How to ssh with key linux

Ubuntu Documentation

Public and Private Keys

Public key authentication is more secure than password authentication. This is particularly important if the computer is visible on the internet. If you don’t think it’s important, try logging the login attempts you get for the next week. My computer — a perfectly ordinary desktop PC — had over 4,000 attempts to guess my password and almost 2,500 break-in attempts in the last week alone.

With public key authentication, the authenticating entity has a public key and a private key. Each key is a large number with special mathematical properties. The private key is kept on the computer you log in from, while the public key is stored on the .ssh/authorized_keys file on all the computers you want to log in to. When you log in to a computer, the SSH server uses the public key to «lock» messages in a way that can only be «unlocked» by your private key — this means that even the most resourceful attacker can’t snoop on, or interfere with, your session. As an extra security measure, most SSH programs store the private key in a passphrase-protected format, so that if your computer is stolen or broken in to, you should have enough time to disable your old public key before they break the passphrase and start using your key. Wikipedia has a more detailed explanation of how keys work.

Public key authentication is a much better solution than passwords for most people. In fact, if you don’t mind leaving a private key unprotected on your hard disk, you can even use keys to do secure automatic log-ins — as part of a network backup, for example. Different SSH programs generate public keys in different ways, but they all generate public keys in a similar format:

Key-Based SSH Logins

Key-based authentication is the most secure of several modes of authentication usable with OpenSSH, such as plain password and Kerberos tickets. Key-based authentication has several advantages over password authentication, for example the key values are significantly more difficult to brute-force, or guess than plain passwords, provided an ample key length. Other authentication methods are only used in very specific situations.

SSH can use either «RSA» (Rivest-Shamir-Adleman) or «DSA» («Digital Signature Algorithm») keys. Both of these were considered state-of-the-art algorithms when SSH was invented, but DSA has come to be seen as less secure in recent years. RSA is the only recommended choice for new keys, so this guide uses «RSA key» and «SSH key» interchangeably.

Key-based authentication uses two keys, one «public» key that anyone is allowed to see, and another «private» key that only the owner is allowed to see. To securely communicate using key-based authentication, one needs to create a key pair, securely store the private key on the computer one wants to log in from, and store the public key on the computer one wants to log in to.

Using key based logins with ssh is generally considered more secure than using plain password logins. This section of the guide will explain the process of generating a set of public/private RSA keys, and using them for logging into your Ubuntu computer(s) via OpenSSH.

Generating RSA Keys

The first step involves creating a set of RSA keys for use in authentication.

This should be done on the client.

To create your public and private SSH keys on the command-line:

You will be prompted for a location to save the keys, and a passphrase for the keys. This passphrase will protect your private key while it’s stored on the hard drive:

Your public key is now available as .ssh/id_rsa.pub in your home folder.

Congratulations! You now have a set of keys. Now it’s time to make your systems allow you to login with them

Choosing a good passphrase

You need to change all your locks if your RSA key is stolen. Otherwise the thief could impersonate you wherever you authenticate with that key.

An SSH key passphrase is a secondary form of security that gives you a little time when your keys are stolen. If your RSA key has a strong passphrase, it might take your attacker a few hours to guess by brute force. That extra time should be enough to log in to any computers you have an account on, delete your old key from the .ssh/authorized_keys file, and add a new key.

Your SSH key passphrase is only used to protect your private key from thieves. It’s never transmitted over the Internet, and the strength of your key has nothing to do with the strength of your passphrase.

The decision to protect your key with a passphrase involves convenience x security. Note that if you protect your key with a passphrase, then when you type the passphrase to unlock it, your local computer will generally leave the key unlocked for a time. So if you use the key multiple times without logging out of your local account in the meantime, you will probably only have to type the passphrase once.

If you do adopt a passphrase, pick a strong one and store it securely in a password manager. You may also write it down on a piece of paper and keep it in a secure place. If you choose not to protect the key with a passphrase, then just press the return when ssh-keygen asks.

Key Encryption Level

Note: The default is a 2048 bit key. You can increase this to 4096 bits with the -b flag (Increasing the bits makes it harder to crack the key by brute force methods).

