Agenda: exercise-ssh-key.txt

SSH with private/public key authentication
==========================================

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

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

Remember: the `$` character before commands indicates that they are to be
run as your normal login user, not as root.

For laptops running Windows
===========================

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

* putty.exe (you should already have this)
* psftp.exe
* pageant.exe
* puttygen.exe

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

1. Generate an ssh public/private key pair
------------------------------------------

Double-click on `puttygen.exe`

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

* Make sure the type of key to generate is "SSH-2 RSA"
* Set the number of bits to 2048

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. Copy the PUBLIC key onto your Unix server
--------------------------------------------

You have two ways of doing this.

### 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.

### 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 pcN.ws.nsrc.org
    login as: sysadm
    sysadm@pcN.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

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.

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 pcN.ws.nsrc.org" to start a connection). No passphrase is needed
until you tell Pageant to forget the private key.


For laptops running Linux (or BSD or OSX)
=========================================

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@pcN.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.

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@pcN.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@pcN.ws.nsrc.org 'cat >>.ssh/authorized_keys'

3. Login using your private key
-------------------------------

Open an ssh connection to your server as normal:

    $ ssh sysadm@pcN.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.

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.

---------------------------------------------------------------------------

Additional information [not part of exercises]
==============================================

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.

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