Generate Rsa Key Pair Ubuntu
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This article shows how to configure a SSH connection for authentication by using the public-key method. To do this, a key pair is created at the client, the public part of the key is transferred to the server, and afterwards the server is set up for key authentication. The user can log on to the server without a login password, only the password is required to protect the private key.The operating systems used in this article are on the one hand a Ubuntu 12.10 at the client side and a Ubuntu 12.04 at the server side.This guide was also validated working with Ubuntu 16.04 as client and server.
- 1On the client
- 2On the server
- 3Notes for other distributions
- Step One: Creation of the RSA Key Pair. The first step in the installation process is to create the key pair on the client machine, which would, more often than not, be your own system. Users need to use the following command: ssh-keygen -o -b 4096 -t rsa. The above command kicks off the SSH Key installation process for users.
- Create an SSH key pair Use the ssh-keygen command to generate SSH public and private key files. By default, these files are created in the /.ssh directory. You can specify a different location, and an optional password (passphrase) to access the private key file.
There is already a command in the ssh suite to do this automatically for you. I.e log into a remote host and add the public key to that computers authorizedkeys file. Ssh-copy-id -i /path/to/key/file user@host.com If the key you are installing is /.ssh/idrsa then you can even drop the -i flag completely. Much better than manually doing it! Amazon EC2 Key Pairs. Amazon EC2 uses public–key cryptography to encrypt and decrypt login information. Public–key cryptography uses a public key to encrypt a piece of data, and then the recipient uses the private key to decrypt the data. The public and private keys are known as a key pair. There are many reasons you might want to create a key pair on Linux, more specifically on Ubuntu. For more information about key pairs, see this. If your server is an Amazon EC2 Server Instance, you might want to look at more specific information here. Key pairs are just one way to log into a system. (Perhaps the one you currently use is regular username and password ssh login).
On the client
The first configuration steps take place at the client side.
Home folder rights
By default, Ubuntu sets the user home directory permissions to 755. Nevertheless, for security reasons, check whether the permissions are set to 755 on your system and change them if necessary:
:~$ sudo chmod 755 /home/<USER>
Generate keypair
In the first step, a key pair with ssh-keygen is created at the client. If you use Ubuntu 18.04 on the server, the package openssh-server will be installed in the version 7.6.[1] Since this version, RSA bit lengths smaller than 1024 bits are no longer accepted.[2] In this example, a bit length of 4096 bits is selected for the RSA keys:
Please note: It is recommended to protect the key with a passphrase for security reasons. This means that the key is not available in plain text, but is AES-CBC encrypted:
If the private key is stolen by an attacker, he has to find out the password of the key in order to access the server with the key. If the key is available in plain text, an attacker can use this stolen key to directly access the server.
Transfer the public key to the server
To transfer the public key to the server, the first step is to use the SSH connection via password authentication yet. The ssh-copy-id tool copies the corresponding identity file to the server:
The above-mentioned procedure has created the following entry in the /home/tktest/.ssh/authorized_keyson the server:
Test the key authentication
Now that the public key is transferred to the server, the connection can be tested from the client. In this case, it is important that the server does not ask for the user password, but of course the passphrase of the protected key is required!
The following dialog box appears for GUI-based systems:
After entering the password that protected the key when it was created, you are authenticated on the system:
On the server
This paragraph shows some additional configuration steps on the server side to harden the public-key authentication.
sshd configuration
In Ubuntu, it is generally sufficient to carry out the above-mentioned procedure for public-key authentication. In some situations it makes sense to deactivate password authentication completely.
Please note: After changing the following setting, it is no longer possible to log in with a password via ssh: PasswordAuthentication no.
From the client, the connection is tested again:
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In the above example, the dialog for entering the key password has been aborted. Since the log-on via password was deactivated, it was not possible to log-on to the system.
Forbid password authentication for just one user
Another way in which password authentication is not completely deactivated is to disable password authentication for specific users. This allows a user who does not have sudo privileges to log on to the server, for example. To gain root privileges, at least one additional password must be found for a user with sudo privileges. Plus, there's a way to completely exclude users from ssh:
This example:
- Prohibits SSH access for the user
test - Deactivates password authentication for the user
tktest - Password authentication is retained for all other users
Notes for other distributions
For other Linux distributions, the required steps may differ slightly. We would be happy to supplement our experiences with other distributions, which you are welcome to share with us via the feedback function.
Red Hat
Generate Rsa Key Pair Ubuntu Software
One reader told us that the procedure described Red Hat does not work 1:1 in Red Hat. In the home directory of the user, the write permission was set for the group. After a chmod 755 it worked to connect via ssh to the server without asking for a password.
References
- ↑Package: openssh-server (1:7.6p1-4) (packages.ubuntu.com)
- ↑OpenSSH 7.6 Release Notes (openssh.com)