\define{versionidpubkey} \versionid $Id: pubkey.but 6905 2006-11-15 12:56:48Z jacob $

\C{pubkey} Using public keys for SSH authentication

\H{pubkey-intro} \ii{Public key authentication} - an introduction

Public key authentication is an alternative means of identifying
yourself to a login server, instead of typing a password. It is more
secure and more flexible, but more difficult to set up.

In conventional password authentication, you prove you are who you
claim to be by proving that you know the correct password. The only
way to prove you know the password is to tell the server what you
think the password is. This means that if the server has been
hacked, or \i\e{spoofed} (see \k{gs-hostkey}), an attacker can learn
your password.

Public key authentication solves this problem. You generate a \i\e{key
pair}, consisting of a \i{public key} (which everybody is allowed to
know) and a \i{private key} (which you keep secret and do not give to
anybody). The private key is able to generate \i\e{signatures}.
A signature created using your private key cannot be forged by
anybody who does not have that key; but anybody who has your public
key can verify that a particular signature is genuine.

So you generate a key pair on your own computer, and you copy the
public key to the server. Then, when the server asks you to prove
who you are, PuTTY can generate a signature using your private key.
The server can verify that signature (since it has your public key)
and allow you to log in. Now if the server is hacked or spoofed, the
attacker does not gain your private key or password; they only gain
one signature. And signatures cannot be re-used, so they have gained
nothing.

There is a problem with this: if your private key is stored
unprotected on your own computer, then anybody who gains access to
\e{that} will be able to generate signatures as if they were you. So
they will be able to log in to your server under your account. For
this reason, your private key is usually \i\e{encrypted} when it is
stored on your local machine, using a \i{passphrase} of your choice. In
order to generate a signature, PuTTY must decrypt the key, so you
have to type your passphrase.

This can make public-key authentication less convenient than
password authentication: every time you log in to the server,
instead of typing a short password, you have to type a longer
passphrase. One solution to this is to use an \i\e{authentication
agent}, a separate program which holds decrypted private keys and
generates signatures on request. PuTTY's authentication agent is
called \i{Pageant}. When you begin a Windows session, you start Pageant
and load your private key into it (typing your passphrase once). For
the rest of your session, you can start PuTTY any number of times
and Pageant will automatically generate signatures without you
having to do anything. When you close your Windows session, Pageant
shuts down, without ever having stored your decrypted private key on
disk. Many people feel this is a good compromise between security
and convenience. See \k{pageant} for further details.

There is more than one \i{public-key algorithm} available. The most
common is \i{RSA}, but others exist, notably \i{DSA} (otherwise known as
DSS), the USA's federal Digital Signature Standard. The key types
supported by PuTTY are described in \k{puttygen-keytype}.

\H{pubkey-puttygen} Using \i{PuTTYgen}, the PuTTY key generator

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

PuTTYgen is a key generator. It \I{generating keys}generates pairs of
public and private keys to be used with PuTTY, PSCP, and Plink, as well
as the PuTTY authentication agent, Pageant (see \k{pageant}).  PuTTYgen
generates RSA and DSA keys.

When you run PuTTYgen you will see a window where you have two
choices: \q{Generate}, to generate a new public/private key pair, or
\q{Load} to load in an existing private key.

\S{puttygen-generating} Generating a new key

This is a general outline of the procedure for generating a new key
pair. The following sections describe the process in more detail.

\b First, you need to select which type of key you want to generate,
and also select the strength of the key. This is described in more
detail in \k{puttygen-keytype} and
\k{puttygen-strength}.

\b Then press the \q{Generate} button, to actually generate the key.
\K{puttygen-generate} describes this step.

\b Once you have generated the key, select a comment field
(\k{puttygen-comment}) and a passphrase (\k{puttygen-passphrase}).

\b Now you're ready to save the private key to disk; press the
\q{Save private key} button. (See \k{puttygen-savepriv}).

Your key pair is now ready for use. You may also want to copy the
public key to your server, either by copying it out of the \q{Public
key for pasting into authorized_keys file} box (see
\k{puttygen-pastekey}), or by using the \q{Save public key} button
(\k{puttygen-savepub}). However, you don't need to do this
immediately; if you want, you can load the private key back into
PuTTYgen later (see \k{puttygen-load}) and the public key will be
available for copying and pasting again.

