rsa):
    
  3. While creating the key pair, you are given the option to encrypt the private key with a passphrase. This means that the key pair cannot be used without entering the passphrase (unless you save that passphrase to your local machine’s keychain manager). We suggest that you use the key pair with a passphrase, but you can leave this field blank if you don’t want to use one.

      Enter passphrase (empty for no passphrase):
    Enter same passphrase again:
    Your identification has been saved in /home/your_username/.ssh/id_rsa.
    Your public key has been saved in /home/your_username/.ssh/id_rsa.pub.
    The key fingerprint is:
    f6:61:a8:27:35:cf:4c:6d:13:22:70:cf:4c:c8:a0:23 [email protected]
    

Upload Your Public Key

There are a few different ways to upload your public key to your Linode from Linux and macOS client systems:

Using ssh-copy-id

ssh-copy-id is a utility available on some operating systems that can copy a SSH public key to a remote server over SSH.

  1. To use ssh-copy-id, pass your username and the IP address of the server you would like to access:

    ssh-copy-id [email protected]
    
  2. You’ll see output like the following, and a prompt to enter your user’s password:

      /usr/bin/ssh-copy-id: INFO: Source of key(s) to be installed: "/home/your_username/.ssh/id_rsa.pub"
    /usr/bin/ssh-copy-id: INFO: attempting to log in with the new key(s), to filter out any that are already installed
    /usr/bin/ssh-copy-id: INFO: 1 key(s) remain to be installed -- if you are prompted now it is to install the new keys
    [email protected]'s password:
    
  3. Verify that you can log in to the server with your key.

Using Secure Copy (scp)

Secure Copy (scp) is a tool that copies files from a local computer to a remote server over SSH:

Caution
These instructions will overwrite any existing contents of the authorized_keys file on your server. If you have already set up other public keys on your server, use the ssh-copy-id command or enter your key manually.
  1. Connect to your server at its IP address via SSH with the user you would like to add your key to:

    ssh [email protected]
    
  2. Create the ~/.ssh directory and authorized_keys file if they don’t already exist:

    mkdir -p ~/.ssh && touch ~/.ssh/authorized_keys
    
  3. Give the ~/.ssh directory and authorized_keys files appropriate file permissions:

    chmod 700 ~/.ssh && chmod 600 ~/.ssh/authorized_keys
    
  4. In another terminal on your local machine, use scp to copy the contents of your SSH public key (id_rsa.pub) into the authorized_keys file on your server. Substitute in your own username and your server’s IP address:

    scp ~/.ssh/id_rsa.pub [email protected]:~/.ssh/authorized_keys
    
  5. Verify that you can log in to the server with your key.

Manually Copy Your Public Key

You can also manually add an SSH key to a server:

  1. Begin by copying the contents of your public SSH key on your local computer. You can use the following command to output the contents of the file:

    cat ~/.ssh/id_rsa.pub
    

    You should see output similar to the following:

      ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQCyVGaw1PuEl98f4/7Kq3O9ZIvDw2OFOSXAFVqilSFNkHlefm1iMtPeqsIBp2t9cbGUf55xNDULz/bD/4BCV43yZ5lh0cUYuXALg9NI29ui7PEGReXjSpNwUD6ceN/78YOK41KAcecq+SS0bJ4b4amKZIJG3JWmDKljtv1dmSBCrTmEAQaOorxqGGBYmZS7NQumRe4lav5r6wOs8OACMANE1ejkeZsGFzJFNqvr5DuHdDL5FAudW23me3BDmrM9ifUzzjl1Jwku3bnRaCcjaxH8oTumt1a00mWci/1qUlaVFft085yvVq7KZbF2OPPbl+erDW91+EZ2FgEi+v1/CSJ5 [email protected]
    

    Note that the public key begins with ssh-rsa and ends with [email protected].

  2. Once you have copied that text, connect to your server via SSH with the user you would like to add your key to:

    ssh [email protected]
    
  3. Create the ~/.ssh directory and authorized_keys file if they don’t already exist:

    mkdir -p ~/.ssh && touch ~/.ssh/authorized_keys
    
  4. Give the ~/.ssh directory and authorized_keys files appropriate file permissions:

    chmod 700 ~/.ssh && chmod 600 ~/.ssh/authorized_keys
    
  5. Open the authorized_keys file with the text editor of your choice ( nano, for example). Then, paste the contents of your public key that you copied in step one on a new line at the end of the file.

  6. Save and close the file.

    Note

    If you initially logged into the server as root but edited the authorized_keys file of another user, then the .ssh/ folder and authorized_keys file of that user may be owned by root. Set that other user as the files' owner:

        chown -R your_username:your_username /home/your_username/.ssh
    
  7. Verify that you can log in to the server with your key.

Connect to the Remote Server

  1. SSH into the server from your local machine:

    ssh [email protected]
    
  2. If you choose to use a passphrase when creating your SSH key, you will be prompted to enter it when you attempt to log in. Depending on your desktop environment, a window may appear:

    Caution
    Do not allow the local machine to remember the passphrase in its keychain unless you are on a private computer which you trust.

