SSH keys

SSH keys serve as a means of identifying yourself to an SSH server using public-key cryptography and challenge-response authentication. One immediate advantange this method has over traditional password authentication is that you can be authenticated by the server without ever having to send your password over the network. Anyone eavesdropping on your connection will not be able to intercept and crack your password because it is never actually transmitted. Additionally, Using SSH keys for authentication virtually eliminates the risk posed by brute-force password attacks by drastically reducing the chances of the attacker correctly guessing the proper credentials.

As well as offering additional security, SSH key authentication can be more convenient than the more traditional password authentication. When used with a program known as an SSH agent, SSH keys can allow you to connect to a server, or multiple servers, without having to remember or enter your password for each system.

SSH keys are not without their drawbacks and may not be appropriate for all environments, but in many circumstances they can offer some strong advantages. A general understanding of how SSH keys work will help you decide how and when to use them to meet your needs. This article assumes you already have a basic understanding of the Secure Shell protocol and have installed the openssh package, available in the Official Repositories.

Background

SSH keys always come in pairs, one private and the other public. The private key is known only to you and it should be safely guarded. By contrast, the public key can be shared freely with any SSH server to which you would like to connect.

When an SSH server has your public key on file and sees you requesting a connection, it uses your public key to construct and send you a challenge. This challenge is like a coded message and it must be met with the appropriate response before the server will grant you access. What makes this coded message particularly secure is that it can only be understood by someone with the private key. While the public key can be used to encrypt the message, it cannot be used to decrypt that very same message. Only you, the holder of the private key, will be able to correctly understand the challenge and produce the correct response.

This challenge-response phase happens behind the scenes and is invisible to the user. As long as you hold the private key, which is typically stored in the ~/.ssh/ directory, your SSH client should be able to reply with the appropriate response to the server.

Because private keys are considered sensitive information, they are often stored on disk in an encrypted form. In this case, when the private key is required, a passphrase must first be entered in order to decrypt it. While this might superficially appear the same as entering a login password on the SSH server, it is only used to decrypt the private key on the local system. This passphrase is not, and should not, be transmitted over the network.

Generating an SSH key pair

An SSH key pair can be generated by running the ssh-keygen command:

$ ssh-keygen -b 521 -t ecdsa -C"$(id -un)@$(hostname)-$(date --rfc-3339=date)"

Generating public/private ecdsa key pair.
Enter file in which to save the key (/home/username/.ssh/id_ecdsa):
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /home/username/.ssh/id_ecdsa.
Your public key has been saved in /home/username/.ssh/id_ecdsa.pub.
The key fingerprint is:
dd:15:ee:24:20:14:11:01:b8:72:a2:0f:99:4c:79:7f username@localhost-2011-12-22
The key's randomart image is:
+--[ECDSA  521]---+
|     ..oB=.   .  |
|    .    . . . . |
|  .  .      . +  |
| oo.o    . . =   |
|o+.+.   S . . .  |
|=.   . E         |
| o    .          |
|  .              |
|                 |
+-----------------+

In the above example, ssh-keygen generates a 521 bit long (-b 521) public/private ECDSA (-t ecdsa) key pair with an extended comment including the data (-C"$(id -un)"@$(hostname)-$(date --rfc-3339=date)). The randomart image was introduced in OpenSSH 5.1 as an easier means of visually identifying they key fingerprint.

Choosing the type of encryption

The Elliptic Curve Digital Signature Algorithm (ECDSA) provides smaller key sizes and faster operations for equivalent estimated security to the previous methods. It was introduced as the preferred algorithm for authentication in OpenSSH 5.7, see OpenSSH 5.7 Release Notes. ECDSA keys might not be compatible with systems that ship old versions of OpenSSH.

If you choose to create an RSA (2048-4096 bit) or DSA (1024 bit) key pair instead of ECDSA, use the -t rsa or -t dsa switches in your ssh-keygen command and do not forget to increase the key size. Running ssh-keygen without the -b switch should provide reasonable defaults.

Note: These keys are used only to authenticate you, choosing stronger keys will not increase CPU load when transferring data over SSH.

Choosing the key location and passphrase

Upon issuing the ssh-keygen command, you will be prompted for the desired name an location of your private key. By default, keys are stored in the ~/.ssh/ directory and named according the type of encryption used. You are advised to accept the default name and location in order for later code examples in this article to work properly.

When prompted for a passphrase, choose something that will be hard to guess if you have the security of your private key in mind. A longer, more random password will generally be stronger and harder to crack should it fall into the wrong hands.

It is also possible to create your private key without a passphrase. While this can be convenient, you need to be aware of the associated risks. Without a passphrase, your private key will be stored on disk in an unencrypted form. Anyone who gains access to your private key file will then be able to assume your identity on any SSH server to which you connect using key-based authentication. Furthermore, without a passphrase, you must also trust the root user, as he can bypass file permissions and will be able to access your unencrypted private key file at any time.

