1 Introduction

1.1 Goals

In this exercise we'll show how you can eliminate passwords by using ssh key authentication.

1.2 Notes

Choose the version of the exercises depending on what OS you are running on your laptop.

2 For laptops running Windows

Download the following onto your desktop or into a downloads folder:

from http://www.chiark.greenend.org.uk/~sgtatham/putty/download.html (Or you can try the installer bundle which gets them all)

2.1 1. Generate an ssh public/private key pair

Double-click on puttygen.exe

At the bottom of the dialog box, under "Parameters":

Click on "Generate". Move the mouse randomly over the blank area until the progress bar reaches 100%

    Key comment:        [Your Name <your@email.address>   ]
    Key passphrase:     [chooose a passphrase             ]
    Confirm passphrase: [choose same passphrase           ]

The passphrase is used to keep your private key encrypted on disk. It can be pretty much anything you want and as long as you want - including spaces - but if you forget it, your key becomes worthless. For now pick something that you will easily remember. You can change it at any time you want in the future.

Click "Save public key". Give a filename of "id_rsa.pub" (please save files into the same directory as where the executables are)

Click "Save private key". Give a filename of "id_rsa.ppk"

Use the mouse to highlight all the text in the box "Public key for pasting into OpenSSH authorized_keys file", and copy it to the clipboard.

Exit puttygen.

NOTE: Key generation is a one-off exercise. The more you deploy your public key, the more work it to be if you were to lose it and have to start again with a new one. I suggest you keep a secure backup of it somewhere, e.g. on a CD-ROM that you lock away.

2.2 2. Copy the PUBLIC key onto your Unix server

You have two ways of doing this.

2.2.1 Copy-paste

Use putty.exe to make a normal ssh connection to your host as the 'sysadm' user.

$ cat >>.ssh/authorized_keys
    *** PASTE KEY FROM CLIPBOARD ***
    *** If the cursor is still at the end of the line, hit Enter ***
    *** hit ctrl-D ***

The key consists of one very long line, which looks like

ssh-rsa <lots of base64 data> <comment>

As a quick check that it hasn't been corrupted, count the lines in the file:

$ wc -l .ssh/authorized_keys
1 .ssh/authorized_keys

If you don't see "1", then you'll need to fix it (possibly with an editor, or else just rm the file and start again)

Now logout.

2.2.2 Alternative way (if you're having problems with copy-paste)

Double-click on psftp.exe. Open a connection to your server, and upload your public key:

psftp> open dmzN.ws.nsrc.org
login as: sysadm
sysadm@dmzN.ws.nsrc.org's password: <usual one>
Remote working directory is /home/sysadm
psftp> put id_rsa.pub
local:id_rsa.pub => remote:/home/sysadm/id_rsa.pub
psftp> quit

Unfortunately, this public key is not in the format which openssh requires, so now login again using putty.exe, and use the following command to convert it and put it in the right place.

$ ssh-keygen -i -f id_rsa.pub >>.ssh/authorized_keys

2.3 3. Login using your private key

Start putty.exe again. Enter the hostname as usual, but before clicking Open, browse in the left hand column to Connection > SSH > Auth

    [-] Connection
         |
        [-] SSH
         |  |- Keyex
         |  |- Auth    <--- CLICK HERE

Next to "Private key for authentication", click Browse. Find your id_rsa.ppk file, open it, then click Open to start the connection.

You should be prompted for your username as before (sysadm), but then instead of being prompted for a password, you are asked for the passphrase for your private key. Enter it, and you should be logged in.

This is quite painful (both locating the private key and entering the passphrase), so as the final step of the exercise we're going automate it using an agent.

2.4 4. Use a passphrase agent

Run pageant.exe

It runs in the background, and adds an icon to your task tray (a PC with a black hat at a jaunty angle). You may need to select "Show hidden icons" to see it.

Right-click on the icon, and select "Add Key". Browse to your id_rsa.ppk and open it. You will be prompted for the passphrase - enter it. (If you make a mistake, you'll be prompted again until you get it right)

Now run putty.exe again, enter your hostname, click Open, and enter your username (sysadm). You should be logged in immediately, with no prompt for either a password or a passphrase!

