SSH keys

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zh-cn: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 advantage 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.


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 -t rsa -b 4096 -C "$(whoami)@$(hostname)-$(date -I)"
Generating public/private rsa key pair.
Enter file in which to save the key (/home/username/.ssh/id_rsa):
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /home/username/.ssh/id_rsa.
Your public key has been saved in /home/username/.ssh/
The key fingerprint is:
dd:15:ee:24:20:14:11:01:b8:72:a2:0f:99:4c:79:7f username@localhost-2014-11-22
The key's randomart image is:
+--[RSA  4096]---+
|     ..oB=.   .  |
|    .    . . . . |
|  .  .      . +  |
| oo.o    . . =   |
|o+.+.   S . . .  |
|=.   . E         |
| o    .          |
|  .              |
|                 |

In the above example, ssh-keygen generates a 4096 bit long (-b 4096) public/private RSA (-t rsa) key pair with an extended comment including the data (-C "$(whoami)@$(hostname)-$(date -I)"). The randomart image was introduced in OpenSSH 5.1 as an easier means of visually identifying the 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. Some vendors also disable the required implementations due to potential patent issues.

Warning: There is reason to be suspicious of the NIST curves used to generate ECDSA keys. Depending on the threat model, it might be advisable to use Ed25519 or RSA.
Note: As of December 28, 2013, the Windows SSH client PuTTY does not support ECDSA and cannot connect to a server that uses ECDSA keys.
Note: As of July 10, 2015, GNOME Keyring does not handle ECDSA[1] and Ed25519[2] keys. Users will have to turn to other SSH agents.

As of OpenSSH 6.5 Ed25519 keys are supported: "Ed25519 is an elliptic curve signature scheme that offers better security than ECDSA and DSA and good performance."[3] They can be generated by ssh-keygen -t ed25519. There is no need to set the key size, as all Ed25519 keys are 256 bits.

If you choose to create an RSA (2048-16384 bit) or DSA (2048 bit) key pair instead, 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: As of August 11, 2015, DSA is deprecated and disabled by default[4] in OpenSSH 7.0 due to discovered vulnerabilities.

Starting with the 7.0 release of OpenSSH, your best option is to generate new keys using strong algorithms such as RSA, or ECDSA, or Ed25519. RSA keys will give you the greatest portability with other clients/servers while Ed25519 will get you the best security with OpenSSH (but requires recent versions of client & server)[5].

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 and location of your private key. By default, keys are stored in the ~/.ssh/ directory and named according to 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.

Changing the private key's passphrase without changing the key

If the originally chosen SSH key passphrase is undesirable or must be changed, one can use the ssh-keygen command to change the passphrase without changing the actual key.

To change the passphrase for the private RSA key, run the following command:

$ ssh-keygen -f ~/.ssh/id_rsa -p

Managing multiple keys

It is possible, and common. to use the same SSH key for multiple host computers. However, if you would like to have a separate key for each host computer, you can create the file ~/.ssh/config. Below is an example of the file.

  IdentitiesOnly yes
  IdentityFile ~/.ssh/id_rsa_SERVER1
  # CheckHostIP yes
  # Port 22
  IdentitiesOnly yes
  IdentityFile ~/.ssh/id_rsa_SERVER2
  # CheckHostIP no
  # Port 2177
ControlMaster auto
ControlPath /tmp/%r@%h:%p

For a full list of options, look at the following man page.

$ man ssh_config

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 it is appended with a .pub extension. Note that the private key is not shared and remains on the local machine.

Simple method

Note: This method might fail if the remote server uses a non-sh shell such as tcsh as default and uses OpenSSH older than 6.6.1p1. See this bug report.

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

$ ssh-copy-id

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

$ ssh-copy-id

If your public key filename is anything other than the default of ~/.ssh/ 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/

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/ -p 221

Manual 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/

The above example copies the public key ( 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's password:
$ mkdir ~/.ssh
$ chmod 700 ~/.ssh
$ cat ~/ >> ~/.ssh/authorized_keys
$ rm ~/
$ 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.