Password Authentication

The main problem with public key authentication is that you need a secure way of getting the public key onto a computer before you can log in with it. If you will only ever use an SSH key to log in to your own computer from a few other computers (such as logging in to your PC from your laptop), you should copy your SSH keys over on a memory stick, and disable password authentication altogether. If you would like to log in from other computers from time to time (such as a friend’s PC), make sure you have a strong password.

Transfer Client Key to Host

The key you need to transfer to the host is the public one. If you can log in to a computer over SSH using a password, you can transfer your RSA key by doing the following from your own computer:

Where and should be replaced by your username and the name of the computer you’re transferring your key to.

Due to this bug, you cannot specify a port other than the standard port 22. You can work around this by issuing the command like this: ssh-copy-id " @ -p

". If you are using the standard port 22, you can ignore this tip.

Another alternative is to copy the public key file to the server and concatenate it onto the authorized_keys file manually. It is wise to back that up first:

You can make sure this worked by doing:

You should be prompted for the passphrase for your key:

Enter passphrase for key ‘/home/ /.ssh/id_rsa’:

Enter your passphrase, and provided host is configured to allow key-based logins, you should then be logged in as usual.

Troubleshooting

Encrypted Home Directory

If you have an encrypted home directory, SSH cannot access your authorized_keys file because it is inside your encrypted home directory and won’t be available until after you are authenticated. Therefore, SSH will default to password authentication.

To solve this, create a folder outside your home named /etc/ssh/ (replace » » with your actual username). This directory should have 755 permissions and be owned by the user. Move the authorized_keys file into it. The authorized_keys file should have 644 permissions and be owned by the user.

Then edit your /etc/ssh/sshd_config and add:

Finally, restart ssh with:

The next time you connect with SSH you should not have to enter your password.

username@host’s password:

If you are not prompted for the passphrase, and instead get just the

username@host’s password:

prompt as usual with password logins, then read on. There are a few things which could prevent this from working as easily as demonstrated above. On default Ubuntu installs however, the above examples should work. If not, then check the following condition, as it is the most frequent cause:

On the host computer, ensure that the /etc/ssh/sshd_config contains the following lines, and that they are uncommented;

If not, add them, or uncomment them, restart OpenSSH, and try logging in again. If you get the passphrase prompt now, then congratulations, you’re logging in with a key!

Permission denied (publickey)

If you’re sure you’ve correctly configured sshd_config, copied your ID, and have your private key in the .ssh directory, and still getting this error:

Permission denied (publickey).

Chances are, your /home/ or

/.ssh/authorized_keys permissions are too open by OpenSSH standards. You can get rid of this problem by issuing the following commands:

Error: Agent admitted failure to sign using the key.

This error occurs when the ssh-agent on the client is not yet managing the key. Issue the following commands to fix:

This command should be entered after you have copied your public key to the host computer.

Debugging and sorting out further problems

The permissions of files and folders is crucial to this working. You can get debugging information from both the client and server.

if you think you have set it up correctly , yet still get asked for the password, try starting the server with debugging output to the terminal.

To connect and send information to the client terminal

Where to From Here?

No matter how your public key was generated, you can add it to your Ubuntu system by opening the file .ssh/authorized_keys in your favourite text editor and adding the key to the bottom of the file. You can also limit the SSH features that the key can use, such as disallowing port-forwarding or only allowing a specific command to be run. This is done by adding «options» before the SSH key, on the same line in the authorized_keys file. For example, if you maintain a CVS repository, you could add a line like this:

When the user with the specified key logged in, the server would automatically run /usr/bin/cvs server, ignoring any requests from the client to run another command such as a shell. For more information, see the sshd man page. /755

SSH/OpenSSH/Keys (последним исправлял пользователь larsnooden 2015-07-30 19:26:16)

The material on this wiki is available under a free license, see Copyright / License for details
You can contribute to this wiki, see Wiki Guide for details

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How To Set up SSH Keys on a Linux / Unix System

I recently read that SSH keys provide a secure way of logging into a Linux and Unix-based server. How do I set up SSH keys on a Linux or Unix based systems? In SSH for Linux/Unix, how do I set up public key authentication?

