SSH keys

This article or section needs expansion.
Reason:
The intro and
Background
section ignore the server perspective. (Discuss in
Talk:SSH keys#Server perspective is ignored
)
SSH keys can serve as a means of identifying yourself to an SSH server using
public-key cryptography
and
challenge-response authentication
. The major advantage of key-based authentication is that, in contrast to password authentication, it is not prone to
brute-force attacks
, and you do not expose valid credentials if the server has been compromised (see
RFC 4251 9.4.4
).
Furthermore, 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.
Key-based authentication is not without its drawbacks and may not be appropriate for all environments, but in many circumstances it 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.
Background
SSH keys are always generated in pairs with one known as the private key and the other as the public key.  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 wish to connect.
If 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 an encrypted 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 the private key holder.  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 proper 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.
A private key is a guarded secret and as such it is advisable to store it on disk in an encrypted form.  When the encrypted private key is required, a passphrase must first be entered in order to decrypt it.  While this might superficially appear as though you are providing a login password to the SSH server, the passphrase is only used to decrypt the private key on the local system.  The passphrase is not transmitted over the network.
Generating an SSH key pair
An SSH key pair can be generated by running the
ssh-keygen
command, see the
ssh-keygen(1)
man page for what is "
generally considered sufficient
" and should be compatible with virtually all clients and servers:
$ ssh-keygen
Generating public/private ed25519 key pair.
Enter file in which to save the key (/home/
username
/.ssh/id_ed25519):
Created directory '/home/
username
/.ssh'.
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /home/
username
/.ssh/id_ed25519
Your public key has been saved in /home/
username
/.ssh/id_ed25519.pub
The key fingerprint is:
SHA256:RLy4JBv7jMK5qYhRKwHB3af0rpMKYwE2PBhALCBV3G8
username
@
hostname
The key's randomart image is:
+--[ED25519 256]--+
|%oooo. ..        |
|== ..o.o.        |
|==  . +o..       |
|+ o o.ooE        |
|...  *.oS        |
| o..o ..         |
|o=.. +o          |
|+o*..+o          |
|+.o+. .          |
+----[SHA256]-----+
The
randomart image
was
introduced in OpenSSH 5.1
as an easier means of visually identifying the key fingerprint.
Note
You can use the
-a
switch to specify the number of KDF rounds on the password encryption.
You can also add an optional comment field to the public key with the
-C
switch, to more easily identify it in places such as
~/.ssh/known_hosts
,
~/.ssh/authorized_keys
and
ssh-add -L
output. For example:
$ ssh-keygen -C "$(whoami)@$(uname -n)-$(date -I)"
will add a comment saying which user created the key on which machine and when.
Choosing the authentication key type
OpenSSH supports several signing algorithms (for authentication keys) which can be divided in two groups depending on the mathematical properties they exploit:
Ed25519
and
ECDSA
, which rely on the elliptic curve
discrete logarithm
problem (ECDLP). (
example
)
RSA
, which relies on the
practical difficulty
of factoring the product of two large prime numbers,
Elliptic curve cryptography
(ECC) algorithms are a
more recent addition
to public key cryptosystems. One of their main advantages is their ability to provide
the same level of security with smaller keys
, which makes for less computationally intensive operations (
i.e.
faster key creation, encryption and decryption) and reduced storage and transmission requirements.
DSA keys are
deprecated
due to their security weaknesses and most SSH implementations do not support them anymore. Dropbear 2022.83 disabled DSA key support while OpenSSH 10.0 and libssh 0.11.0 removed support for DSA keys entirely. Therefore the choice of
cryptosystem
lies within RSA or one of the two types of ECC.
The default Ed25519 will give you the best security and good performance. ECDSA is slower than Ed25519, but faster than RSA; concerns exist about its security (see below). RSA keys will give you the greatest compatibility with old servers, but it requires a larger key size to provide sufficient security.
Note
These keys are used only to authenticate you; choosing stronger keys will not increase CPU load when transferring data over SSH.
Tip
To protect SSH keys against exfiltration from the machine, it is possible to store them in
FIDO/U2F hardware authenticators
or
Trusted Platform Modules
.
Ed25519 and ECDSA keys can be stored in FIDO/U2F hardware authenticators by using the special "security key" key types when generating the keys. See
#FIDO/U2F
.
