Ssh Using Public Key

1. Ssh Using Public Key Command Line Ubuntu
2. Get Ssh Public Key
3. Ssh With Public Key Ubuntu
4. Sftp Using Public Key
5. Public Key Private Key Ssh

If you interact regularly with SSH commands and remote hosts, you may find that using a key pair instead of passwords can be convenient. Instead of the remote system prompting for a password with each connection, authentication can be automatically negotiated using a public and private key pair.

Public-key cryptography, or asymmetric cryptography, is a cryptographic system that uses pairs of keys: public keys (which may be known to others), and private keys (which may never be known by any except the owner). The generation of such key pairs depends on cryptographic algorithms which are based on mathematical problems termed one-way functions. SSH to Linux using Public KeyHelpful? Please support me on Patreon: thanks & praise to God, and with thanks to the.

The private key remains secure on your own workstation, and the public key gets placed in a specific location on each remote system that you access. Your private key may be secured locally with a passphrase. A local caching program such as ssh-agent or gnome-keyring allows you to enter that passphrase periodically, instead of each time you use the key to access a remote system.

[ Free download: Advanced Linux commands cheat sheet. ]

Generating a key pair and propagating the public key

Generating your key pair and propagating your public key is simpler than it sounds. Let’s walk through it.

Generating the key

The minimum effort to generate a key pair involves running the ssh-keygen command, and choosing the defaults at all the prompts:

The default location to store the keys is in the ~/.ssh directory, which will be created if it does not exist:

Allowing this command to create the directory also ensures that the owner and permissions are set correctly. Some applications will not use keys if the permissions to the private key are too open.

The file ending in .pub is the public key that needs to be transferred to the remote systems. It is a file containing a single line: The protocol, the key, and an email used as an identifier. Options for the ssh-keygen command allow you to specify a different identifier:

After generating the key pair, the ssh-keygen command also displays the fingerprint and randomart image that are unique to this key. This information can be shared with other people who may need to verify your public key.

Later you can view these with:

The -l option lists the fingerprint, and the -v option adds the ASCII art.

Propagating the public key to a remote system

If password authentication is currently enabled, then the easiest way to transfer the public key to the remote host is with the ssh-copy-id command. If you used the default name for the key all you need to specify is the remote user and host:

Following the instructions from the output, verify that you can connect using the key pair. If you implemented a passphrase, you will be prompted for the passphrase to use the private key:

Examine the resulting authorized key file. This is where the public key was appended. If the directory or file did not exist, then it was (or they were) created with the correct ownership and permissions. Each line is a single authorized public key:

To revoke access for this key pair, remove the line for the public key.

There are many other options that can be added to this line in the authorized key file to control access. These options are usually used by administrators placing the public keys on a system with restrictions. These restrictions may include where the connection may originate, what command(s) may be run, and even a date indicating when to stop accepting this key. These and more options are listed in the sshd man page.

Changing the passphrase

If you need to change a passphrase on your private key or if you initially set an empty passphrase and want that protection at a later time, use the ssh-keygen command with the -p option:

You can add additional options to specify the key (-f), and the old (-P) or new (-N) passphrases on the command line. Remember that any passwords specified on the command line will be saved in your shell history.

See the ssh-keygen man page for additional options.

Rotating keys

While the public key by itself is meant to be shared, keep in mind that if someone obtains your private key, they can then use that to access all systems that have the public key. These key pairs also do not have a period of validity like GNU Privacy Guard (GPG) keys or public key infrastructure (PKI) certificates.

If you have any reason to suspect that a private key has been stolen or otherwise compromised, you should replace that key pair. The old public key has to be removed from all systems, a new key has to be generated with ssh-keygen, and the new public key has to be transferred to the desired remote systems.

If you are rotating keys as a precaution and without any concern of compromise, you can use the old key pair to authenticate the transfer of the new public key before removing the old key.

Is using empty passphrases ever a good idea?

There are several things to think about when considering an empty passphrase for your SSH private key.

How secure is the private key file?

If you tend to work from multiple client systems and want to either have multiple copies of your key or keep a copy on removable media, then it really is a good idea to have a passphrase on the private key. This practice is in addition to protecting access to the key file with encrypted media.

However, if you have only one copy of the private key and it is kept on a system that is well secured and not shared, then having a passphrase is simply one more level of protection just in case.

Remember that changing the passphrase on one copy does not change the passphrase on other copies. The passphrase is simply locking access to a specific key file.

Why do think you need an empty passphrase?

There are cases for keys with empty passphrases. Some utilities that need to automatically transfer files between systems need a passwordless method to authenticate. The kdump utility, when configured to dump the kernel to a remote system using SSH, is one example.

Another common use is to generate a key pair for a script that is designed to run unattended, such as from a cron job.

How about a middle ground alternative?

By itself, a passphrase-protected private key requires the passphrase to be entered each time the key is used. This setup does not feel like passwordless SSH. However, there are caching mechanisms that allow you to enter the key passphrase once and then use the key over and over without reentering that passphrase.

More Linux resources

OpenSSH comes with an ssh-agent daemon and an ssh-add utility to cache the unlocked private key. The GNOME desktop also has a keyring daemon that stores passwords and secrets but also implements an SSH agent.

The lifetime of the cached key can be configured with each of the agents or when the key is added. In many cases, it defaults to an unlimited lifetime, but the cache is cleared when the user logs out of the system. You will be prompted for the passphrase only once per login session.

