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 implementaion 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 server, 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 and more.
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.
This tutorial assumes that you are familiar with our guide on How to Log Into Your Droplet with PuTTY (for windows users).
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.
To generate a set of RSA keys with PuTTYgen:
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 ...."
Now, you need to paste the copied public key in the file ~/.ssh/authorized_keys on your server.
mkdir ~/.ssh chmod 0700 ~/.ssh touch ~/.ssh/authorized_keys chmod 0644 ~/.ssh/authorized_keys
sudo vim ~/.ssh/authorized_keys
ikey on your keyboard & right-click your mouse to paste.
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.
Now you can go ahead and log in to email@example.com 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).
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
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):
Enter. Now, reload
the SSH server's configuration:
sudo reload ssh