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UbuntuHelp:SSH/OpenSSH/Keys

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Parent page: Internet and Networking >> SSH

Public and Private Keys

If your SSH server is visible over the Internet, you should use public key authentication instead of passwords if at all possible. If you don't think it's important, try logging all of the malicious 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. How many thousand random guesses do you think it will take before an attacker stumbles across your password? With public key authentication, every computer has a public and a private "key" (a large number with particular 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:

<ssh-rsa or ssh-dsa> <really long string of nonsense> <username>@<host>

Key-Based SSH Logins

Key-based authentication is the most secure of several modes of authentication usable with OpenSSH, such as plain password (the default with Ubuntu) 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, you need to create a public key for the computer you're logging in from, and securely transmit it to the computer you're logging in to. Wikipedia has a good explanation of the theory 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:

mkdir ~/.ssh
chmod 700 ~/.ssh
ssh-keygen -t rsa

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 and be required to use the keys every time you need to login to a key-based system:

Generating public/private rsa key pair.
Enter file in which to save the key (/home/b/.ssh/id_rsa):
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /home/b/.ssh/id_rsa.
Your public key has been saved in /home/b/.ssh/id_rsa.pub.

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

Just like with physical keys, you need to change all your locks if your RSA key is stolen. Otherwise, your thief will be able to get access to all your stuff. 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. You have to choose for yourself whether to use a passphrase with your RSA key. Ultimately, it's a choice between cursing the difficulty every time you have to type it in, or cursing your glibness when someone logs in to all your accounts and changes your password so you can't get in any more. If you choose to use a passphrase, pick something strong and write it down on a piece of paper that you keep in a safe place. If you choose not to use a password, just press the `return` key without typing a password - you'll never be asked for one again.

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).

ssh-keygen -t rsa -b 4096

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.

Saving Keys on Remote Computers

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:

ssh-copy-id <username>@<host>

Where `<username>` and `<host>` should be replaced by your username and the name of the computer you're transferring your key to. You can make sure this worked by doing:

ssh <username>@<host>

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 your encrypted home directory and won't be available until after you are authenticated. Therefore, SSH will default to password authentication. To solve, create a folder outside your home. For example, /etc/<username> and move the authorized_keys file to it. Then change the line in your /etc/ssh/sshd_config to:

AuthorizedKeysFile	/etc/<username>/authorized_keys

Then

sudo /etc/init.d/ssh restart

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

[email protected]'s password:

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

[email protected]'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 remote computer, ensure that the `/etc/ssh/sshd_config` contains the following lines, and that they are uncommented;

PubkeyAuthentication yes
RSAAuthentication yes

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/<user>` or `~/.ssh/authorized_keys` permissions are too open by OpenSSH standards. You can get rid of this problem by issuing the following commands:

chmod go-w ~/
chmod 700 ~/.ssh
chmod 600 ~/.ssh/authorized_keys

<<Anchor(keys-with-specific-commands)>>

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:

command="/usr/bin/cvs server",no-agent-forwarding,no-port-forwarding,no-X11-forwarding,no-user-rc ssh-dss <string of nonsense>...

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.