Note about separate client-server and server-client encryption in SSH-2

[u/mdw/putty] / doc / pageant.but

\versionid $Id: pageant.but,v 1.7 2001/12/20 15:27:40 simon Exp $

\C{pageant} Using Pageant for authentication

\cfg{winhelp-topic}{pageant.general}

Pageant is an SSH authentication agent. It holds your private keys
in memory, already decoded, so that you can use them often without
needing to type a passphrase.

\H{pageant-start} Getting started with Pageant

Before you run Pageant, you need to have a private key. See
\k{pubkey} to find out how to generate and use one.

When you run Pageant, it will put an icon of a computer wearing a
hat into the System tray. It will then sit and do nothing, until you
load a private key into it.

If you click the Pageant icon with the right mouse button, you will
see a menu. Select \q{View Keys} from this menu. The Pageant main
window will appear. (You can also bring this window up by
double-clicking on the Pageant icon.)

The Pageant window contains a list box. This shows the private keys
Pageant is holding. When you start Pageant, it has no keys, so the
list box will be empty. After you add one or more keys, they will
show up in the list box.

To add a key to Pageant, press the \q{Add Key} button. Pageant will
bring up a file dialog, labelled \q{Select Private Key File}. Find
your private key file in this dialog, and press \q{Open}.

Pageant will now load the private key. If the key is protected by a
passphrase, Pageant will ask you to type the passphrase. When the
key has been loaded, it will appear in the list in the Pageant
window.

Now start PuTTY and open an SSH session to a site that accepts your
key. PuTTY will notice that Pageant is running, retrieve the key
automatically from Pageant, and use it to authenticate. You can now
open as many PuTTY sessions as you like without having to type your
passphrase again.

When you want to shut down Pageant, click the right button on the
Pageant icon in the System tray, and select \q{Exit} from the menu.
Closing the Pageant main window does \e{not} shut down Pageant.

\H{pageant-mainwin} The Pageant main window

The Pageant main window appears when you left-click on the Pageant
system tray icon, or alternatively right-click and select \q{View
Keys} from the menu. You can use it to keep track of what keys are
currently loaded into Pageant, and to add new ones or remove the
existing keys.

\S{pageant-mainwin-keylist} The key list box

\cfg{winhelp-topic}{pageant.keylist}

The large list box in the Pageant main window lists the private keys
that are currently loaded into Pageant. The list might look
something like this:

\c ssh1     1024  22:c3:68:3b:09:41:36:c3:39:83:91:ae:71:b2:0f:04  key1
\c ssh-rsa  1023  74:63:08:82:95:75:e1:7c:33:31:bb:cb:00:c0:89:8b  key2

For each key, the list box will tell you:

\b The type of the key. Currently, this can be \c{ssh1} (an RSA key
for use with the SSH v1 protocol), \c{ssh-rsa} (an RSA key for use
with the SSH v2 protocol), or \c{ssh-dss} (a DSA key for use with
the SSH v2 protocol).

\b The size (in bits) of the key.

\b The fingerprint for the public key. This should be the same
fingerprint given by PuTTYgen, and (hopefully) also the same
fingerprint shown by remote utilities such as \c{ssh-keygen} when
applied to your \c{authorized_keys} file.

\b The comment attached to the key.

\S{pageant-mainwin-addkey} The \q{Add Key} button

\cfg{winhelp-topic}{pageant.addkey}

To add a key to Pageant by reading it out of a local disk file,
press the \q{Add Key} button in the Pageant main window, or
alternatively right-click on the Pageant icon in the system tray and
select \q{Add Key} from there.

Pageant will bring up a file dialog, labelled \q{Select Private Key
File}. Find your private key file in this dialog, and press
\q{Open}. If you want to add more than one key at once, you can
select multiple files using Shift-click (to select several adjacent
files) or Ctrl-click (to select non-adjacent files).

Pageant will now load the private key(s). If a key is protected by a
passphrase, Pageant will ask you to type the passphrase.

(This is not the only way to add a private key to Pageant. You can
also add one from a remote system by using agent forwarding; see
\k{pageant-forward} for details.)

\S{pageant-mainwin-remkey} The \q{Remove Key} button

\cfg{winhelp-topic}{pageant.remkey}

If you need to remove a key from Pageant, select that key in the
list box, and press the \q{Remove Key} button. Pageant will remove
the key from its memory.

You can apply this to keys you added using the \q{Add Key} button,
or to keys you added remotely using agent forwarding (see
\k{pageant-forward}); it makes no difference.

\H{pageant-cmdline} The Pageant command line

Pageant can be made to do things automatically when it starts up, by
specifying instructions on its command line. If you're starting
Pageant from the Windows GUI, you can arrange this by editing the
properties of the Windows shortcut that it was started from.

\S{pageant-cmdline-loadkey} Making Pageant automatically load keys
on startup

Pageant can automatically load one or more private keys when it
starts up, if you provide them on the Pageant command line. Your
command line might then look like:

\c C:\PuTTY\pageant.exe d:\main.key d:\secondary.key

If the keys are stored encrypted, Pageant will request the
passphrases on startup.

