checking in all the old panacean stuff
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puttysrc/DOC/PAGEANT.BUT
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puttysrc/DOC/PAGEANT.BUT
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\define{versionidpageant} \versionid $Id: pageant.but 6610 2006-03-14 11:21:59Z jacob $
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\C{pageant} Using \i{Pageant} for authentication
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\cfg{winhelp-topic}{pageant.general}
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Pageant is an SSH \i{authentication agent}. It holds your \i{private key}s
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in memory, already decoded, so that you can use them often
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\I{passwordless login}without needing to type a \i{passphrase}.
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\H{pageant-start} Getting started with Pageant
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Before you run Pageant, you need to have a private key in \c{*.\i{PPK}}
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format. See \k{pubkey} to find out how to generate and use one.
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When you run Pageant, it will put an icon of a computer wearing a
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hat into the \ii{System tray}. It will then sit and do nothing, until you
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load a private key into it.
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If you click the Pageant icon with the right mouse button, you will
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see a menu. Select \q{View Keys} from this menu. The Pageant main
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window will appear. (You can also bring this window up by
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double-clicking on the Pageant icon.)
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The Pageant window contains a list box. This shows the private keys
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Pageant is holding. When you start Pageant, it has no keys, so the
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list box will be empty. After you add one or more keys, they will
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show up in the list box.
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To add a key to Pageant, press the \q{Add Key} button. Pageant will
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bring up a file dialog, labelled \q{Select Private Key File}. Find
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your private key file in this dialog, and press \q{Open}.
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Pageant will now load the private key. If the key is protected by a
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passphrase, Pageant will ask you to type the passphrase. When the
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key has been loaded, it will appear in the list in the Pageant
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window.
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Now start PuTTY and open an SSH session to a site that accepts your
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key. PuTTY will notice that Pageant is running, retrieve the key
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automatically from Pageant, and use it to authenticate. You can now
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open as many PuTTY sessions as you like without having to type your
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passphrase again.
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(PuTTY can be configured not to try to use Pageant, but it will try
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by default. See \k{config-ssh-tryagent} and
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\k{using-cmdline-agentauth} for more information.)
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When you want to shut down Pageant, click the right button on the
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Pageant icon in the System tray, and select \q{Exit} from the menu.
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Closing the Pageant main window does \e{not} shut down Pageant.
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\H{pageant-mainwin} The Pageant main window
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The Pageant main window appears when you left-click on the Pageant
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system tray icon, or alternatively right-click and select \q{View
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Keys} from the menu. You can use it to keep track of what keys are
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currently loaded into Pageant, and to add new ones or remove the
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existing keys.
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\S{pageant-mainwin-keylist} The key list box
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\cfg{winhelp-topic}{pageant.keylist}
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The large list box in the Pageant main window lists the private keys
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that are currently loaded into Pageant. The list might look
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something like this:
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\c ssh1 1024 22:c3:68:3b:09:41:36:c3:39:83:91:ae:71:b2:0f:04 k1
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\c ssh-rsa 1023 74:63:08:82:95:75:e1:7c:33:31:bb:cb:00:c0:89:8b k2
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For each key, the list box will tell you:
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\b The type of the key. Currently, this can be \c{ssh1} (an RSA key
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for use with the SSH-1 protocol), \c{ssh-rsa} (an RSA key for use
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with the SSH-2 protocol), or \c{ssh-dss} (a DSA key for use with
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the SSH-2 protocol).
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\b The size (in bits) of the key.
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\b The \I{key fingerprint}fingerprint for the public key. This should be
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the same fingerprint given by PuTTYgen, and (hopefully) also the same
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fingerprint shown by remote utilities such as \i\c{ssh-keygen} when
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applied to your \c{authorized_keys} file.
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\b The comment attached to the key.
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\S{pageant-mainwin-addkey} The \q{Add Key} button
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\cfg{winhelp-topic}{pageant.addkey}
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To add a key to Pageant by reading it out of a local disk file,
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press the \q{Add Key} button in the Pageant main window, or
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alternatively right-click on the Pageant icon in the system tray and
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select \q{Add Key} from there.
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Pageant will bring up a file dialog, labelled \q{Select Private Key
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File}. Find your private key file in this dialog, and press
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\q{Open}. If you want to add more than one key at once, you can
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select multiple files using Shift-click (to select several adjacent
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files) or Ctrl-click (to select non-adjacent files).
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Pageant will now load the private key(s). If a key is protected by a
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passphrase, Pageant will ask you to type the passphrase.
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(This is not the only way to add a private key to Pageant. You can
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also add one from a remote system by using agent forwarding; see
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\k{pageant-forward} for details.)
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\S{pageant-mainwin-remkey} The \q{Remove Key} button
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\cfg{winhelp-topic}{pageant.remkey}
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If you need to remove a key from Pageant, select that key in the
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list box, and press the \q{Remove Key} button. Pageant will remove
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the key from its memory.
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You can apply this to keys you added using the \q{Add Key} button,
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or to keys you added remotely using agent forwarding (see
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\k{pageant-forward}); it makes no difference.
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\H{pageant-cmdline} The Pageant command line
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Pageant can be made to do things automatically when it starts up, by
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\I{command-line arguments}specifying instructions on its command line.
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If you're starting Pageant from the Windows GUI, you can arrange this
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by editing the properties of the \i{Windows shortcut} that it was
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started from.
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If Pageant is already running, invoking it again with the options
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below causes actions to be performed with the existing instance, not a
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new one.
