Dec 01, 2017 ssh-keygen can create RSA keys for use by SSH protocol version 1 and DSA, ECDSA or RSA keys for use by SSH protocol version 2. The type of key to be generated is specified with the -t option. If invoked without any arguments, ssh-keygen will generate an RSA key for use in SSH protocol 2 connections. SSH Key Generation: nsk@nsk-linux $ ssh. Quick steps: Create and use an SSH public-private key pair for Linux VMs in Azure.; 4 minutes to read +4; In this article. With a secure shell (SSH) key pair, you can create virtual machines (VMs) in Azure that use SSH keys for authentication, eliminating the need for passwords to sign in. Red Hat Enterprise Linux 6 uses SSH Protocol 2 and RSA keys by default (see Section 14.1.3, “Protocol Versions” for more information). Important Do not generate key pairs as root, as only root would be able to use those 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.
(Redirected from SSH Keys)
This article or section needs expansion.
Reason: The intro and Background section ignore the server perspective. (Discuss in Talk:SSH keys#)
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.[1]
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.
- 2Generating an SSH key pair
- 2.1Choosing the authentication key type
- 2.2Choosing the key location and passphrase
- 3Copying the public key to the remote server
- 4SSH agents
- 4.1ssh-agent
- 4.3Keychain
- 4.4x11-ssh-askpass
- 4.5pam_ssh
- 5Troubleshooting
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, defaulting to 3072-bit RSA (and SHA256) which the ssh-keygen(1) man page says is 'generally considered sufficient' and should be compatible with virtually all clients and servers: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: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:
- DSA and RSA, which rely on the practical difficulty of factoring the product of two large prime numbers,
- ECDSA and Ed25519, which rely on the elliptic curve discrete logarithm problem. (example)
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.
OpenSSH 7.0 deprecated and disabled support for DSA keys due to discovered vulnerabilities, therefore the choice of cryptosystem lies within RSA or one of the two types of ECC.
#RSA keys will give you the greatest portability, while #Ed25519 will give you the best security but requires recent versions of client & server[2][dead link 2020-04-02 ⓘ]. #ECDSA is likely more compatible than Ed25519 (though still less than RSA), but suspicions exist about its security (see below).
Note: These keys are used only to authenticate you; choosing stronger keys will not increase CPU load when transferring data over SSH.
RSA
ssh-keygen
defaults to RSA therefore there is no need to specify it with the -t
option. It 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.
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:Be aware though that there are diminishing returns in using longer keys.[3][4] 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'.[5]
On the other hand, the latest iteration of the NSA Fact Sheet Suite B Cryptography[dead link 2020-04-02 ⓘ] suggests a minimum 3072-bit modulus for RSA while '[preparing] for the upcoming quantum resistant algorithm transition'.[6]
ECDSA
The Elliptic Curve Digital Signature Algorithm (ECDSA) was introduced as the preferred algorithm for authentication in OpenSSH 5.7. Some vendors also disable the required implementations due to potential patent issues.
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 haveexpresseddoubts about how the NIST curves were designed, and voluntary tainting has already beenproved 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 are still subject to debate, there is a clear consensus that #Ed25519 is technically superior and should therefore be preferred.
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.[7] See also this blog post by a Mozilla developer on how it works.
It is already implemented in many applications and libraries and is the default key exchange algorithm (which is different from key signature) in OpenSSH.
Ed25519 key pairs can be generated with:
There is no need to set the key size, as all Ed25519 keys are 256 bits.
Keep in mind that older SSH clients and servers may not support these keys.
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.
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.Managing multiple keys
It is possible — although controversial [8][9] — to use the same SSH key pair for multiple hosts.
On the other hand, it is rather easy to maintain distinct keys for multiple hosts by using the
IdentityFile
directive in your openSSH config file: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 are hardware tokens are described in:
- YubiKey#Using a YubiKey with SSH, and
Copying the public key to the remote server
This article or section needs expansion.
Reason: How to do this if you force public key authentication? (Discuss in Talk:SSH keys#)
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/id_rsa.pub
you can simply enter the following command.If your username differs on remote machine, be sure to prepend the username followed by
@
to the server name.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.If the ssh server is listening on a port other than default of 22, be sure to include it within the host argument.