Tutorial details
Difficulty level	Easy
Root privileges	No
Requirements	OpenSSH client and server
Est. reading time	8 mintues

This page explains a public key and shows you how to set up SSH keys on a Linux or Unix-like server. I am assuming that you are using Linux or Unix-like server and client with the following software:

• OpenSSH SSHD server
• OpenSSH ssh client and friends on Linux (Ubuntu, Debian, BSD, RHEL, CentOS, MacOS/OSX, AIX, HP-UX and co).

What is a public key authentication?

OpenSSH server supports various authentication schema. The two most popular are as follows:

1. Passwords based authentication
2. Public key based authentication. It is an alternative security method to using passwords. This method is recommended on a VPS, cloud, dedicated or even home based server.

How to set up SSH keys

Steps to setup secure ssh keys:

1. Create the ssh key pair using ssh-keygen command.
2. Copy and install the public ssh key using ssh-copy-id command on a Linux or Unix server.
3. Add yourself to sudo or wheel group admin account.
4. Disable the password login for root account.
5. Test your password less ssh keys login using ssh user@server-name command.

Let us see all steps in details.

How do I set up public key authentication?

You must generate both a public and a private key pair. For example:

Fig.01: Our sample setup

In public key based method you can log into remote hosts and server, and transfer files to them, without using your account passwords. Feel free to replace server1.cyberciti.biz and client1.cyberciti.biz names with your actual setup. Enough talk, let’s set up public key authentication. Open the Terminal and type following commands if .ssh directory does not exists:

1: Create the key pair

On the computer (such as client1.cyberciti.biz), generate a key pair for the protocol.

You need to set the Key Pair location and name. I recommend you use the default location if you do not yet have another key there, for example: $HOME/.ssh/id_rsa. You will be prompted to supply a passphrase (password) for your private key. I suggest that you setup a passphrase when prompted. You should see two new files in $HOME/.ssh/ directory:

1. $HOME/.ssh/id_rsa – contains your private key.
2. $HOME/.ssh/id_rsa.pub – contain your public key.

Optional syntax for advance users

The following syntax specifies the 4096 of bits in the RSA key to creation (default 2048):
$ ssh-keygen -t rsa -b 4096 -f

/.ssh/vps-cloud.web-server.key -C «My web-server key»
Where,

• -t rsa : Specifies the type of key to create. The possible values are “rsa1” for protocol version 1 and “dsa”, “ecdsa”, “ed25519”, or “rsa” for protocol version 2.
• -b 4096 : Specifies the number of bits in the key to create
• -f

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How To Create SSH Keys with OpenSSH on macOS, Linux, or Windows Subsystem for Linux

Published on December 1, 2020

Introduction

When setting up a remote Linux server, you’ll need to decide upon a method for securely connecting to it. While passwords are one way of verifying a user’s identity, passwords have multiple vulnerabilities and can be cracked by a brute force attack. Secure Shell keys — better known as SSH keys — are often used instead of passwords, as they offer a more secure method of connecting to remote Linux servers. As part of the Secure Shell cryptographic network protocol, SSH keys also enable users to securely perform network services over an unsecured network, such as delivering text-based commands to a remote server or configuring its services.

This tutorial will guide you through the steps of creating SSH keys with OpenSSH, a suite of open source SSH tools, on macOS and Linux, as well as on Windows through the use of the Windows Subsystem for Linux. It is written for an audience that is just getting started with the command line and will provide guidance on accessing the terminal on a personal computer. After completing this tutorial, you will have SSH keys that can be used to securely set up a server in the tutorial How To Set Up an Ubuntu 20.04 Server on a DigitalOcean Droplet that is part of the Introduction to the Cloud Curriculum.

If you are already familiar with the command line and looking for instructions on using SSH to connect to a remote server, please see our collection of tutorials on Setting Up SSH Keys for a range of Linux operating systems.

Prerequisites

To complete this tutorial, you will need:

• A local machine running one of the following operating systems: macOS, Linux, or Windows with Windows Subsystem for Linux installed. If you are using Windows, you can find instructions for downloading or updating the Windows Subsystem for Linux on Microsoft’s documentation page. Note that while OpenSSH should work for a range of Linux distributions, this tutorial has been tested using Ubuntu 20.04.