ECDSA and RSA are supported by Trusted Platform Modules making it possible to seal SSH keys inside the TPM. See
Trusted Platform Module#SSH
.
Ed25519
Ed25519
was introduced in
OpenSSH 6.5
of January 2014: "
Ed25519 is an elliptic curve signature scheme that offers better security than ECDSA and DSA and good performance
". Its main strengths are its speed, its constant-time run time (and resistance against side-channel attacks), and its lack of nebulous hard-coded constants.
[1]
See also
this blog post
by a Mozilla developer on how it works.
It is implemented in
many applications and libraries
and is the default key type in
ssh-keygen(1)
and
dropbearkey(1)
.
ssh-keygen(1)
defaults to Ed25519 therefore there is no need to specify it with the
-t ed25519
option. The key pairs can be simply generated with:
$ ssh-keygen
There is no need to set the key size, as all Ed25519 keys are 256 bits.
Keep in mind that ancient SSH clients and servers may not support these keys.
ECDSA
The Elliptic Curve Digital Signature Algorithm (ECDSA) was the preferred algorithm for authentication (key exchange algorithm) from
OpenSSH 5.7
(2011-01-24) to OpenSSH 6.5 (2014-01-30).
There are two sorts of concerns with it:
Political concerns
, the trustworthiness of NIST-produced curves
being questioned
after revelations that the NSA willingly inserts backdoors into softwares, hardware components and published standards were made; well-known cryptographers
have
expressed
doubts
about how the NIST curves were designed, and voluntary tainting has already
been
proven
in the past.
Technical concerns
, about the
difficulty to properly implement the standard
and the
slowness and design flaws
which reduce security in insufficiently precautious implementations.
Both of those concerns are best summarized in
libssh curve25519 introduction
. Although the political concerns will always be subject to debate, there is a
clear consensus
that
#Ed25519
is technically superior and should therefore be preferred.
ECDSA key pairs can be generated with:
$ ssh-keygen -t ecdsa
Three elliptic curve sizes are supported for ECDSA keys: 256, 384 and 521 bits. The default is 256 bits. If you wish to generate a stronger ECDSA key pair, simply specify the
-b
option:
$ ssh-keygen -t ecdsa -b 384
RSA
RSA provides the best compatibility of all algorithms but requires the key size to be larger to provide sufficient security. Minimum key size is 1024 bits, default is 3072 (see
ssh-keygen(1)
) and maximum is 16384.
RSA key pairs can be generated with:
$ ssh-keygen -t rsa
If you wish to generate a stronger RSA key pair (
e.g.
to guard against cutting-edge or unknown attacks and more sophisticated attackers), simply specify the
-b
option with a higher bit value than the default:
$ ssh-keygen -t rsa -b 4096
Be aware though that there are diminishing returns in using longer keys.
[2]
[3]
The GnuPG FAQ reads: "
If you need more security than RSA-2048 offers, the way to go would be to switch to elliptical curve cryptography — not to continue using RSA
."
[4]
On the other hand, the latest iteration of the
NSA Fact Sheet Suite B Cryptography
suggests a minimum 3072-bit modulus for RSA while "
[preparing] for the upcoming quantum resistant algorithm transition
".
[5]
FIDO/U2F
FIDO/
U2F
hardware authenticator
support was added in
OpenSSH version 8.2
for both of the elliptic curve signature schemes mentioned above. It allows for a hardware token attached via USB or other means to act a second factor alongside the private key.
The
libfido2
is required for hardware token support.
Note
Both the client and server must support the
ed25519-sk
and
ecdsa-sk
key types.
OpenSSH uses a middleware library to communicate with the hardware token and comes with an internal middleware which supports USB tokens. Other middleware may be specified by the
sshd_config(5) § SecurityKeyProvider
directive or the
SSH_SK_PROVIDER
environment variable for
ssh-keygen
and
ssh-add
.
After attaching a compatible FIDO key, a key pair may be generated with:
$ ssh-keygen -t ed25519-sk
You will usually be required to enter your PIN and/or tap your token to confirm the generation.
resident
By default, the generated SSH key consist of two parts: a key handle on disk, and a private key that is unique for each security key. To easily move your FIDO key between machines, generate a key with the
ssh-keygen(1) § resident
option,
-O resident
. This indicates "that the key handle should be stored on the FIDO authenticator itself."