\S{pageant-cmdline-command} Making Pageant run another program

You can arrange for Pageant to start another program once it has
initialised itself and loaded any keys specified on its command
line. This program (perhaps a PuTTY, or a WinCVS making use of
Plink, or whatever) will then be able to use the keys Pageant has
loaded.

You do this by specifying the \c{-c} option followed by the command,
like this:

\c C:\PuTTY\pageant.exe d:\main.key -c C:\PuTTY\putty.exe

\H{pageant-forward} Using agent forwarding

Agent forwarding is a mechanism that allows applications on your SSH
server machine to talk to the agent on your client machine.

Note that at present, agent forwarding in SSH2 is only available
when your SSH server is OpenSSH. The \cw{ssh.com} server uses a
different agent protocol which they have not published. If you would
like PuTTY to be able to support agent forwarding to an \cw{ssh.com}
server, please write to \cw{ssh.com} and explain to them that they
are hurting themselves and their users by keeping their protocol
secret.

To enable agent forwarding, first start Pageant. Then set up a PuTTY
SSH session in which \q{Allow agent forwarding} is enabled (see
\k{config-ssh-agentfwd}). Open the session as normal.

If this has worked, your applications on the server should now have
access to a Unix domain socket which the SSH server will forward
back to PuTTY, and PuTTY will forward on to the agent. To check that
this has actually happened, you can try this command on Unix server
machines:

\c unixbox:~$ echo $SSH_AUTH_SOCK
\c /tmp/ssh-XXNP18Jz/agent.28794
\c unixbox:~$

If the result line comes up blank, agent forwarding has not been
enabled at all.

Now if you run \c{ssh} on the server and use it to connect through
to another server that accepts one of the keys in Pageant, you
should be able to log in without a password:

\c unixbox:~$ ssh -v otherunixbox
\c [...]
\c debug: next auth method to try is publickey
\c debug: userauth_pubkey_agent: trying agent key my-putty-key
\c debug: ssh-userauth2 successful: method publickey
\c [...]

If you enable agent forwarding on \e{that} SSH connection as well
(see the manual for your server-side SSH client to find out how to
do this), your authentication keys will still be available on the
next machine you connect to - two SSH connections away from where
they're actually stored.

In addition, if you have a private key on one of the SSH servers,
you can send it all the way back to Pageant using the local
\c{ssh-add} command:

\c unixbox:~$ ssh-add ~/.ssh/id_rsa
\c Need passphrase for /home/fred/.ssh/id_rsa
\c Enter passphrase for /home/fred/.ssh/id_rsa:
\c Identity added: /home/fred/.ssh/id_rsa (/home/simon/.ssh/id_rsa)
\c unixbox:~$

and then it's available to every machine that has agent forwarding
available (not just the ones downstream of the place you added it).

\H{pageant-security} Security considerations

Using Pageant for public-key authentication gives you the
convenience of being able to open multiple SSH sessions without
having to type a passphrase every time, but also gives you the
security benefit of never storing a decrypted private key on disk.
Many people feel this is a good compromise between security and
convenience.

It \e{is} a compromise, however. Holding your decrypted private keys
in Pageant is better than storing them in easy-to-find disk files,
but still less secure than not storing them anywhere at all. This is
for two reasons:

\b Windows unfortunately provides no way to protect pieces of memory
from being written to the system swap file. So if Pageant is holding
your private keys for a long period of time, it's possible that
decrypted private key data may be written to the system swap file,
and an attacker who gained access to your hard disk later on might
be able to recover that data. (However, if you stored an unencrypted
key in a disk file they would \e{certainly} be able to recover it.)

\b Although, like most modern operating systems, Windows prevents
programs from accidentally accessing one another's memory space, it
does allow programs to access one another's memory space
deliberately, for special purposes such as debugging. This means
that if you allow a virus, trojan, or other malicious program on to
your Windows system while Pageant is running, it could access the
memory of the Pageant process, extract your decrypted authentication
keys, and send them back to its master.

Similarly, use of agent \e{forwarding} is a security improvement on
other methods of one-touch authentication, but not perfect. Holding
your keys in Pageant on your Windows box has a security advantage
over holding them on the remote server machine itself (either in an
agent or just unencrypted on disk), because if the server machine
ever sees your unencrypted private key then the sysadmin or anyone
who cracks the machine can steal the keys and pretend to be you for
as long as they want.

However, the sysadmin of the server machine can always pretend to be
you \e{on that machine}. So if you forward your agent to a server
machine, then the sysadmin of that machine can access the forwarded
agent connection and request signatures from your public keys, and
can therefore log in to other machines as you. They can only do this
to a limited extent - when the agent forwarding disappears they lose
the ability - but using Pageant doesn't actually \e{prevent} the
sysadmin (or hackers) on the server from doing this.

Therefore, if you don't trust the sysadmin of a server machine, you
should \e{never} use agent forwarding to that machine. (Of course
you also shouldn't store private keys on that machine, type
passphrases into it, or log into other machines from it in any way
at all; Pageant is hardly unique in this respect.)