Securing the authorized_keys file

For additional protection, you can prevent users from adding new public keys and connecting from them.

In the server, make the authorized_keys file read-only for the user and deny all other permissions:

$ chmod 400 ~/.ssh/authorized_keys

To keep the user from simply changing the permissions back, set the immutable bit on the authorized_keys file. After that the user could rename the ~/.ssh directory to something else and create a new ~/.ssh directory and authorized_keys file. To prevent this, set the immutable bit on the ~/.ssh directory too.

Note: If you find yourself needing to add a new key, you will first have to remove the immutable bit from authorized_keys and make it writable. Follow the steps above to secure it again.

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 absence 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.

PasswordAuthentication no
ChallengeResponseAuthentication no

Two-factor authentication and public keys

Since OpenSSH 6.2, you can add your own chain to authenticate with using the AuthenticationMethods option. This enables you to use public keys as well as a two-factor authorization.

See Google Authenticator to set up Google Authenticator.

To use PAM with OpenSSH, edit the following files:

ChallengeResponseAuthentication yes
AuthenticationMethods publickey keyboard-interactive:pam

Then you can log in with either a publickey or the user authentication as required by your PAM setup.

If, on the other hand, you want to authenticate the user on both a publickey and the user authentication as required by your PAM setup, use a comma instead of a space to separate the AuthenticationMethods:

ChallengeResponseAuthentication yes
AuthenticationMethods publickey,keyboard-interactive:pam

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 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 forks to background and prints necessary environment variables. E.g.

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

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

$ eval $(ssh-agent)
Agent pid 2157

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 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 your passphrase.

In order to have all this happen automatically, and make sure that only one ssh-agent process runs at a time, add the following to your ~/.bashrc:

if ! pgrep -u $USER ssh-agent > /dev/null; then
    ssh-agent > ~/.ssh-agent-thing
if [[ "$SSH_AGENT_PID" == "" ]]; then
    eval $(<~/.ssh-agent-thing)
ssh-add -l >/dev/null || alias ssh='ssh-add -l >/dev/null || ssh-add && unalias ssh; ssh'

This will run a ssh-agent process if there isn't one already, and save the output thereof. If there is one running already, we retrieve the cached ssh-agent output and evaluate it which will set the necessary environment variables. Also, if needed, we create an alias around ssh to add the key to the agent, then remove the alias. One downside to this approach is that the key will not be added by commands that use the private key other than ssh, such as git.

There also exist a number of front-ends to ssh-agent and alternative agents described later in this section which avoid this problem.

Start ssh-agent with systemd user

It is possible to use the systemd/User facilities to start the agent.

Description=SSH key agent

ExecStart=/usr/bin/ssh-agent -a $SSH_AUTH_SOCK


Add export SSH_AUTH_SOCK="$XDG_RUNTIME_DIR/ssh-agent.socket" to your shell's startup file, for example .bash_profile for Bash. Then enable or start the service.

ssh-agent as a wrapper program

An alternative way to start ssh-agent (with, say, each X session) is described in this ssh-agent tutorial by UC Berkeley Labs. A basic use case is if you normally begin X with the startx command, you can instead prefix it with ssh-agent like so:

$ ssh-agent startx

And so you do not even need to think about it you can put an alias in your .bash_aliases file or equivalent:

alias startx='ssh-agent startx'

Doing it this way avoids the problem of having extraneous ssh-agent instances floating around between login sessions. Exactly one instance will live and die with the entire X session.

Note: You can also add eval $(ssh-agent) to ~/.xinitrc.

See the below notes on using x11-ssh-askpass with ssh-add for an idea on how to immediately add your key to the agent.

GnuPG Agent

Merge-arrows-2.pngThis article or section is a candidate for merging with GnuPG#gpg-agent.Merge-arrows-2.png

Notes: This section should just refer to the main article. (Discuss in Talk:SSH keys#)

The GnuPG agent, distributed with the gnupg package, available in the official repositories, has OpenSSH agent emulation. If you already use the GnuPG suite, you might consider using its agent to also cache your SSH keys. Additionally, some users may prefer the PIN entry dialog GnuPG agent provides as part of its passphrase management.