Note: If you are looking for instructions on how to create SSH keys on a Windows machine that does not have the Windows Subsystem for Linux, please visit our product documentation How to Create SSH Keys with PuTTY on Windows.

• Some familiarity with working with a terminal and the command line. If you need an introduction to working with terminals and the command line, you can visit our guide A Linux Command Line Primer.

Step 1 — Understanding SSH Keys

SSH keys are two long strings of characters that can be used to authenticate the identity of a user requesting access to a remote server. These keys are generated by the user on their local computer using a SSH utility. One key is private and stored on the user’s local machine. The other key is public and shared with the remote server or any other entity the user wishes to securely communicate with.

When a user requests to connect to a server with SSH, the server sends a message encrypted with the public key that can only be decrypted by the associated private key. The user’s local machine then uses its private key to attempt to decrypt the message. If the message is successfully decrypted, the server grants the user access without the need of a password. Once authenticated, users can launch a remote shell session in their local terminal to deliver text-based commands to the remote server.

In the next step, you will open a terminal on your computer so that you can access the SSH utility used to generate a pair of SSH keys.

Step 2 — Opening a Terminal on Your Computer

A terminal allows you to interact with your computer through text-based commands rather than a graphical user interface. The way you access the terminal on your computer will depend on what type of operating system you are using.

On machines running macOS, the Terminal application is typically located in the Utilities folder inside the Applications folder. You can also find it by searching for “terminal” in the Search Spotlight.

If you are working on a Linux computer, your distribution’s default terminal application is also typically located in the Utilities folder inside the Applications folder. You can also find it by searching for “terminal” with the Desktop search functionality.

If you are working on a Windows machine running Windows Subsystem for Linux, a Linux terminal should open immediately after installation. You can also find it by searching for “Ubuntu” with the Desktop search functionality.

Once you have located your system’s terminal application, open up a new terminal window. Your terminal should display your user name, a dollar sign ( $ ), and a cursor. This is where you will begin to type commands to tell the terminal what to do.

In the next step, you will enter a text-based command to generate a pair of SSH keys.

Step 3 — Generating Keys With OpenSSH

Your macOS or Linux operating system should have the standard OpenSSH suite of tools already installed. This suite of tools includes the utility ssh-keygen , which you will use to generate a pair of SSH keys.

Type the following command into your terminal:

You will then be prompted to select a location for the keys. By default, the keys are stored in the

/.ssh directory with the filenames id_rsa for the private key and id_rsa.pub for the public key. Using the default locations allows your SSH client to automatically find your SSH keys when authenticating, so we recommend accepting these default options. To do so, press ENTER :

Warning: If you have previously generated a key pair, you will be prompted to confirm that you actually want to overwrite the existing key:

If you choose to overwrite the key on disk, you will not be able to authenticate using the previous key anymore. Selecting “yes” is an irreversible destructive process.

If you’re certain that you want to overwrite the existing key on disk, you can do so by pressing Y and then ENTER .

If you choose the default location, your public key will be located in /home/ sammy /.ssh/id_rsa.pub and your private key will be located in /home/ sammy /.ssh/id_rsa . Note that in your filepath, sammy will be replaced with your username.

After selecting a location for the key, you’ll be prompted to enter an optional passphrase which encrypts the private key file on disk.

If you enter a passphrase, you will have to provide it every time you use this key (unless you are running SSH agent software that stores the decrypted key). We recommend using a passphrase, but you can just press ENTER to bypass this prompt:

Following that final prompt, your system will generate the SSH key pair:

You now have a public and private key that you can use to authenticate.

Conclusion

Congratulations, you have now generated a pair of SSH keys. These keys can be used to securely connect with a remote server and are necessary for the tutorial How To Set Up an Ubuntu 20.04 Server on a DigitalOcean Droplet that follows this tutorial in the Introduction to the Cloud Curriculum.

For a deeper dive on working using SSH, please visit our guide SSH Essentials: Working With SSH Servers, Clients, and Keys.

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