Try logging in again. Also try using psftp.exe (when it runs, enter "open dmzN.ws.nsrc.org" to start a connection). No passphrase is needed until you tell Pageant to forget the private key.

3 For laptops running Linux (or BSD or OSX)

3.1 1. Generate an ssh public/private key pair

$ ssh-keygen -t rsa -b 2048
Generating public/private rsa key pair.
Enter file in which to save the key (/home/sysadm/.ssh/id_rsa): <HIT ENTER>
Created directory '/home/sysadm/.ssh'.
Enter passphrase (empty for no passphrase): <CHOOSE PASSPHRASE>
Enter same passphrase again: <SAME PASSPHRASE>
Your identification has been saved in /home/sysadm/.ssh/id_rsa.
Your public key has been saved in /home/sysadm/.ssh/id_rsa.pub.
The key fingerprint is:
32:2b:e3:0e:14:fb:60:38:a6:e2:73:95:53:9d:a8:0f sysadm@dmzN.ws.nsrc.org

The passphrase is used to keep your private key encrypted on disk. It can be pretty much anything you want and as long as you want - including spaces - but if you forget it, your key becomes worthless. For now pick something that you will easily remember. You can change it at any time you want in the future (using ssh-keygen -p)

NOTE: Key generation is a one-off exercise. The more you deploy your public key, the more work it to be if you were to lose it and have to start again with a new one. I suggest you keep a secure backup of it somewhere, e.g. on a CD-ROM that you lock away.

3.2 2. Copy the PUBLIC key onto your Unix server

The simplest way to copy the public key is with scp:

$ scp .ssh/id_rsa.pub sysadm@dmzN.ws.nsrc.org:.ssh/authorized_keys

Note that .ssh/authorized_keys can contain multiple keys, one per line, so on a shared system you might want to append your key instead:

$ cat .ssh/id_rsa.pub | ssh sysadm@dmzN.ws.nsrc.org 'cat >>.ssh/authorized_keys'

3.3 3. Login using your private key

Open an ssh connection to your server as normal:

$ ssh sysadm@dmzN.ws.nsrc.org

This time, instead of being prompted for your password, you should be prompted for the passphrase on your private key. Enter it. You should be logged in.

3.4 4. Use a passphrase agent

Entering a passphrase every time you connect would be painful, but this isn't necessary if you have an agent which decrypts the private key and keeps it in memory.

If you are running under a modern graphical environment like Gnome, you probably already got a dialog box prompting you for a passphrase, and this means you're already running an agent. You should be able to logout and login to the remote server, without being prompted for your passphrase again.

To see what identities (decrypted private keys) your agent has in memory:

$ ssh-add -l

To forget all identities:

$ ssh-add -d

If you don't have an agent, then you can start a new subshell with ssh-agent as its parent:

$ ssh-agent bash
$ ssh-add
 ... prompted for your passphrase
$ 

Now the agent will handle future connections for you.

If you are running an older graphical environment, and you normally start X using startx, then start it using ssh-agent startx instead. Then type 'ssh-add' in an xterm.


4 Additional information [not part of exercises]

4.1 Agent forwarding

Using an agent, you can access across multiple ssh hops without having to copy your key or enter your passphrase anywhere.

If you enable "agent forwarding" when you login to host X, you can then login from X to Y without any prompting (assuming Y has your public key in authorized_keys). The request to authenticate is forwarded securely back along your original ssh session to the agent running on your workstation.

Under Unix:

$ ssh -o ForwardAgent=yes user@host

If you do this frequently, it's easier to configure it in .ssh/hosts

host foo
hostname foo.example.com
user sysadm
ForwardAgent yes

Then you only have to type ssh foo to get a connection with those options.

4.2 Advanced: X11 forwarding and port forwarding

ssh can securely carry arbitrary forwarded TCP connections and X11 graphics.

$ ssh -L8080:some.where:80 user@remote.host
... while ssh connection is open, a connection to 127.0.0.1 port 8080
... will be tunneled, and the far end will open a connection to
... some.where port 80

$ ssh -X user@remote.host
...
$ xclock   # graphical output redirected back through ssh tunnel