Note: If you are using KDE and have kde-agent[broken link: package not found] installed you only need to set enable-ssh-support into ~/.gnupg/gpg-agent.conf! Otherwise, continue reading.

To start using GnuPG agent for your SSH keys, you should enable enable-ssh-support in the ~/.gnupg/gpg-agent.conf file.

  # Enable SSH support

Next, start gpg-agent when using gpg-connect-agent, set SSH_AUTH_SOCK so that SSH will use gpg-agent instead of ssh-agent, set the GPG TTY and refresh the TTY in case user has switched into an X session. Example:


  # Start the gpg-agent if not already running
  if ! pgrep -x -u "${USER}" gpg-agent >/dev/null 2>&1; then
    gpg-connect-agent /bye >/dev/null 2>&1

  # Set SSH to use gpg-agent
  if [ "${gnupg_SSH_AUTH_SOCK_by:-0}" -ne $$ ]; then
    export SSH_AUTH_SOCK="${HOME}/.gnupg/S.gpg-agent.ssh"

  # Set GPG TTY
  export GPG_TTY

  # Refresh gpg-agent tty in case user switches into an X session
  gpg-connect-agent updatestartuptty /bye >/dev/null

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...

  # Keyboard control
  # PIN entry program
  #pinentry-program /usr/bin/pinentry-curses
  #pinentry-program /usr/bin/pinentry-qt4
  #pinentry-program /usr/bin/pinentry-kwallet
  #pinentry-program /usr/bin/pinentry-gtk-2


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.


Warning: As of 2015-09-26, the -Q, --quick option has the unexpected side-effect of making keychain switch to a newly-spawned ssh-agent upon relogin (at least on systems using GNOME), forcing you to re-add all the previously registered keys.

Add a line similar to the following to your shell congifuration file, e.g. if using Bash:

eval $(keychain --eval --quiet id_ed25519 id_rsa ~/.keys/my_custom_key)
Note: ~/.bashrc is used instead of the upstream suggested ~/.bash_profile because on Arch it is sourced by both login and non-login shells, making it suitable for textual and graphical environments alike. See Bash#Invocation for more information on the difference between those.

In the above example,

  • the --eval switch 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.
  • --quiet will limit output to warnings, errors, and user prompts.

Multiple keys can be specified on the command line, as shown in the example. By default keychain will look for key pairs in the ~/.ssh/ directory, but absolute path can be used for keys in non-standard location. You may also use the --confhost option to inform keychain to look in ~/.ssh/config for IdentityFile settings defined for particular hosts, and use these paths to locate keys.

See keychain --help or man keychain for details on setting keychain for other shells.

To test Keychain, simply open a new terminal emulator or log out and back in your session. It should prompt you for the passphrase of the specified private key(s) (if applicable), either using the program set in $SSH_ASKPASS or on the terminal.

Because Keychain reuses the same ssh-agent process on successive logins, you should not have to enter your passphrase the next time you log in or open a new terminal. You will only be prompted for your passphrase once each time the machine is rebooted.


  • keychain expects public key files to exist in the same directory as their private counterparts, with a .pub extension. If the private key is a symlink, the public key can be found alongside the symlink or in the same directory as the symlink target (this capability requires the readlink command to be available on the system).
  • to disable the graphical prompt and always enter your passphrase on the terminal, use the --nogui option. This allows to copy-paste long passphrases from a password manager for example.
  • if you do not want to be immediately prompted for unlocking the keys but rather wait until they are needed, use the --noask option.
Note: Keychain is able to manage GPG keys in the same fashion. By default it attempts to start ssh-agent only, but you can modify this behavior using the --agents option, e.g. --agents ssh,gpg. See man keychain.