    You may also see the passphrase prompt at your command line:

      Enter passphrase for key '/root/.ssh/id_rsa':
    
  3. Enter your passphrase. You should see the connection established in the local terminal.

Public Key Authentication on Windows

The following instructions use the PuTTY software to connect over SSH, but other options are available on Windows too.

Generate a Key Pair with PuTTY

  1. Download PuTTYgen (puttygen.exe) and PuTTY (putty.exe) from the official site.

  2. Launch puttygen.exe. The RSA key type at the bottom of the window is selected by default for an RSA key pair but ED25519 ( EdDSA using Curve25519) is a comparable option if your remote machine’s SSH server supports DSA signatures. Do not use the SSH-1(RSA) key type unless you know what you’re doing.

  3. Increase the RSA key size from 2048 bits 4096 and click Generate:

  4. PuTTY uses the random input from your mouse to generate a unique key. Once key generation begins, keep moving your mouse until the progress bar is filled:

  5. When finished, PuTTY displays the new public key. Right-click on it and select Select All, then copy the public key into a Notepad file.

  6. Save the public key as a .txt file or some other plaintext format. This is important–a rich text format such as .rtf or .doc can add extra formatting characters and then your private key won’t work:

  7. Enter a passphrase for the private key in the Key passphrase and Confirm passphrase text fields:

    Note
    Important: Make a note of your passphrase for later use.

  8. Click Save private key. Choose a filename and location in Explorer while keeping the ppk file extension. If you plan to create multiple key pairs for different servers, be sure to give them different names so that you don’t overwrite old keys with new:

Manually Copy the SSH Key with PuTTY

  1. Launch putty.exe. Find the Connection tree in the Category window, expand SSH and select Auth. Click Browse and navigate to the private key you created above:

  2. Scroll back to the top of the Category window and click Session. Enter the hostname or IP address of your Linode. PuTTY’s default TCP port is 22, the IANA assigned port for SSH traffic. Change it if your server is listening on a different port. Name the session in the Saved Sessions text bar and click Save:

  3. Click the Open button to establish a connection. You are prompted to enter a login name and password for the remote server.

  4. Once you’re logged in to the remote server, configure it to authenticate with your SSH key pair instead of a user’s password. Create an .ssh directory in your home directory on your Linode, create a blank authorized_keys file inside, and set their access permissions:

    mkdir -p ~/.ssh && touch ~/.ssh/authorized_keys
    chmod 700 ~/.ssh && chmod 600 ~/.ssh/authorized_keys
    
  5. Open the authorized_keys file with the text editor of your choice ( nano, for example). Then, paste the contents of your public key that you copied in step one on a new line at the end of the file.

  6. Save, close the file, and exit PuTTY.

  7. Verify that you can log in to the server with your key.

Manually Copy the SSH Key with WinSCP

Uploading a public key from Windows can also be done using WinSCP:

Caution
These instructions will overwrite any existing contents of the authorized_keys file on your server. If you have already set up other public keys on your server, use the PuTTY instructions instead.
  1. In the login window, enter your Linode’s public IP address as the hostname, the user you would like to add your key to, and your user’s password. Click Login to connect.

  2. Once connected, WinSCP shows two file tree sections. The left shows files on your local computer and the right shows files on your Linode. Using the file explorer on the left, navigate to the file where you saved your public key in Windows. Select the public key file and click Upload in the toolbar above.

  3. You are prompted to enter a path on your Linode where you want to upload the file. Upload the file to /home/your_username/.ssh/authorized_keys.

  4. Verify that you can log in to the server with your key.

Connect to the Remote Server with PuTTY

Start PuTTY and Load your saved session. You are be prompted to enter your server user’s login name as before. However, this time you are prompted for your private SSH key’s passphrase rather than the password for your server’s user. Enter the passphrase and press Enter.

Upload Your SSH Key to Linode Cloud Manager

It is possible to provision each new Linode you create with an SSH public key automatically through the Cloud Manager.

  1. Log in to the Cloud Manager.

  2. Click on your username or profile image at the top bar of the page. Then click on SSH Keys in the dropdown menu that appears:

  3. From the SSH Keys page, click Add a SSH Key:

  4. Create a label for your key, then paste in the contents of your public SSH key (id_rsa.pub):

  5. Click Add Key.

  6. When you next create a Linode you are given the opportunity to include your SSH key in the Linode’s creation. This key is added to the root user of the new Linode.

    In the Create Linode form, select the SSH key to include. This field appears below the Root Password field:

Troubleshooting

If your SSH connections are not working as expected, or if you have locked yourself out of your system, review the Troubleshooting SSH guide for troubleshooting help.

Next Steps

After you set up your SSH keys and confirm they are working as expected, review our How to Secure Your Server guide for instructions on disabling password authentication for your server.

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