Tutorial

How To Create SSH Keys With PuTTY to Connect to a VPS

Published on July 19, 2013
author

Pablo Carranza

How To Create SSH Keys With PuTTY to Connect to a VPS

Benefits of Public & Private SSH Keys

If your headless, or remote, VPS is visible over the Internet, you should use public key authentication instead of passwords, if at all possible. This is because SSH keys provide a more secure way of logging in compared to using a password alone. While a password can eventually be cracked with a brute-force attack, SSH keys are nearly impossible to decipher by brute force alone. With public key authentication, every computer has (i) a public and (ii) a private "key" (two mathematically-linked algorithms that are effectively impossible to crack).

Today, OpenSSH is the default SSH implementation on Unix-like systems such as Linux and OS X. Key-based authentication is the most secure of several modes of authentication usable with OpenSSH, such as plain passwords and Kerberos tickets. 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 tutorial uses "RSA key" and "SSH key" interchangeably.

When you log in to your DigitalOcean VPS, 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 cannot snoop on, or interfere with, your session. As an extra security measure, some users and most SSH programs store the private key in a passphrase-protected format, to provide a window of time in which you can disable your compromised public key, should your computer be stolen or broken in to. For these reasons, public key authentication is a much better solution than passwords for most people. In fact, by not employing a passphrase on your private key, you will have the ability to automate parts of your configuration management with secure, automatic log-ins, such as incremental off-site backups, manage your DigitalOcean assets via the DigitalOcean API, and more.

Key-Based SSH Logins

You can save the same public key on as many cloud servers as you'd like, while your private key is saved on a client from which you log in to the server. Then, you can disable the normal username/password login procedure, which means that only people with a valid private/public key pair can log in; making your system more secure, because it will be impervious to brute-force attacks.

Automate the Creation of New Droplets

Another useful purpose that SSH keys can serve is in the creation of DigitalOcean droplets. As you know, when you spin up a droplet, you have to wait for an e-mail with your password. Although this email is very convenient, there is a more secure (and faster) way of gaining access to your new cloud server without the need for email. This can be done by saving your public key in the DigitalOcean Control Panel. To accomplish this:

Prerequisites

This tutorial assumes that you are familiar with DigitalOcean's guide on How to Log Into Your Droplet with PuTTY (for windows users).

PuTTY Key Generator (a.k.a. PuTTYgen)

While PuTTY is a client program for SSH (in addition to Telnet and Rlogin), it is not a port of or otherwise based on OpenSSH. Consequently, PuTTY does not have native support for reading OpenSSH's SSH-2 private key files. However, PuTTY does have a companion named PuTTYgen (an RSA and DSA key generation utility), that can convert OpenSSH private key files into PuTTY's format; allowing you to connect to your cloud server from a Windows machine, with the added security that SSH keys provide.

PuTTYgen is a (free) open-source utility and can be downloaded from the maintainer's website. PuTTYgen is what you will use to generate your SSH keys for use in PuTTY. To start, all you need to do is download the exectuable files (.exe) and save them on the computer that you'll use to connect to your VPS, e.g. on the desktop. You will not need to "install" PuTTYgen, because it is a standalone application.

Generating OpenSSH-compatible Keys for Use with PuTTY

To generate a set of RSA keys with PuTTYgen:

  1. Start the PuTTYgen utility, by double-clicking on its .exe file;
  2. For Type of key to generate, select RSA;
  3. In the Number of bits in a generated key field, specify either 2048 or 4096 (increasing the bits makes it harder to crack the key by brute-force methods);
  4. Click the Generate button;
  5. Move your mouse pointer around in the blank area of the Key section, below the progress bar (to generate some randomness) until the progress bar is full;
  6. A private/ public key pair has now been generated;
  7. In the Key comment field, enter any comment you'd like, to help you identify this key pair, later (e.g. your e-mail address; home; office; etc.) -- the key comment is particularly useful in the event you end up creating more than one key pair;
  8. Optional: Type a passphrase in the Key passphrase field & re-type the same passphrase in the Confirm passphrase field (if you would like to use your keys for automated processes, however, you should not create a passphrase);
  9. Click the Save public key button & choose whatever filename you'd like (some users create a folder in their computer named my_keys);
  10. Click the Save private key button & choose whatever filename you'd like (you can save it in the same location as the public key, but it should be a location that only you can access and that you will NOT lose! If you lose your keys and have disabled username/password logins, you will no longer be able log in!);
  11. Right-click in the text field labeled Public key for pasting into OpenSSH authorized_keys file and choose Select All;
  12. Right-click again in the same text field and choose Copy.