[6]
$ ssh-keygen -O resident -t ed25519-sk
Afterwards, on a new machine, the key can be downloaded using
ssh-keygen(1) § K
$ ssh-keygen -K
no-touch-required
Connecting to a server will usually require tapping your token unless the
-O no-touch-required
command line option is used during generation and the
sshd(8) § no-touch-required
authorized_keys
option is set on the server.
To create keys that do not require touch events, generate a key pair with the
no-touch-required
option. For example:
$ ssh-keygen -O no-touch-required -t ed25519-sk
Note
Not all hardware tokens support this option. If you are using a YubiKey, firmware version 5.2.3 is needed for the ed25519-sk key type.
[7]
Additionally,
sshd
rejects
no-touch-required
keys by default. To allow keys generated with this option, either enable it for an individual key in the
authorized_keys
file:
~/.ssh/authorized_keys
no-touch-required sk-ssh-ed25519@openssh.com AAAAInN... user@example.com
Or for the whole system by editing
/etc/ssh/sshd_config
with:
PubkeyAuthOptions none
Tip
GitHub
and
GitLab
do not support
no-touch-required
.
An ECDSA-based keypair may also be generated with the
ecdsa-sk
keytype, but the relevant concerns in the
#ECDSA
section above still apply.
$ ssh-keygen -t ecdsa-sk
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 they can bypass file permissions and will be able to access your unencrypted private key file at any time.
Note
Previously, the private key password was encoded in an insecure way: only a single round of an MD5 hash. OpenSSH 6.5 and later support a new, more secure format to encode your private key. This format is the default since
OpenSSH version 7.8
. Ed25519 keys have always used the new encoding format. To upgrade to the new format, simply change the key's passphrase, as described in the next section.
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. This can also be used to change the password encoding format to the new standard.
$ ssh-keygen -f ~/.ssh/id_rsa -p
Managing multiple keys
If you have multiple SSH identities, you can set different keys to be used for different hosts or remote users by using the
Host
and
IdentityFile
directives in your configuration:
~/.ssh/config
Host SERVER1
   IdentitiesOnly yes
   IdentityFile ~/.ssh/id_rsa_IDENTITY1

Host SERVER2 SERVER3
   IdentitiesOnly yes
   IdentityFile ~/.ssh/id_ed25519_IDENTITY2
See
ssh_config(5)
for full description of these options.
Storing SSH keys on hardware tokens
SSH keys can also be stored on a security token like a smart card or a USB token. This has the advantage that the private key is stored securely on the token instead of being stored on disk. When using a security token the sensitive private key is also never present in the RAM of the PC; the cryptographic operations are performed on the token itself. A cryptographic token has the additional advantage that it is not bound to a single computer; it can easily be removed from the computer and carried around to be used on other computers.
Examples of hardware tokens are described in:
#FIDO/U2F
YubiKey#SSH notes
Native OpenSSH support for FIDO/U2F keys
YubiKey#SSH keys
Trusted Platform Module#SSH
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.
There are essentially two scenarios:
Scenario 1 is that password authentification is not disabled yet, in this case continue with section
#Scenario 1
Scenario 2 is that public key authentication is forced and password authentication is disabled already, in this case continue with section
#Scenario 2
Scenario 1
In case you can regularly login to a server with ssh via password authentication, you can either use the
#Simple method
or the
#Manual method
below.
Simple method
If your key file name matches
~/.ssh/id*.pub
pattern (which is the default name format for
ssh-keygen
) you can simply enter the following command:
$ ssh-copy-id remote-server.example
Note
ssh-copy-id
chooses the most recent file that matches. If you have multiple files that would match the pattern, either update the timestamp of the desired file with
touch
or see the next part for further instructions.