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\S{pageant-cmdline-loadkey} Making Pageant automatically load keys
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on startup
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Pageant can automatically load one or more private keys when it
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starts up, if you provide them on the Pageant command line. Your
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command line might then look like:
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\c C:\PuTTY\pageant.exe d:\main.ppk d:\secondary.ppk
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If the keys are stored encrypted, Pageant will request the
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passphrases on startup.
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If Pageant is already running, this syntax loads keys into the
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existing Pageant.
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\S{pageant-cmdline-command} Making Pageant run another program
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You can arrange for Pageant to start another program once it has
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initialised itself and loaded any keys specified on its command
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line. This program (perhaps a PuTTY, or a WinCVS making use of
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Plink, or whatever) will then be able to use the keys Pageant has
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loaded.
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You do this by specifying the \I{-c-pageant}\c{-c} option followed
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by the command, like this:
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\c C:\PuTTY\pageant.exe d:\main.ppk -c C:\PuTTY\putty.exe
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\H{pageant-forward} Using \i{agent forwarding}
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Agent forwarding is a mechanism that allows applications on your SSH
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server machine to talk to the agent on your client machine.
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Note that at present, agent forwarding in SSH-2 is only available
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when your SSH server is \i{OpenSSH}. The \i\cw{ssh.com} server uses a
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different agent protocol, which PuTTY does not yet support.
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To enable agent forwarding, first start Pageant. Then set up a PuTTY
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SSH session in which \q{Allow agent forwarding} is enabled (see
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\k{config-ssh-agentfwd}). Open the session as normal. (Alternatively,
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you can use the \c{-A} command line option; see
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\k{using-cmdline-agent} for details.)
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If this has worked, your applications on the server should now have
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access to a Unix domain socket which the SSH server will forward
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back to PuTTY, and PuTTY will forward on to the agent. To check that
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this has actually happened, you can try this command on Unix server
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machines:
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\c unixbox:~$ echo $SSH_AUTH_SOCK
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\c /tmp/ssh-XXNP18Jz/agent.28794
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\c unixbox:~$
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If the result line comes up blank, agent forwarding has not been
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enabled at all.
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Now if you run \c{ssh} on the server and use it to connect through
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to another server that accepts one of the keys in Pageant, you
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should be able to log in without a password:
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\c unixbox:~$ ssh -v otherunixbox
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\c [...]
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\c debug: next auth method to try is publickey
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\c debug: userauth_pubkey_agent: trying agent key my-putty-key
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\c debug: ssh-userauth2 successful: method publickey
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\c [...]
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If you enable agent forwarding on \e{that} SSH connection as well
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(see the manual for your server-side SSH client to find out how to
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do this), your authentication keys will still be available on the
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next machine you connect to - two SSH connections away from where
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they're actually stored.
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In addition, if you have a private key on one of the SSH servers,
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you can send it all the way back to Pageant using the local
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\i\c{ssh-add} command:
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\c unixbox:~$ ssh-add ~/.ssh/id_rsa
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\c Need passphrase for /home/fred/.ssh/id_rsa
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\c Enter passphrase for /home/fred/.ssh/id_rsa:
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\c Identity added: /home/fred/.ssh/id_rsa (/home/simon/.ssh/id_rsa)
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\c unixbox:~$
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and then it's available to every machine that has agent forwarding
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available (not just the ones downstream of the place you added it).
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\H{pageant-security} Security considerations
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\I{security risk}Using Pageant for public-key authentication gives you the
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convenience of being able to open multiple SSH sessions without
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having to type a passphrase every time, but also gives you the
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security benefit of never storing a decrypted private key on disk.
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Many people feel this is a good compromise between security and
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convenience.
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It \e{is} a compromise, however. Holding your decrypted private keys
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in Pageant is better than storing them in easy-to-find disk files,
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but still less secure than not storing them anywhere at all. This is
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for two reasons:
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\b Windows unfortunately provides no way to protect pieces of memory
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from being written to the system \i{swap file}. So if Pageant is holding
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your private keys for a long period of time, it's possible that
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decrypted private key data may be written to the system swap file,
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and an attacker who gained access to your hard disk later on might
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be able to recover that data. (However, if you stored an unencrypted
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key in a disk file they would \e{certainly} be able to recover it.)
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\b Although, like most modern operating systems, Windows prevents
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programs from accidentally accessing one another's memory space, it
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does allow programs to access one another's memory space
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deliberately, for special purposes such as debugging. This means
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that if you allow a virus, trojan, or other malicious program on to
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your Windows system while Pageant is running, it could access the
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memory of the Pageant process, extract your decrypted authentication
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keys, and send them back to its master.
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Similarly, use of agent \e{forwarding} is a security improvement on
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other methods of one-touch authentication, but not perfect. Holding
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your keys in Pageant on your Windows box has a security advantage
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over holding them on the remote server machine itself (either in an
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agent or just unencrypted on disk), because if the server machine
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ever sees your unencrypted private key then the sysadmin or anyone
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who cracks the machine can steal the keys and pretend to be you for
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as long as they want.
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However, the sysadmin of the server machine can always pretend to be
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you \e{on that machine}. So if you forward your agent to a server
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machine, then the sysadmin of that machine can access the forwarded
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agent connection and request signatures from your private keys, and
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can therefore log in to other machines as you. They can only do this
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to a limited extent - when the agent forwarding disappears they lose
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the ability - but using Pageant doesn't actually \e{prevent} the
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sysadmin (or hackers) on the server from doing this.
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Therefore, if you don't trust the sysadmin of a server machine, you
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should \e{never} use agent forwarding to that machine. (Of course
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you also shouldn't store private keys on that machine, type
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passphrases into it, or log into other machines from it in any way
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at all; Pageant is hardly unique in this respect.)
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