NOTE: PuTTY and OpenSSH use different formats for public SSH keys. If the SSH Key you copied starts with "---- BEGIN SSH2 PUBLIC KEY ...", it is in the wrong format. Be sure to follow the instructions carefully. Your key should start with "ssh-rsa AAAA ...."

Save The Public Key On The Server

Now, you need to paste the copied public key in the file ~/.ssh/authorized_keys on your server.

  1. Log in to your destination server; see How to Log Into Your Droplet with PuTTY (for windows users)
  2. If your SSH folder does not yet exist, create it manually:
  3. mkdir ~/.ssh
    chmod 0700 ~/.ssh
    touch ~/.ssh/authorized_keys
    chmod 0644 ~/.ssh/authorized_keys
  4. Paste the SSH public key into your ~/.ssh/authorized_keys file (see Installing and Using the Vim Text Editor on an Cloud Server):
  5. sudo vim ~/.ssh/authorized_keys
  6. Tap the i key on your keyboard & right-click your mouse to paste.
  7. To save, tap the following keys on your keyboard (in this order): Esc, :, w, q, Enter.

Create a PuTTY Profile to Save Your Server's Settings

In PuTTY, you can create (and save) profiles for connections to your various SSH servers, so you don't have to remember, and continually re-type, redundant information.

  1. Start PuTTY by double-clicking its executable file;
  2. PuTTY's initial window is the Session Category (navigate PuTTY's various categories, along the left-hand side of the window);
  3. In the Host Name field, enter the IP address of your VPS or its fully qualified domain name (FQDN); see How to Set Up a Host Name with DigitalOcean
  4. Enter the port number in the Port field (for added security, consider changing your server's SSH port to a non-standard port. See Step Five of Initial Server Setup with Ubuntu 12.04
  5. Select SSH under Protocol;
  6. Along the left-hand side of the window, select the Data sub-category, under Connection;
  7. Specify the username that you plan on using, when logging in to the SSH server, and whose profile you're saving, in the Auto-login username field;
  8. Expand the SSH sub-category, under Connection;
  9. Highlight the Auth sub-category and click the Browse button, on the right-hand side of the PuTTY window;
  10. Browse your file system and select your previously-created private key;
  11. Return to the Session Category and enter a name for this profile in the Saved Sessions field, e.g. user@123.456.78.9 or user@host.yourdomain.tld;
  12. Click the Save button for the Load, Save or Delete a stored session area.

Now you can go ahead and log in to user@1.2.3.4 and you will not be prompted for a password. However, if you had set a passphrase on your public key, you will be asked to enter the passphrase at that time (and every time you log in, in the future).

Disable Username/Password Logins

Once you have verified that your key-based logins are working, you may elect to disable username/password logins to achieve better security. To do this, you need to edit your SSH server's configuration file. On Debian/ Ubuntu systems, this file is located at /etc/ssh/sshd_config.

sudo vim /etc/ssh/sshd_config

Tap the i key on your keyboard and edit the lines, referenced below:

[...]
PasswordAuthentication no
[...]
UsePAM no
[...]

To save, tap the following keys on your keyboard (in this order): Esc, :, w, q, Enter. Now, reload the SSH server's configuration:

sudo reload ssh

Additional Resources

As always, if you need help with the steps in this HowTo, look to the DigitalOcean Community for assistance by posing your question(s), below.

Article Submitted by: Pablo Carranza

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For a fresh install of CentOS 6.4 64-bit, I had to use “vim” wherever “vi” appears to edit files and I used this comment to restart sshd:

sudo service sshd restart

Otherwise, very helpful. Thanks!

I got a server ‘server refused our key’ message in PuTTY…

I got this message due to me creating the keys as “root” for a user. This appends the owner of “root” to the created folder/file. I had to add this command to succeed:

Note: this “user” was created beforehand without password.

mkdir /home/user/.ssh
chmod 0700 /home/user/.ssh
touch /home/user/.ssh/authorized_keys
chmod 0644 /home/user/.ssh/authorized_keys
chown user:user /home/rsadmin/.ssh/ -R
Kamal Nasser
DigitalOcean Employee
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October 18, 2013

@Nicholas: Have you uploaded the public to your droplet?

@Kamal,

You mean by means of sudo nano ~/.ssh/authorized_keys? Yes, I did that.

Strangely, it’s accepting my ‘user’ but not my key. I don’t have to type my username but still have to type my password to logon.