An alternative to keychain is envoy. Envoy is available as envoy , or the Git version as envoy-gitAUR.

After installing it, set up the envoy socket by enabling envoy@ssh-agent.socket.

And add to your shell's rc file:

 envoy -t ssh-agent -a ssh_key
 source <(envoy -p)

If this syntax for sourcing causes errors (in mksh, for example), you can replace it with an equivalent for loop:

 envoy -t ssh-agent -a ssh_key
 eval "$(envoy -p)"

If the key is ~/.ssh/id_rsa, ~/.ssh/id_dsa, ~/.ssh/id_ecdsa, or ~/.ssh/identity, the -a ssh_key parameter is not needed.

envoy with key passphrases stored in kwallet

If you have long passphrases for your SSH keys, remembering them can be a pain. So let us tell kwallet to store them! Along with envoy, install ksshaskpass and kwalletmanager from the official repositories. Next, enable the envoy socket in systemd (see above).

Note: As of April 30, 2015, if after installation ksshaskpass keeps asking for access to your wallet even after having submitted the password, you might have [this] problem. The proposed solution is to install ksshaskpass4AUR, though this might break your login.

First, you will add this script to ~/.kde4/Autostart/

envoy -t ssh-agent -a ssh_key

Then, make sure the script is executable by running: chmod +x ~/.kde4/Autostart/

And add this to ~/.kde4/env/

eval $(envoy -p)

When you log into KDE, it will execute the script. This will call ksshaskpass, which will prompt you for your kwallet password when envoy calls ssh-agent.


The x11-ssh-askpass package provides a graphical dialog for entering your passhrase when running an X session. x11-ssh-askpass depends only on the libx11 and libxt libraries, and the appearance of x11-ssh-askpass is customizable. While it can be invoked by the ssh-add program, which will then load your decrypted keys into ssh-agent, the following instructions will, instead, configure x11-ssh-askpass to be invoked by the aforementioned Keychain script.

Install keychain and x11-ssh-askpass, both available in the official repositories.

Edit your ~/.xinitrc file to include the following lines, replacing the name and location of your private key if necessary. Be sure to place these commands before the line which invokes your window manager.

keychain ~/.ssh/id_ecdsa
[ -f ~/.keychain/$HOSTNAME-sh ] && . ~/.keychain/$HOSTNAME-sh 2>/dev/null
[ -f ~/.keychain/$HOSTNAME-sh-gpg ] && . ~/.keychain/$HOSTNAME-sh-gpg 2>/dev/null
exec openbox-session

In the above example, the first line invokes keychain and passes the name and location of your private key. If this is not the first time keychain was invoked, the following two lines load the contents of $HOSTNAME-sh and $HOSTNAME-sh-gpg, if they exist. These files store the environment variables of the previous instance of keychain.

Calling x11-ssh-askpass with ssh-add

The ssh-add manual page specifies that, in addition to needing the DISPLAY variable defined, you also need SSH_ASKPASS set to the name of your askpass program (in this case x11-ssh-askpass). It bears keeping in mind that the default Arch Linux installation places the x11-ssh-askpass binary in /usr/lib/ssh/, which will not be in most people's PATH. This is a little annoying, not only when declaring the SSH_ASKPASS variable, but also when theming. You have to specify the full path everywhere. Both inconveniences can be solved simultaneously by symlinking:

$ ln -sv /usr/lib/ssh/x11-ssh-askpass ~/bin/ssh-askpass

This is assuming that ~/bin is in your PATH. So now in your .xinitrc, before calling your window manager, one just needs to export the SSH_ASKPASS environment variable:

$ export SSH_ASKPASS=ssh-askpass

and your X resources will contain something like:

ssh-askpass*background: #000000

Doing it this way works well with the above method on using ssh-agent as a wrapper program. You start X with ssh-agent startx and then add ssh-add to your window manager's list of start-up programs.


The appearance of the x11-ssh-askpass dialog can be customized by setting its associated X resources. The x11-ssh-askpass home page[dead link 2015-04-01] presents some example themes[dead link 2015-04-01]. See the x11-ssh-askpass manual page for full details.