\K{pubkey-gettingready} describes the typical process of configuring
PuTTY to attempt public-key authentication, and configuring your SSH
server to accept it.

\S{puttygen-keytype} Selecting the type of key

\cfg{winhelp-topic}{puttygen.keytype}

Before generating a key pair using PuTTYgen, you need to select
which type of key you need. PuTTYgen currently supports three types
of key:

\b An \i{RSA} key for use with the SSH-1 protocol.

\b An RSA key for use with the SSH-2 protocol.

\b A \i{DSA} key for use with the SSH-2 protocol.

The SSH-1 protocol only supports RSA keys; if you will be connecting
using the SSH-1 protocol, you must select the first key type or your
key will be completely useless.

The SSH-2 protocol supports more than one key type. The two types
supported by PuTTY are RSA and DSA.

The PuTTY developers \e{strongly} recommend you use RSA.
\I{security risk}\i{DSA} has an intrinsic weakness which makes it very
easy to create a signature which contains enough information to give
away the \e{private} key!
This would allow an attacker to pretend to be you for any number of
future sessions. PuTTY's implementation has taken very careful
precautions to avoid this weakness, but we cannot be 100% certain we
have managed it, and if you have the choice we strongly recommend
using RSA keys instead.

If you really need to connect to an SSH server which only supports
DSA, then you probably have no choice but to use DSA. If you do use
DSA, we recommend you do not use the same key to authenticate with
more than one server.

\S{puttygen-strength} Selecting the size (strength) of the key

\cfg{winhelp-topic}{puttygen.bits}

The \q{Number of bits} input box allows you to choose the strength
of the key PuTTYgen will generate.

Currently 1024 bits should be sufficient for most purposes.

Note that an RSA key is generated by finding two primes of half the
length requested, and then multiplying them together. For example,
if you ask PuTTYgen for a 1024-bit RSA key, it will create two
512-bit primes and multiply them. The result of this multiplication
might be 1024 bits long, or it might be only 1023; so you may not
get the exact length of key you asked for. This is perfectly normal,
and you do not need to worry. The lengths should only ever differ by
one, and there is no perceptible drop in security as a result.

DSA keys are not created by multiplying primes together, so they
should always be exactly the length you asked for.

\S{puttygen-generate} The \q{Generate} button

\cfg{winhelp-topic}{puttygen.generate}

Once you have chosen the type of key you want, and the strength of
the key, press the \q{Generate} button and PuTTYgen will begin the
process of actually generating the key.

First, a progress bar will appear and PuTTYgen will ask you to move
the mouse around to generate randomness. Wave the mouse in circles
over the blank area in the PuTTYgen window, and the progress bar
will gradually fill up as PuTTYgen collects enough randomness. You
don't need to wave the mouse in particularly imaginative patterns
(although it can't hurt); PuTTYgen will collect enough randomness
just from the fine detail of \e{exactly} how far the mouse has moved
each time Windows samples its position.

When the progress bar reaches the end, PuTTYgen will begin creating
the key. The progress bar will reset to the start, and gradually
move up again to track the progress of the key generation. It will
not move evenly, and may occasionally slow down to a stop; this is
unfortunately unavoidable, because key generation is a random
process and it is impossible to reliably predict how long it will
take.

When the key generation is complete, a new set of controls will
appear in the window to indicate this.

\S{puttygen-fingerprint} The \q{\ii{Key fingerprint}} box

\cfg{winhelp-topic}{puttygen.fingerprint}

The \q{Key fingerprint} box shows you a fingerprint value for the
generated key. This is derived cryptographically from the \e{public}
key value, so it doesn't need to be kept secret.

The fingerprint value is intended to be cryptographically secure, in
the sense that it is computationally infeasible for someone to
invent a second key with the same fingerprint, or to find a key with
a particular fingerprint. So some utilities, such as the Pageant key
list box (see \k{pageant-mainwin-keylist}) and the Unix \c{ssh-add}
utility, will list key fingerprints rather than the whole public key.