Copying the public key to the remote server

Once you have generated a key pair, you will need to copy the public key to the remote server so that it will use SSH key authentication. The public key file shares the same name as the private key except that is appended with a .pub extension. Note that the private key is not shared and remains on the local machine.

Simple method

If your key file is ~/.ssh/id_rsa.pub you can simply enter the following command.

$ ssh-copy-id remote-server.org

If your username differs on remote machine, be sure to prepend the username followed by @ to the server name.

$ ssh-copy-id username@remote-server.org

If your public key filename is anything other than the default of ~/.ssh/id_rsa.pub you will get an error stating /usr/bin/ssh-copy-id: ERROR: No identities found. In this case, you must explicitly provide the location of the public key.

$ ssh-copy-id -i ~/.ssh/id_ecdsa.pub username@remote-server.org

If the ssh server is listening on a port other than default of 22, be sure to include it within the host argument.

$ ssh-copy-id -i ~/.ssh/id_rsa.pub '-p 221 username@remote-server.org'

Traditional method

By default, for OpenSSH, the public key needs to be concatenated with ~/.ssh/authorized_keys. Begin by copying the public key to the remote server.

$ scp ~/.ssh/id_ecdsa.pub username@remote-server.org:

The above example copies the public key (id_ecdsa.pub) to your home directory on the remote server via scp. Do not forget to include the : at the end of the server address. Also note that the name of your public key may differ from the example given.

On the remote server, you will need to create the ~/.ssh directory if it does not yet exist and append your public key to the authorized_keys file.

$ ssh username@remote-server.org
username@remote-server.org's password:
$ mkdir ~/.ssh
$ cat ~/id_ecdsa.pub >> ~/.ssh/authorized_keys
$ rm ~/id_ecdsa.pub
$ chmod 600 ~/.ssh/authorized_keys

The last two commands remove the public key file from the server and set the permissions on the authorized_keys file such that it is only readable and writable by you, the owner.

Disconnect from the server and then attempt to reconnect. You should now be asked for the passphrase of the key.

$ ssh username@remote-server.org
Enter passphrase for key '/home/username/.ssh/id_ecdsa':

If you are unable to login with the key, double check the permissions on the authorized_keys file.

Also check the permissions on the ~/.ssh directory, which should have write permissions off for 'group' and 'other'. Run the following command to disable 'group' and 'other' write permissions for the ~/.ssh directory.

$ chmod go-w ~/.ssh

Disabling password logins

While copying your public key to the remote SSH server eliminates the need to transmit your password over the network, it does not give any added protection against a brute-force password attack. In the absense of a private key, the SSH server will fall back to password authentication by default, thus allowing a malicious user to attempt to gain access by guessing your password. To disable this behavior, edit the following lines in the /etc/ssh/sshd_config file on the remote server.

/etc/ssh/sshd_config

PasswordAuthentication no
ChallengeResponseAuthentication no

SSH agents

If your private key is encrypted with a passphrase, this passphrase must be entered every time you attempt to connect to an SSH server using public-key authentication. Each individual invocation of ssh or scp will need the passphrase in order to decrypt your private key before authentication can proceed.

An SSH agent is a program which caches your decrypted private keys and provides them to SSH client programs on your behalf. In this arrangement, you must only provide your passphrase once, when adding your private key to the agent's cache. This facility can be of great convenience when making frequent SSH connections.

An agent is typically configured to run automatically upon login and persist for the duration of your login session. A variety of agents, front-ends, and configurations exist to achieve this effect. This section provides an overview of a number of different solutions which can be adapted to meet your specific needs.

ssh-agent

ssh-agent is the default agent included with OpenSSH. It can be used directly or serve as the back-end to a few of the front-end solutions mentioned later in this section. When ssh-agent is run, it will fork itself to the background and print out the environment variables it would use.

$ ssh-agent
SSH_AUTH_SOCK=/tmp/ssh-vEGjCM2147/agent.2147; export SSH_AUTH_SOCK;
SSH_AGENT_PID=2148; export SSH_AGENT_PID;
echo Agent pid 2148;

To make use of these variables, run the command through the eval command.

$ eval $(ssh-agent)
Agent pid 2157

You can append the above command to your ~/.bash_profile script so that it will run automatically when starting a login shell.

$ echo 'eval $(ssh-agent)' >> ~/.bash_profile

If you would rather have ssh-agent run automatically for all users append the command to /etc/profile instead.

# echo 'eval $(ssh-agent)' >> /etc/profile

Once ssh-agent is running, you will need to add your private key to its cache.

