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8.2.4. Using Key-based Authentication

To improve the system security even further, generate SSH key pairs and then enforce key-based authentication by disabling password authentication. To do so, open the /etc/ssh/sshd_config configuration file in a text editor such as vi or nano, and change the PasswordAuthentication option as follows:
PasswordAuthentication no
If you are working on a system other than a new default installation, check that PubkeyAuthentication no has not been set. If connected remotely, not using console or out-of-band access, testing the key-based log in process before disabling password authentication is advised.
To be able to use ssh, scp, or sftp to connect to the server from a client machine, generate an authorization key pair by following the steps below. Note that keys must be generated for each user separately.
Fedora 24 uses SSH Protocol 2 and RSA keys by default (see Section 8.1.3, “Protocol Versions” for more information).

Do not generate key pairs as root

If you complete the steps as root, only root will be able to use the keys.

Backup your ~/.ssh/ directory

If you reinstall your system and want to keep previously generated key pairs, backup the ~/.ssh/ directory. After reinstalling, copy it back to your home directory. This process can be done for all users on your system, including root.

8.2.4.1. Generating Key Pairs

To generate an RSA key pair for version 2 of the SSH protocol, follow these steps:
  1. Generate an RSA key pair by typing the following at a shell prompt:
    ~]$ ssh-keygen -t rsa
    Generating public/private rsa key pair.
    Enter file in which to save the key (/home/USER/.ssh/id_rsa):
  2. Press Enter to confirm the default location, ~/.ssh/id_rsa, for the newly created key.
  3. Enter a passphrase, and confirm it by entering it again when prompted to do so. For security reasons, avoid using the same password as you use to log in to your account.
    After this, you will be presented with a message similar to this:
    Your identification has been saved in /home/USER/.ssh/id_rsa.
    Your public key has been saved in /home/USER/.ssh/id_rsa.pub.
    The key fingerprint is:
    e7:97:c7:e2:0e:f9:0e:fc:c4:d7:cb:e5:31:11:92:14 USER@penguin.example.com
    The key's randomart image is:
    +--[ RSA 2048]----+
    |             E.  |
    |            . .  |
    |             o . |
    |              . .|
    |        S .    . |
    |         + o o ..|
    |          * * +oo|
    |           O +..=|
    |           o*  o.|
    +-----------------+
  4. By default, the permissions of the ~/.ssh/ directory are set to rwx------ or 700 expressed in octal notation. This is to ensure that only the USER can view the contents. If required, this can be confirmed with the following command:
    ~]$ ls -ld ~/.ssh
    drwx------. 2 USER USER 54 Nov 25 16:56 /home/USER/.ssh/
  5. To copy the public key to a remote machine, issue a command in the following format:
     ssh-copy-id user@hostname
    This will copy the most recently modified ~/.ssh/id*.pub public key if it is not yet installed. Alternatively, specify the public key's file name as follows:
    ssh-copy-id -i ~/.ssh/id_rsa.pub user@hostname
    This will copy the content of ~/.ssh/id_rsa.pub into the ~/.ssh/authorized_keys file on the machine to which you want to connect. If the file already exists, the keys are appended to its end.
To generate an ECDSA key pair for version 2 of the SSH protocol, follow these steps:
  1. Generate an ECDSA key pair by typing the following at a shell prompt:
    ~]$ ssh-keygen -t ecdsa
    Generating public/private ecdsa key pair.
    Enter file in which to save the key (/home/USER/.ssh/id_ecdsa):
  2. Press Enter to confirm the default location, ~/.ssh/id_ecdsa, for the newly created key.
  3. Enter a passphrase, and confirm it by entering it again when prompted to do so. For security reasons, avoid using the same password as you use to log in to your account.
    After this, you will be presented with a message similar to this:
    Your identification has been saved in /home/USER/.ssh/id_ecdsa.
    Your public key has been saved in /home/USER/.ssh/id_ecdsa.pub.
    The key fingerprint is:
    fd:1d:ca:10:52:96:21:43:7e:bd:4c:fc:5b:35:6b:63 USER@penguin.example.com
    The key's randomart image is:
    +--[ECDSA  256]---+
    |       .+ +o     |
    |       . =.o     |
    |        o o +  ..|
    |         + + o  +|
    |        S o o oE.|
    |           + oo+.|
    |            + o  |
    |                 |
    |                 |
    +-----------------+
  4. By default, the permissions of the ~/.ssh/ directory are set to rwx------ or 700 expressed in octal notation. This is to ensure that only the USER can view the contents. If required, this can be confirmed with the following command:
    ~]$ ls -ld ~/.ssh
                  ~]$ ls -ld ~/.ssh/
    drwx------. 2 USER USER 54 Nov 25 16:56 /home/USER/.ssh/
  5. To copy the public key to a remote machine, issue a command in the following format:
    ssh-copy-id USER@hostname
    This will copy the most recently modified ~/.ssh/id*.pub public key if it is not yet installed. Alternatively, specify the public key's file name as follows:
    ssh-copy-id -i ~/.ssh/id_ecdsa.pub USER@hostname
    This will copy the content of ~/.ssh/id_ecdsa.pub into the ~/.ssh/authorized_keys on the machine to which you want to connect. If the file already exists, the keys are appended to its end.
See Section 8.2.4.2, “Configuring ssh-agent” for information on how to set up your system to remember the passphrase.

Never share your private key

The private key is for your personal use only, and it is important that you never give it to anyone.

8.2.4.2. Configuring ssh-agent

To store your passphrase so that you do not have to enter it each time you initiate a connection with a remote machine, you can use the ssh-agent authentication agent. To save your passphrase for a certain shell prompt, use the following command:
~]$ ssh-add
Enter passphrase for /home/USER/.ssh/id_rsa:
Note that when you log out, your passphrase will be forgotten. You must execute the command each time you log in to a virtual console or a terminal window.