If your public key filename differs from the default of
~/.ssh/id_{ed25519,ed25519_sk,rsa,ecdsa,ecdsa_sk}
, or if you have multiple files that match the default format, you need to explicitly specify the location of the public key:
$ ssh-copy-id -i ~/.ssh/
your_key_name
.pub remote-server.example
If your username differs on remote machine, be sure to prepend the username followed by
@
to the server name:
$ ssh-copy-id -i ~/.ssh/
your_key_name
.pub
username
@remote-server.example
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/
your_key_name
.pub -p 39901
username
@remote-server.example
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/id_ed25519.pub username@remote-server.org
:
The above example copies the public key (
id_ed25519.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:
$ install -dm700 ~/.ssh
$ cat ~/id_ed25519.pub >> ~/.ssh/authorized_keys
$ rm ~/id_ed25519.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.
Scenario 2
In case the server forces public key authentication and disabled password authentication already, you have the following potential options, depending on what the server and/or server provider supports.
In general you need to copy the public key to
~/.ssh/authorized_keys
on the server (for the user you want to connect to), using one of the below mentioned options:
modify the OS image before it is installed on the server
add
cloud-init
instructions, e.g. on OS installation
use an Intelligent Platform Management Interface (IPMI) that allows for modifying a system independently of OS-level SSH
if installed beforehand, several alternatives such as FTP access, other Remote Access protocols etc. can be used
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 forks to background and prints necessary environment variables. E.g.
$ 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. Use
ssh-agent -c
instead if using the
fish
shell.
$ 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_ed25519
Enter passphrase for /home/user/.ssh/id_ed25519:
Identity added: /home/user/.ssh/id_ed25519 (/home/user/.ssh/id_ed25519)
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.
Tip
To make all
ssh
clients (including
git
) store keys in the agent on first use, add the configuration setting
AddKeysToAgent yes
to
~/.ssh/config
. See
ssh_config(5) § AddKeysToAgent
for other possible values.
In order to start the agent automatically and make sure that only one
ssh-agent
process runs at a time,
touch $XDG_RUNTIME_DIR/ssh-agent.env
and add the following to your
~/.bashrc
:
if ! pgrep -u "$USER" ssh-agent > /dev/null; then
    ssh-agent -t 1h > "$XDG_RUNTIME_DIR/ssh-agent.env"
fi
if [ ! -f "$SSH_AUTH_SOCK" ]; then
    source "$XDG_RUNTIME_DIR/ssh-agent.env" >/dev/null
fi
This will run an
ssh-agent
process if there is not 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. The lifetime of the unlocked keys is set to 1 hour.
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
If you would like your ssh agent to run when you are logged in, regardless of whether X is running, a handy
ssh-agent.service
is included in
openssh
since the version 9.4p1-3, which can be
enabled
as a
user unit
.
Then
set the environment variable
SSH_AUTH_SOCK
to
$XDG_RUNTIME_DIR/ssh-agent.socket
.
Note
If you use GNOME, this environment variable is overridden by default. See
GNOME/Keyring#Disabling
.
Forwarding ssh-agent
This article or section needs language, wiki syntax or style improvements. See
Help:Style
for reference.
Reason:
This is not specific to ssh-agent, e.g. gpg-agent uses the same environment variable:
GnuPG#Forwarding gpg-agent and ssh-agent to remote
(Discuss in
Talk:SSH keys
)
When
forwarding
a local
ssh-agent
to remote (e.g., through command-line argument
ssh -A remote
or through
ForwardAgent yes
in the configuration file), it is important for the remote machine not to overwrite the environment variable
SSH_AUTH_SOCK
. So if the remote machine uses a
systemd unit
shown previously to start the agent,
SSH_AUTH_SOCK
must not be set in the environment when a user is logged in through SSH. Otherwise, the forwarding may fail, and you may see errors (for example:
The agent has no identities
) when checking the existing keys with
ssh-add -l
on the remote machine.
For example, if using bash, the
.bashrc
could be something like:
~/.bashrc
...
if [[ -z "${SSH_CONNECTION}" ]]; then
    export SSH_AUTH_SOCK="$XDG_RUNTIME_DIR/ssh-agent.socket"
fi
...
In this way,
SSH_AUTH_SOCK
is only set when the current session is
not
an SSH login. And when this is a SSH session,
SSH_AUTH_SOCK
on the remote machine is then set by the local machine to make the forwarding work.