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.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.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.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.To make use of these variables, run the command through the
eval
command.Once
ssh-agent
is running, you will need to add your private key to its cache: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
. Other possible values are confirm
, ask
and no
(default).In order to start the agent automatically and make sure that only one
ssh-agent
process runs at a time, add the following to your ~/.bashrc
:This will run a
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.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. Use this if you would like your ssh agent to run when you are logged in, regardless of whether x is running.
Add
SSH_AUTH_SOCK DEFAULT='${XDG_RUNTIME_DIR}/ssh-agent.socket'
to ~/.pam_environment
. Then enable or start the service with the --user
flag.Note: If you use GNOME, this environment variable is overridden by default. See GNOME/Keyring#Disable keyring daemon components.
Tip: When starting the agent via systemd as described above, it is possible to automatically enter the passphrase of your default key and add it to the agent. See systemd-user-pam-ssh for details.
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:And so you do not even need to think about it you can put an alias in your
.bash_aliases
file or equivalent: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: As an alternative to calling
ssh-agent startx
, you can 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
The gpg-agent has OpenSSH agent 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:
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 openingeval
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 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 thereadlink
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 keychain(1).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.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: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:![Ssh Ssh](/uploads/1/2/6/0/126058669/458513469.jpg)
and your X resources will contain something like:
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)[dead link 2019-05-05] 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 uses the KDE Wallet.
- openssh-askpass uses the Qt library.
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_sshAUR 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_rsa
in the example below with the name of your own private key file.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.
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(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
toFor an explanation, see [10].
Mac Generate Ssh Key
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.
pam_exec-ssh
As an alternative to pam_ssh you can use pam_exec-sshAUR. It is a shell script that uses pam_exec. Help for configuration can be found upstream.
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 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 supports being used as an SSH agent by default. 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:
- For the remote machine:
- If that does not solve the problem you may try temporarily setting
StrictModes
tono
in/etc/ssh/sshd_config
. If authentication withStrictModes 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 or DSA keys instead, see #Generating an SSH key pair.
- You may want to use debug mode and monitor the output while connecting:
See also
- OpenSSH key management: Part 1, Part 2, Part 3
Retrieved from 'https://wiki.archlinux.org/index.php?title=SSH_keys&oldid=604264'
Generate Ssh Key Putty
What is SSH Key-based authentication?
As we all know, Secure Shell, shortly SSH, is the cryptographic network protocol that allows you to securely communicate/access a remote system over unsecured network, for example Internet. Whenever you send a data over an unsecured network using SSH, the data will be automatically encrypted in the source system, and decrypted in the destination side. SSH provides four authentication methods namely password-based authentication, key-based authentication, Host-based authentication, and Keyboard authentication. The most commonly used authentication methods are password-based and key-based authentication.
Download – Free Guide: “Introduction to Linux – A Hands on Guide”
In password-based authentication, all you need is the password of the remote system’s user. If you know the password of remote user, you can access the respective system using “ssh [email protected]” command. On the other hand, in key-based authentication, you need to generate SSH key pairs and upload the SSH public key to the remote system in order to communicate it via SSH. Each SSH key pair consists of a private key and public key. The private key should be kept within the client system, and the public key should uploaded to the remote systems. You shouldn’t disclose the private key to anyone. Hope you got the basic idea about SSH and its authentication methods. In this tutorial, we will be discussing how to configure SSH key-based authentication in Linux.
Configure SSH Key-based Authentication In Linux
For the purpose of this guide, I will be using Arch Linux system as local system and Ubuntu 18.04 LTS as remote system.
Local system details:
- OS : Arch Linux Desktop
- IP address : 192.168.225.37 /24
Remote system details:
- OS : Ubuntu 18.04 LTS Server
- IP address : 192.168.225.22/24
Local system configuration
![Generate Ssh Key Arch Linux Generate Ssh Key Arch Linux](/uploads/1/2/6/0/126058669/246459711.jpg)
Like I said already, in SSH key-based authentication method, the public key should be uploaded to the remote system that you want to access via SSH. The public keys will usually be stored in a file called ~/.ssh/authorized_keys in the remote SSH systems.