Kamal Nasser
DigitalOcean Employee
DigitalOcean Employee badge
October 20, 2013

@Nicholas: Hmm. Can you please pastebin the contents of ~/.ssh/authorized_keys?

question. do i sudo nano ~/.ssh/authorized_keys in the root directory or do i do that in /home/user/?

Kamal Nasser
DigitalOcean Employee
DigitalOcean Employee badge
October 23, 2013

@Nicholas: /home/user/. Also, what’s the output of <code>wc -l /home/user/.ssh/authorized_keys</code>?

@Nicholas: I was having the same issue as you (“server refused our key” in PuTTY). I followed the steps of this article as a user with root access in CentOS 6. To be able to log in, I had to set the permissions of ~/.ssh to 755 instead of 700. In other words: sudo chmod 755 ~/.ssh sudo chmod 644 ~/.ssh/authorized_keys

@Kamal: Is the above acceptable? Or does it pose a risk, compared to setting the permissions of ~/.ssh to 700?

The CentOS wiki mentions that the following “permissions are required if StrictModes is set to yes in /etc/ssh/sshd_config (the default).” 700 ~/.ssh 600 ~/.ssh/authorized_keys

If I understand correctly… 700 means the owner can read/write/execute 600 means the owner can read/write

I assume the owner is root. But even as root, if I set the above permissions (or the ones mentioned in the guide), I receive “server refused our key”.

755 means owner can read/write/execute, group can read/execute, and public can read/execute. 644 means owner can read/write, group can read, and public can read.

Which is I’m not too comfortable setting permissions to… 755 ~/.ssh 644 ~/.ssh/authorized_keys

Then again, if I understand correctly, the above is still safer than logging in with username/password. Is that right?

Another thing: I followed another DigitalOcean guide by Etel Sverdlov (https://www.digitalocean.com/community/articles/initial-server-setup-with-centos-6) to create a user and change PermitRootLogin to no in order to prevent root log in. Could this be one of the reasons why I cannot enter the server through keys under permissions 700/644 above?

Thanks for your help.

@Kamal,

It works now. Thanks for pointing out my mistake. Had ‘sudo nano ~/.ssh/authorized_keys?’ in root. did that in /home/user and it is working

Hi, does anyone know how can I automate the passphrase entering ? in unix I just accept the passphrase for the first time and it will never ask me again, on windows I accept the first time, but it will ask me again and again

a better solution is not to do pasphrase but I had to (for other computers)

any idea ? any configuration in Putty ?

thanks

Hi, I would like to disable password login. This is what I have done:

[…] PasswordAuthentication no […] UsePAM no […]

To confirm that password has been disabled, I removed private key from Pageant and reconnected to putty. This is what I see in Putty:

login as: random random@123.123.123.12’s password:

I was prompted for the password. Can i safely assume that all passwords will not work since I have disabled passwords?

Fyi, when i created my droplet, i used SSH Key thus no email (containing the password) was sent by Digital Ocean.

Can I again assume that there is no password set for this droplet?

Sorry if i seem to sound like i don’t know what is going on as I am totally new to all this. Thanks in advance!

how do i get this software to sends bulk emails for marketing

Kamal Nasser
DigitalOcean Employee
DigitalOcean Employee badge
December 4, 2013

@hopefloatt: <blockquote>I was prompted for the password. Can i safely assume that all passwords will not work since I have disabled passwords? </blockquote> Yes, as long as you restarted SSH, it will reject all passwords.

<blockquote>Fyi, when i created my droplet, i used SSH Key thus no email (containing the password) was sent by Digital Ocean.

Can I again assume that there is no password set for this droplet? </blockquote> It does have a random password set however it wasn’t sent to you. You can always change it by running the <code>passwd</code> command.

Thanks very much Kamal!! :D

<b>“It does have a random password set however it wasn’t sent to you. You can always change it by running the <code>passwd</code> command.”</b>

Is it possible to remove the password altogether?

Kamal Nasser
DigitalOcean Employee
DigitalOcean Employee badge
December 6, 2013

@Pablo: I think you can remove the password (<code>sudo passwd -d user</code>) and then lock the user:

<a href=“http://www.cyberciti.biz/faq/linux-locking-an-account/”>http://www.cyberciti.biz/faq/linux-locking-an-account/</a>

Hey DO, great tutorial as usual, FYI the link to installing vim is broken

I guess it is better to use sudo /etc/init.d/ssh restart to restart SSH

That command in the tut doesn’t work on my setup so I had to use the one I wrote in my first comment line.

Good luck, and Thanks by the way for the nice tut!

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