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
ssh-askpass
requires the
DISPLAY
or
WAYLAND_DISPLAY
environment variable to work, so you may want to run
ssh-agent
in
~/.xinitrc
instead of
ssh-agent startx
, where
DISPLAY
is set. For example, you can add
exec ssh-agent dbus-launch i3
to
~/.xinitrc
. Or as an alternative to using
ssh-agent
as a wrapper program, you can add
eval $(ssh-agent)
to
~/.xinitrc
.
See
the notes on using x11-ssh-askpass with ssh-add
for an idea on how to immediately add your key to the agent.
OpenPGP card ssh-agent
This ssh-agent specializes on
OpenPGP
card integration. It uses private keys that are stored in
OpenPGP
card authentication slots.
Note
When also using
GnuPG
while running this agent, it is required to
use it with pcscd
and configure
shared access with pcscd
.
Install
openpgp-card-ssh-agent
and
enable
and
start
the
openpgp-card-ssh-agent.socket
user unit.
Afterwards add the relevant
environment variable
for this agent:
SSH_AUTH_SOCK="$XDG_RUNTIME_DIR/openpgp-card/ssh-agent.sock"
User PIN handling
The user PIN for the
OpenPGP
card is persisted via an
org.freedesktop.secrets
provider (such as
GNOME Keyring
,
KeePassXC
or
KDE Wallet
) by default. The PIN storage backend is
configurable and extendable
.
The user PIN needs to be persisted only once for each
OpenPGP
card. Prior to the first SSH connection with this agent, list the available SSH public keys and add their respective card identifiers:
$ ssh-add -L
ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIJUz6VnFprMe33G88Pq8NLw3wnIKOsBg0CDrwFeUVrU6
FFFE:01234567
$ ssh-add -s
FFFE:01234567
Enter passphrase for PKCS#11:
Note
The prompt mentions
PKCS#11
as it is a hardcoded message in
ssh-add(1)
. However, in this context the OpenPGP card user PIN must be entered.
GnuPG Agent
The
gpg-agent
has OpenSSH Agent protocol emulation. See
GnuPG#SSH agent
for necessary configuration.
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.
Installation
Install
the
keychain
package.
Configuration
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
configuration file,
e.g.
if using
Bash
:
~/.bashrc
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 environment variables for an SSH client to be able to find your agent.
the
--eval
switch generates the commands for the shell specified in the
SHELL
environent variable. It might save you some trouble to specify it for the command using e.g.
SHELL=/bin/bash
before the
keychain
command.
--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
keychain(1)
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.
Tips
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. For keychain versions up to 2.9.0 the default is to attempt to start
ssh-agent
only, but you can modify this behavior using the
--agents
option,
e.g.
--agents ssh,gpg
. See
keychain(1)
.
As of Keychain 2.9.0, the
--agents
option is deprecated. See
keychain(1)
.
If you are on Wayland, you might have to add
--inherit any-once
as per
keychain issue 148
.
x11-ssh-askpass
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 the
keychain
and
x11-ssh-askpass
packages.
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.
~/.xinitrc
keychain ~/.ssh/id_ed25519
[ -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
or
WAYLAND_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.
Theming
The appearance of the
x11-ssh-askpass
dialog can be customized by setting its associated
X resources
. Some examples are the .ad files at
https://github.com/sigmavirus24/x11-ssh-askpass
. See
x11-ssh-askpass(1)
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.
seahorse
(provides
/usr/lib/seahorse/ssh-askpass
) uses the
GTK
library.
gnome-ssh-askpass3
AUR
uses the
GTK
library.
ksshaskpass
uses the
KDE Wallet
.
openssh-askpass
AUR
uses the
Qt5
library.
lxqt-openssh-askpass
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.
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_ed25519
in the example below with the name of your own private key file, if needed.
$ mkdir ~/.ssh/login-keys.d/
$ cd ~/.ssh/login-keys.d/
$ ln -s ../id_ed25519
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       include      system-local-login
auth       optional     pam_ssh.so        try_first_pass
account    include      system-local-login
session    include      system-local-login
session    optional     pam_ssh.so
In the above example, login authentication initially proceeds as it normally would, with the user being prompted to enter their 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(8)
man page.