$ ssh-add ~/.ssh/id_ecdsa
Enter passphrase for /home/user/.ssh/id_ecdsa:
Identity added: /home/user/.ssh/id_ecdsa (/home/user/.ssh/id_ecdsa)

If you would like your private keys to be added automatically on login. Append the following command to your ~/.bash_profile as well.

$ echo 'ssh-add' >> ~/.bash_profile

If your private key is encrypted ssh-add will prompt you to enter your passphrase. Once your private key has been successfully added to the agent you will be able to make SSH connections without having to enter a passphrase.

One downside to this approach is that a new instance of ssh-agent is created for every login shell and each instance will persist between login sessions. Over time you can wind up with dozens of needless {ic|ssh-agent}} processes running. There exist a number of front-ends to ssh-agent and alternative agents described later in this section which avoid this problem.

GnuPG Agent

The GnuPG agent, distributed with the gnupg2 package, available in the Official Repositories, has OpenSSH agent emulation. If you use GPG you might consider using its agent to take care of all of your keys. Otherwise you might like the PIN entry dialog it provides and its passphrase management, which is different from Keychain.

To start using GPG agent for your SSH keys you should first start the gpg-agent with the --enable-ssh-support option. Example (don't forget to make the file executable):

/etc/profile.d/gpg-agent.sh

#!/bin/sh

# Start the GnuPG agent and enable OpenSSH agent emulation
gnupginf="${HOME}/.gnupg/gpg-agent.info"

if pgrep -u "${USER}" gpg-agent >/dev/null 2>&1; then
    eval `cat $gnupginf`
    eval `cut -d= -f1 $gnupginf | xargs echo export`
else
    eval `gpg-agent --enable-ssh-support --daemon`
fi

Once gpg-agent is running you can use ssh-add to approve keys, just like you did with plain ssh-agent. The list of approved keys is stored in the ~/.gnupg/sshcontrol file. Once your key is approved you will get a PIN entry dialog every time your passphrase is needed. You can control passphrase caching in the ~/.gnupg/gpg-agent.conf file. The following example would have gpg-agent cache your keys for 3 hours:

 # Cache settings
 default-cache-ttl 10800
 default-cache-ttl-ssh 10800

Other useful settings for this file include the PIN entry program (GTK, QT or ncurses version), keyboard grabbing and so on...:

 # Environment file
 write-env-file /home/username/.gnupg/gpg-agent.info
 
 # Keyboard control
 #no-grab
   
 # PIN entry program
 #pinentry-program /usr/bin/pinentry-curses
 #pinentry-program /usr/bin/pinentry-qt4
 pinentry-program /usr/bin/pinentry-gtk-2

Keychain

Keychain is a program designed to help you easily manage your SSH keys with minimal user interaction. It is implemented as a shell script which drives both ssh-agent and ssh-add. A notable feature of Keychain is that it can maintain a single ssh-agent process across multiple login sessions. This means that you only need to enter your passphrase once each time your local machine is booted.

Install the keychain package, available from the official repositories.

Append the following line to ~/.bash_profile, or create /etc/profile.d/keychain.sh as root and make it executable (e.g. chmod 755 keychain.sh):

~/.bash_profile

eval $(keychain --eval --agents ssh -Q --quiet id_ecdsa)

In the above example, the --eval flag outputs lines to be evaluated by the opening eval command. This sets the necessary environments variables for SSH client to be able to find your agent. The --agents flag is not strictly necessary, because Keychain will build the list automatically based on the existence of ssh-agent or gpg-agent on the system. Adding the --quiet flag will limit output to warnings, errors, and user prompts. If you want greater security replace -Q with --clear but will be less convenient.

If necessary, replace ~/.ssh/id_ecdsa with the path to your private key. For those using a non-Bash compatible shell, see keychain --help or man keychain for details on other shells.

To test Keychain, log out from your session and log back in. If this is your first time running Keychain, it will prompt you for the passphrase of the specified private key. Because Keychain reuses the same ssh-agent process on successive logins, you shouldn't have to enter your passphrase the next time you log in. You will only ever be prompted for your passphrase once each time the machine is rebooted.

Alternate startup methods

There are numerous ways in which Keychain can be invoked and you are encouraged to experiment to find a method that works for you. The keychain command itself comes with dozens of command-line options which are described in the Keychain man page.

One alternative implementation of a Keychain startup script could be to create the file /etc/profile.d/keychain.sh as the root user and add the following lines.

/etc/profile.d/keychain.sh

/usr/bin/keychain -Q -q --nogui ~/.ssh/id_ecdsa
[[ -f $HOME/.keychain/$HOSTNAME-sh ]] && source $HOME/.keychain/$HOSTNAME-sh

Be sure to also make /etc/profile.d/keychain.sh executable by changing its file permissions.

# chmod 755 /etc/profile.d/keychain.sh

x11-ssh-askpass

You need to start the ssh-agent everytime you start a new Xsession. The ssh-agent will be closed when the X session ends.