If there is a scheduled application that needs to run outside of a user login session, it may be possible to use a secret or other password manager to automate the unlocking of the key. For example, Ansible Tower stores credentials in a secure database. This database includes an SSH private key used to connect to the remote systems (managed nodes), and any passphrases necessary for those private keys. Once those credentials are stored, a job can be scheduled to run a playbook on a regular schedule.

Ssh Using Public Key Command Line Ubuntu

Automating propagation

A centralized identity manager such as FreeIPA can assist with key propagation. Upload the public key to the server as an attribute of a user account, and then propagate it to the hosts in the domain as needed. FreeIPA can also provide additional host-based access control for where a key may be used. No page breaks in word.

Keys can also be distributed using Ansible modules. The openssh_keypair module uses ssh-keygen to generate keys and the authorized_key module adds and removes SSH authorized keys for particular user accounts.

Wrapping up

SSH key pairs are only one way to automate authentication without passwords. Using the Generic Security Services Application Program Interface (GSSAPI) authentication is also common when trying to reduce the use of passwords on a network with centralized user management. SSH key pairs are the easier option to implement when single sign-on (SSO) is not already available.

Many source code repositories grant access using SSH keys. You can upload a public key to an account in the hosting organization such as the Fedora Account System, GitLab, or GitHub sites and use that key pair to authenticate when pulling and pushing content to repositories.

Free Event: Red Hat Summit 2021 Virtual Experience

Join Red Hat Summit Virtual Experience for live demos, keynotes, and technical
sessions from experts around the globe—happening April 27–28 and June 15–16.

Related Content

More Linux resources

If you have ever worked as a sysadmin (or you want to in the future), you need a good grasp of SSH. I will not run you through the general concept as it has already been hashed out here at Enable Sysadmin. However, I do want to look at a potentially better way to use it. SSH is the single most used remote access protocol in the world. Therefore, it makes sense that we should try to improve its use as much as possible.

I used SSH to remotely connect to thousands of customer machines during my time as a support engineer, and I am sure that others have had a similar experience. With traditional SSH authentication, you need the username and password for the account you want to log in to every time that you wish to access a system. Doesn't sound that bad, right? But, what happens when you need to jump back and forth between systems regularly? Or what if your responsibilities include remote sessions to the same 100 systems throughout the day for health checks? There is another way to accomplish the log in, and with a little upfront investment, it can be far more efficient overall.

Process hardening

It is objectively true that an encrypted key is a much harder target than a username and password for those with ill intentions. Although it can take a little learning, creating and using SSH key-based authentication is worth the investment for every sysadmin.

Here is how it works. You generate a public key and a matching private key. The private key file acts as a password and should be kept safe. However, the public key is copied to the target systems that you connect to regularly. You place the public key in your account home directory on the target server. When you try to log in, the keys are verified, and access is granted.

Now, there are two ways that you can do this. One is more convenient, and the other is a bit tedious but with added protection to you. The convenient way is not to specify a password along with the private key. The result is that you do not have to enter a password when you use your private key for authentication. This means that if someone gets their hands on your private key, they can use it to authenticate, as well. The other method is to password-protect your private key so that you are prompted for the password when authenticating (think two-factor authentication using both the private key and the password).

ssh-keygen without a password

To generate an SSH key pair, use the following command:

By default, your private and public keys are saved in your ~/.ssh/id_rsa and ~/.ssh/id_rsa.pub files, respectively.

ssh-keygen with a password

Creating a password-protected key looks something like this:

Use the -f option to specify the file where the keys will be saved. In the example above, the private and public keys are stored in the /home/user/.ssh/key-with-pass and /home/user/.ssh/key-with-pass.pub files, respectively.

Warning

During further SSH key pair generation, if you do not specify a unique file name, you are prompted for permission to overwrite the existing id_rsa and id_rsa.pub files. If you overwrite the existing id_rsa and id_rsa.pub files, you must then replace the old public key with the new one on ALL of the SSH servers that have your old public key.

Once you have generated the keys, they are stored in the /user/home/.ssh/ directory with the following permissions:

• Private key - 600
• Public key - 644

Get Ssh Public Key

You aren't done yet. Let's look at the final step in successful SSH key-based authentication.

Sharing keys

For all of this to work, you need to share your public key with the remote machines you are trying to SSH to. Use the ssh-copy-id command to copy your public key over to the destination system. By default, the file path is /home/user/.ssh/id_rsa.pub. You issue the command, specify the file you are sharing, then the user/host we are sharing it with. It should look like this:

Ssh With Public Key Ubuntu

Now that you have shared the public key with the destination host, you can authenticate to the remote server by passing the matching private key. If you specified a file path for your private key, you need to give it here. Otherwise, it defaults to /home/_user_/.ssh/id_rsa.

Sftp Using Public Key

Seen here:

Advantages and summary

The advantages of using SSH key-based authentication are clear. Passwords are stolen every day, mainly due to human error but also due to attacker skill and determination. An encrypted key, and more specifically, a password-protected encrypted key, makes your SSH authentication even more difficult to attack. You still need to strike a balance of availability and security, but that is handled differently in every environment.

[ Free online course: Red Hat Enterprise Linux technical overview. ]

Public Key Private Key Ssh