Alternative passphrase dialogs

There are other passphrase dialog programs which can be used instead of x11-ssh-askpass. The following list provides some alternative solutions.

  • ksshaskpass is available in the official repositories. It is dependent on kdelibs and is suitable for the KDE Desktop Environment.
  • openssh-askpass depends on the qt4 libraries and is available from the official repositories.


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_sshAUR package, available in the Arch User Repository.

Note: pam_ssh 2.0 now requires that all private keys used in the authentication process be located under ~/.ssh/login-keys.d/.

Create a symlink to your private key file and place it in ~/.ssh/login-keys.d/. Replace the id_rsa in the example below with the name of your own private key file.

$ mkdir ~/.ssh/login-keys.d/
$ cd ~/.ssh/login-keys.d/
$ ln -s ../id_rsa

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.

auth       required
auth       requisite
auth       include      system-local-login
auth       optional        try_first_pass
account    include      system-local-login
session    include      system-local-login
session    optional

In the above example, login authentication initially proceeds as it normally would, with the user being prompted to enter his user password. The additional auth authentication rule added to the end of the authentication stack then instructs the pam_ssh module to try to decrypt any private keys found in the ~/.ssh/login-keys.d directory. The try_first_pass option is passed to the pam_ssh module, instructing it to first try to decrypt any SSH private keys using the previously entered user password. If the user's private key passphrase and user password are the same, this should succeed and the user will not be prompted to enter the same password twice. In the case where the user's private key passphrase user password differ, the pam_ssh module will prompt the user to enter the SSH passphrase after the user password has been entered. The optional control value ensures that users without an SSH private key are still able to log in. In this way, the use of pam_ssh will be transparent to users without an SSH private key.

If you use 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.

  • Versions of pam_ssh prior to version 2.0 do not support SSH keys employing the newer option of ECDSA (elliptic curve) cryptography. If you are using earlier versions of 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. See the GNOME Keyring article for further details.

Store SSH keys with Kwallet

For instructions on how to use kwallet to store your SSH keys, see KDE Wallet#Using the KDE Wallet to store ssh keys.


Key ignored by the server

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

If that does not solve the problem you may try temporarily setting StrictModes to no in sshd_config. If authentication with StrictModes off is successful, it is likely an issue with file permissions persists.

Tip: Do not forget to set StrictModes to yes for added security.

Make sure the remote machine supports the type of keys you are using. Try using RSA or DSA keys instead #Generating an SSH key pair

Some servers do not support ECDSA keys. 

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

# /usr/bin/sshd -d

Using KDM

KDM does not launch the ssh-agent process directly, kde-agent[broken link: package not found] used to start ssh-agent on login, but since version 20140102-1 it got removed.

In order to start ssh-agent on KDE startup for a user, create scripts to start ssh-agent on startup and one to kill it on logoff:

$ echo -e '#!/bin/sh\n[ -n "$SSH_AGENT_PID" ] || eval "$(ssh-agent -s)"' > ~/.kde4/env/
$ echo -e '#!/bin/sh\n[ -z "$SSH_AGENT_PID" ] || eval "$(ssh-agent -k)"' > ~/.kde4/shutdown/
$ chmod 755 ~/.kde4/env/ ~/.kde4/shutdown/

If you are using Plasma 5, you must create the scripts in the ~/.config/plasma-workspace/ directory, instead of ~/.kde4:

$ echo -e '#!/bin/sh\n[ -n "$SSH_AGENT_PID" ] || eval "$(ssh-agent -s)"' > ~/.config/plasma-workspace/env/
$ echo -e '#!/bin/sh\n[ -z "$SSH_AGENT_PID" ] || eval "$(ssh-agent -k)"' > ~/.config/plasma-workspace/shutdown/
$ chmod 755 ~/.config/plasma-workspace/env/ ~/.config/plasma-workspace/shutdown/

See also