Install a variant of x11-ssh-askpass which will ask your passphrase everytime you open a new Xsession. Either use the original x11-ssh-askpass, go with ksshaskpass (depends on kdelibs) or with openssh-askpass (depends on qt), all available from the official repositories.

After installing these, closing your Xsession and relogging you will henceforth be asked your passphrase on Xsession init without any further work.

pam_ssh

The pam_ssh project exists to provide a Pluggable Authentication Module (PAM) for SSH private keys. This module can provide single sign-on behavior for your SSH connections. On login, your SSH private key passphrase can be entered in place of, or in addition to, your traditional system password. Once you have been authenticated, the pam_ssh module spawns ssh-agent to store your decrypted private key for the duration of the session.

To enable single sign-on behavior at the tty login prompt, install the unofficial pam_ssh^AUR package, available in the Arch User Repository.

Edit the /etc/pam.d/login configuration file to include the text highlighted in bold in the example below. The order in which these lines appear is significiant and can affect login behavior.

Warning: Misconfiguring PAM can leave the system in a state where all users become locked out. Before making any changes, you should have an understanding of how PAM configuration works as well as a backup means of accessing the PAM configuration files, such as an Arch Live CD, in case you become locked out and need to revert any changes. An IBM developerWorks article is available which explains PAM configuration in further detail.

/etc/pam.d/login

#%PAM-1.0
auth            required        pam_securetty.so
auth            requisite       pam_nologin.so
auth            sufficient      pam_ssh.so
auth            required        pam_unix.so nullok use_first_pass
auth            required        pam_tally.so onerr=succeed file=/var/log/faillog
# use this to lockout accounts for 10 minutes after 3 failed attempts
#auth           required        pam_tally.so deny=2 unlock_time=600 onerr=succeed file=/var/log/faillog
account         required        pam_access.so
account         required        pam_time.so
account         required        pam_unix.so
#password       required        pam_cracklib.so difok=2 minlen=8 dcredit=2 ocredit=2 retry=3
#password       required        pam_unix.so sha512 shadow use_authtok
session         required        pam_unix.so
session         required        pam_env.so
session         required        pam_motd.so
session         required        pam_limits.so
session         optional        pam_mail.so dir=/var/spool/mail standard
session         optional        pam_lastlog.so
session         optional        pam_loginuid.so
session         optional        pam_ssh.so
-session        optional        pam_ck_connector.so nox11
-session        optional        pam_systemd.so

In the above example, login uses pam_ssh to check the entered password against the user's SSH private key passphrase. If the password matches, the user is immediately granted access to the system. If the password does not match, control falls to the pam_unix module which provides traditional password authentication. Because the use_first_pass option is set, the pam_unix module checks the previously entered password against the /etc/passwd file instead of asking for a password again. In this way, ssh_pam will be transparent to users without an SSH private key.

If you use a another means of logging in, such as an X11 display manager like SLiM or XDM and you would like it to provide similar functionality, you must edit its associated PAM configuration file in a similar fashion. Packages providing support for PAM typically place a default configuration file in the /etc/pam.d/ directory.

Further details on how to use pam_ssh and a list of its options can be found in the pam_ssh man page.

Known issues with pam_ssh

Work on the pam_ssh project is infrequent and the documentation provided is sparse. You should be aware of some of its limitations which are not mentioned in the package itself.

• SSH keys employing the newer option of ECDSA (elliptic curve) cryptography do not appear to be supported by pam_ssh. You must use either RSA or DSA keys.

• The ssh-agent process spawned by pam_ssh does not persist between user logins. If you like to keep a GNU Screen session active between logins you may notice when reattaching to your screen session that it can no longer communicate with ssh-agent. This is because the GNU Screen environment and those of its children will still reference the instance of ssh-agent which existed when GNU Screen was invoked but was subsequently killed in a previous logout. The Keychain front-end avoids this problem by keeping the ssh-agent process alive between logins.

GNOME Keyring

If you use the GNOME desktop, the GNOME Keyring tool can be used as an SSH agent. Visit the GNOME Keyring article.

Troubleshooting

If it appears that the SSH server is ignoring your keys, ensure that you have the proper permissions set on all relevant files.

For the local machine:

$ chmod 700 ~/
$ chmod 700 ~/.ssh
$ chmod 600 ~/.ssh/id_ecdsa

For the remote machine:

$ chmod 700 ~/
$ chmod 700 ~/.ssh
$ chmod 600 ~/.ssh/authorized_keys

Failing this, run the sshd in debug mode and monitor the output while connecting:

# /usr/sbin/sshd -d

See also

• OpenSSH key management, Part 1
• OpenSSH key management, Part 2
• OpenSSH key management, Part 3
• Getting started with SSH
• OpenSSH 5.7 Release Notes