Using a different password to unlock the SSH key
If you want to unlock the SSH keys or not depending on whether you use your key's passphrase or the (different!) login password, you can modify
/etc/pam.d/system-auth
to
/etc/pam.d/system-auth
#%PAM-1.0
auth      [success=1 new_authtok_reqd=1 ignore=ignore default=ignore]  pam_unix.so     try_first_pass nullok
auth      required  pam_ssh.so      use_first_pass
auth      optional  pam_permit.so
auth      required  pam_env.so

account   required  pam_unix.so
account   optional  pam_permit.so
account   required  pam_time.so

password  required  pam_unix.so     try_first_pass nullok shadow
password  optional  pam_permit.so

session   required  pam_limits.so
session   required  pam_unix.so
session   optional  pam_permit.so
session   optional  pam_ssh.so
For an explanation, see
[8]
.
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 RSA 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.
pam_exec-ssh
As an alternative to
pam_ssh
you can use
pam_exec-ssh-git
AUR
. It is a shell script that uses pam_exec. Help for configuration can be found
upstream
.
GNOME Keyring
The
GNOME Keyring
tool can act as a wrapper around ssh-agent, providing GUI and/or automatic key unlocking. See
GNOME Keyring#SSH keys
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 key passphrases
.
KeePass2 with KeeAgent plugin
KeeAgent
is a plugin for
KeePass
that allows SSH keys stored in a KeePass database to be used for SSH authentication by other programs.
Supports both PuTTY and OpenSSH private key formats.
Works with native SSH agent on Linux/Mac and with PuTTY on Windows.
See
KeePass#Plugin installation in KeePass
or
install
the
keepass-plugin-keeagent
package.
This agent can be used directly, by matching KeeAgent socket:
KeePass -> Tools -> Options -> KeeAgent -> Agent mode socket file -> %XDG_RUNTIME_DIR%/keeagent.socket
and environment variable:
export SSH_AUTH_SOCK="$XDG_RUNTIME_DIR"'/keeagent.socket'
.
KeePassXC
The KeePassXC fork of KeePass
can act as a client for an existing SSH agent
. SSH keys stored in its database can be automatically (or manually) added to the agent. It is also compatible with KeeAgent's database format.
Troubleshooting
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 ~/.ssh
$ chmod 600 ~/.ssh/
key
For the remote machine:
$ chmod 700 ~/.ssh
$ chmod 600 ~/.ssh/authorized_keys
For the remote machine, also check that the target user's home directory has the correct permissions (it must
not
be writable by the group and others):
$ chmod go-w ~
target_user
If that does not solve the problem you may try temporarily setting
StrictModes
to
no
in
/etc/ssh/sshd_config
. If authentication with
StrictModes off
is successful, it is likely an issue with file permissions persists.
Make sure keys in
~/.ssh/authorized_keys
are entered correctly and only use one single line.
Make sure the remote machine supports the type of keys you are using: some servers do not support ECDSA keys, try using RSA keys instead, see
#Generating an SSH key pair
.
You may want to use debug mode and monitor the output while connecting:
# /usr/bin/sshd -d
If you gave another name to your key, for example
id_rsa_server
, you need to connect with the
-i
option:
$ ssh -i id_rsa_server user@server
agent refused operation
If your private key requires a password (or, for instance, you have a hardware key with a PIN) but ssh-agent is not provided with one,
ssh
will fail:
sign_and_send_pubkey: signing failed for ECDSA-SK
user
@
host
from agent: agent refused operation
One potential cause for this is ssh-agent being unable to prompt for a password. Ensure that ssh-agent has access to either a display server (via the
DISPLAY
environment variable) or a TTY. For some graphical environments you might only need to
install
x11-ssh-askpass
, for other setups also follow
#x11-ssh-askpass
instructions.
Another cause, if using a hardware authenticator, could be the key malfunctioning or being unplugged.
There is currently an open
bug
that triggers with the "agent refused operation" error when using authenticator keys like ED25519-sk and ECDSA-SK that were created with the option
-O verify-required
. To avoid this issue, use the
-o IdentityAgent=none -o IdentitiesOnly=yes
option for the
ssh
command or add it to your
ssh_config
file for the relevant hosts:
Host myserver.tld
    IdentityAgent none
    IdentitiesOnly yes
See also
OpenSSH key management:
Part 1
,
Part 2
,
Part 3
Secure Secure Shell