- 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.
- Nov 26, 2018 How to use SSH keys with Windows on Azure.; 6 minutes to read +2; In this article. This article describes ways to generate and use secure shell (SSH) keys on a Windows computer to create and connect to a Linux virtual machine (VM) in Azure. To use SSH keys from a Linux or macOS client, see the quick or detailed guidance.
- Save the text file in the same folder where you saved the private key, using the.pub extension to indicate that the file contains a public key. If you or others are going to use an SSH client that requires the OpenSSH format for private keys (such as the ssh utility on Linux), export the private key: On the Conversions menu, choose Export.
- If you don't already have an SSH key, you must generate a new SSH key.If you're unsure whether you already have an SSH key, check for existing keys. If you don't want to reenter your passphrase every time you use your SSH key, you can add your key to the SSH agent, which manages your SSH keys and remembers your passphrase.
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.
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.
Generating a key pair and propagating the public key
Aug 19, 2019 Secure Shell (SSH) is an encrypted protocol used by Linux users to connect to their remote servers. Generally, there are two ways for clients to access their servers – using password based authentication or public key based authentication. Using SSH keys for authentication is highly recommended, as a safer alternative to passwords.
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.
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.
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.
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Introduction
Secure Shell (SSH) is an encrypted protocol used by Linux users to connect to their remote servers.
Generally, there are two ways for clients to access their servers – using password based authentication or public key based authentication.
Using SSH keys for authentication is highly recommended, as a safer alternative to passwords.
This tutorial will guide you through the steps on how to generate and set up SSH keys on CentOS 7. We also cover connecting to a remote server using the keys and disabling password authentication.
1. Check for Existing Keys
Prior to any installation, it is wise to check whether there are any existing keys on the client machines.
Open the terminal and list all public keys stored with the following command:
The output informs you about any generated keys currently on the system. If there aren’t any, the message tells you it cannot access /.ssh/id_*.pub , as there is no such file or directory.
2. Verify SSH is Installed
To check if thw package is installed, run the command:
If you already have SSH, the output tells you which version it is running. Currently, the latest version is OpenSSH 8.0/8.0p1.
Note: Refer to our guide If you need to install and enable SSH on your CentOS system.
Steps to Creating SSH keys on CentOS
Step 1: Create SSH Key Pair
1. Start by logging into the source machine (local server) and creating a 2048-bit RSA key pair using the command:
If you want to tighten up security measures, you can create a 4096-bit key by adding the -b 4096 flag:
2. After entering the command, you should see the following prompt:
3. To save the file in the suggested directory, press Enter. Alternatively, you can specify another location.
Note: If you already have a key pair in the proposed location, it is advisable to pick another directory. Otherwise it will overwrite existing SSH keys.
4. Next, the prompt will continue with:
Although creating a passphrase isn’t mandatory, it is highly advisable.
5. Finally, the output will end by specifying the following information:
Now you need to add the public key to the remote CentOS server.
You can copy the public SSH key on the remote server using several different methods:
- using the ssh-copy-id script
- using Secure Copy (scp)
- manually copying the key
The fastest and easiest method is by utilizing ssh-copy-id. If the option is available, we recommend using it. Otherwise, try any of the other two noted.
1. Start by typing the following command, specifying the SSH user account, and the IP address of the remote host:
If it is the first time your local computer is accessing this specific remote server you will receive the following output:
2. Confirm the connection – type yes and hit Enter.
3. Once it locates the id_rsa.pub key created on the local machine, it will ask you to provide the password for the remote account. Type in the password and hit Enter.
4. Once the connection has been established, it adds the public key on the remote server. This is done by copying the ~/.ssh/id_rsa.pub file to the remote server’s ~/.ssh directory. You can locate it under the name authorized_keys.
5. Lastly, the output tells you the number of keys added, along with clear instructions on what to do next: Professional plus 2010 key generator.
1. First, set up an SSH connection with the remote user:
2. Next, create the ~/.ssh directory as well as the authorized_keys file:
3. Use the chmod command to change the file permission:
chmod 700 makes the file executable, while chmod 600 allows the user to read and write the file.
4. Now, open a new terminal session, on the local computer.
5. Copy the content from id_rsa.pub (the SSH public key) to the previously created authorized_keys file on the remote CentOS server by typing the command:
With this, the public key has been safely stored on the remote account.
1. To manually add the public SSH key to the remote machine, you first need to open the content from the ~/.ssh/id_rsa.pub file:
2. As in the image below, the key starts with ssh-rsa and ends with the username of the local computer and hostname of the remote machine:
3. Copy the content of the file, as you will need later.
4. Then, in the terminal window, connect to the remote server on which you wish to copy the public key. Use the following command to establish the connection:
5. Create a ~/.ssh directory and authorized_keys file on the CentOS server with the following command:
6. Change their file permission by typing:
7. Next, open the authorized_keys file with an editor of your preference. For example, to open it with Nano, type:
8. Add the public key, previously copied in step 2 of this section, in a new line in (under the existing content).
9. Save the changes and close the file.
10. Finally, log into the server to verify that everything is set up correctly.
Once you have completed the previous steps (creating an RSA Key Pair and copying the Public Key to the CentOS server), you will be able to connect to the remote host without typing the password for the remote account.
All you need to do is type in the following command:
If you didn’t specify a passphrase while creating the SSH key pair, you will automatically log in the remote server.
Otherwise, type in the passphrase you supplied in the initial steps and press Enter.
Once the shell confirms the key match, it will open a new session for direct communication with the server.
Although you managed to access the CentOS server without having to provide a password, it still has a password-based authentication system running on the machine. This makes it a potential target for brute force attacks.
You should disable password authentication entirely by following the outlined steps.
Note: Consider performing the following steps through a non-root account with sudo privileges, as an additional safety layer.
1. Using the SSH keys, log into the remote CentOS server which has administrative privileges:
2. Next, open the SSH daemon configuration file using a text editor of your choice:
3. Look for the following line in the file:
4. Edit the configuration by changing the yes value to no. Thus, the directive should be as following:
Generate A Ssh Key On Linux Download
5. Save the file and exit the text editor.
6. To enable the changes, restart the sshdservice using the command:
7. Verify the SSH connection to the server is still functioning correctly. Open a new terminal window and type in the command:
In this article, you learned how to generate SSH key pairs and set up an SSH key-based authentication. We also covered copying keys to your remote CentOS server, and disabling SSH password authentication.
Create New Ssh Key
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