Important note: Do not generate key pairs as root, as only root would be able to use those keys. Create key pairs as normal user.
Now, let us create the SSH key pair in the local system. To do so, run the following command in your local client system.
The above command will create 2048 bit RSA key pair. Enter the passphrase twice. More importantly, Remember your passphrase. You’ll need it later.
Sample output:
In case you have already created the key pair, you will see the following message. Just type “y” to create overwrite the existing key .
Please note that passphrase is optional. If you give one, you’ll be asked to enter the password every time when you try to SSH into a remote system unless you are using any SSH agent to store the password. If you don’t want passphrase(not safe though), simply press ENTER key twice when you’ll be asked to enter the passphrase. However, we recommend you to use passphrase. Using a password-less ssh key is generally not a good idea from a security point of view. They should be limited to very specific cases such as services having to access a remote system without the user intervention (e.g. remote backups with rsync, …).
If you already have a ssh key without a passphrase in private file ~/.ssh/id_rsa and wanted to update key with passphrase, use the following command:
Sample output:
Now, we have created the key pair in the local system. Now, copy the SSH public key to your remote SSH server using command:
Here, I will be copying the local (Arch Linux) system’s public key to the remote system (Ubuntu 18.04 LTS in my case). Technically speaking, the above command will copy the contents of local system’s ~/.ssh/id_rsa.pub key into remote system’s ~/.ssh/authorized_keys file. Clear? Good.
Type yes to continue connecting to your remote SSH server. And, then Enter the sudo (or root user’s) password of the remote system.
If you have already copied the key, but want to update the key with new passphrase, use -f option to overwrite the existing key like below.
We have now successfully added the local system’s SSH public key to the remote system. Now, let us disable the password-based authentication completely in the remote system. Because, we have configured key-based authentication, so we don’t need password-base authentication anymore.
Disable SSH Password-based authentication in remote system
You need to perform the following commands as root or sudo user.
To disable password-based authentication, go to your remote system’s console and edit /etc/ssh/sshd_config configuration file using any editor:
Find the following line. Uncomment it and set it’s value as no.
Restart ssh service to take effect the changes.
Access Remote system from local system
Go to your local system and SSH into your remote server using command:
Enter the passphrase.
Sample output:
Now, you’ll be able to SSH into your remote system. As you noticed, we have logged-in to the remote system’s account using passphrase which we created earlier using ssh-keygen command, not using the actual account’s password.
If you try to ssh from another client system, you will get this error message. Say for example, I am tried to SSH into my Ubuntu system from my CentOS using command:
Sample output:
As you see in the above output, I can’t SSH into my remote Ubuntu 18.04 systems from any other systems, except the CentOS system.
Adding more Client system’s keys to SSH server
This is very important. Like I said already, you can’t access the remote system via SSH, except the one you configured (In our case, it’s Ubuntu). I want to give permissions to more clients to access the remote SSH server. What should I do? Simple. You need to generate the SSH key pair in all your client systems and copy the ssh public key manually to the remote server that you want to access via SSH.
To create SSH key pair on your client system’s, run:
Enter the passphrase twice. Now, the ssh key pair is generated. You need to copy the public ssh key (not private key) to your remote server manually.
Display the pub key using command:
You should an output something like below.
Copy the entire contents (via USB drive or any medium) and go to your remote server’s console. Create a directory called ssh in the home directory as shown below. You need to execute the following commands as root user.
Now, append the your client system’s pub key which you generated in the previous step in a file called
Restart ssh service on the remote system. Now, you’ll be able to SSH to your server from the new client.
If manually adding ssh pubkey seems difficult, enable password-based authentication temporarily in the remote system and copy the key using “ssh-copy-id” command from your local system and finally disable the password-based authentication.
Suggested read:
And, that’s all for now. SSH Key-based authentication provides an extra layer protection from brute-force attacks. As you can see, configuring key-based authentication is not that difficult either. It is one of the recommended method to keep your Linux servers safe and secure.
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Generate Ssh Key Arch Linux Version
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