\S{puttygen-comment} Setting a comment for your key

\cfg{winhelp-topic}{puttygen.comment}

If you have more than one key and use them for different purposes,
you don't need to memorise the key fingerprints in order to tell
them apart. PuTTYgen allows you to enter a \e{comment} for your key,
which will be displayed whenever PuTTY or Pageant asks you for the
passphrase.

The default comment format, if you don't specify one, contains the
key type and the date of generation, such as \c{rsa-key-20011212}.
Another commonly used approach is to use your name and the name of
the computer the key will be used on, such as \c{simon@simons-pc}.

To alter the key comment, just type your comment text into the
\q{Key comment} box before saving the private key. If you want to
change the comment later, you can load the private key back into
PuTTYgen, change the comment, and save it again.

\S{puttygen-passphrase} Setting a \i{passphrase} for your key

\cfg{winhelp-topic}{puttygen.passphrase}

The \q{Key passphrase} and \q{Confirm passphrase} boxes allow you to
choose a passphrase for your key. The passphrase will be used to
\i{encrypt} the key on disk, so you will not be able to use the key
without first entering the passphrase.

When you save the key, PuTTYgen will check that the \q{Key passphrase}
and \q{Confirm passphrase} boxes both contain exactly the same
passphrase, and will refuse to save the key otherwise.

If you leave the passphrase fields blank, the key will be saved
unencrypted. You should \e{not} do this without good reason; if you
do, your private key file on disk will be all an attacker needs to
gain access to any machine configured to accept that key. If you
want to be able to \i{passwordless login}log in without having to
type a passphrase every time, you should consider using Pageant
(\k{pageant}) so that your decrypted key is only held in memory
rather than on disk.

Under special circumstances you may genuinely \e{need} to use a key
with no passphrase; for example, if you need to run an automated
batch script that needs to make an SSH connection, you can't be
there to type the passphrase. In this case we recommend you generate
a special key for each specific batch script (or whatever) that
needs one, and on the server side you should arrange that each key
is \e{restricted} so that it can only be used for that specific
purpose. The documentation for your SSH server should explain how to
do this (it will probably vary between servers).

Choosing a good passphrase is difficult. Just as you shouldn't use a
dictionary word as a password because it's easy for an attacker to
run through a whole dictionary, you should not use a song lyric,
quotation or other well-known sentence as a passphrase. \i{DiceWare}
(\W{http://www.diceware.com/}\cw{www.diceware.com}) recommends using
at least five words each generated randomly by rolling five dice,
which gives over 2^64 possible passphrases and is probably not a bad
scheme. If you want your passphrase to make grammatical sense, this
cuts down the possibilities a lot and you should use a longer one as
a result.

\e{Do not forget your passphrase}. There is no way to recover it.

\S{puttygen-savepriv} Saving your private key to a disk file

\cfg{winhelp-topic}{puttygen.savepriv}

Once you have generated a key, set a comment field and set a
passphrase, you are ready to save your private key to disk.

Press the \q{Save private key} button. PuTTYgen will put up a dialog
box asking you where to save the file. Select a directory, type in a
file name, and press \q{Save}.

This file is in PuTTY's native format (\c{*.\i{PPK}}); it is the one you
will need to tell PuTTY to use for authentication (see
\k{config-ssh-privkey}) or tell Pageant to load (see
\k{pageant-mainwin-addkey}).

\S{puttygen-savepub} Saving your public key to a disk file

\cfg{winhelp-topic}{puttygen.savepub}

RFC 4716 specifies a \I{SSH-2 public key format}standard format for
storing SSH-2 public keys on disk. Some SSH servers (such as
\i\cw{ssh.com}'s) require a public key in this format in order to accept
authentication with the corresponding private key. (Others, such as
OpenSSH, use a different format; see \k{puttygen-pastekey}.)

To save your public key in the SSH-2 standard format, press the
\q{Save public key} button in PuTTYgen. PuTTYgen will put up a
dialog box asking you where to save the file. Select a directory,
type in a file name, and press \q{Save}.

You will then probably want to copy the public key file to your SSH
server machine. See \k{pubkey-gettingready} for general instructions
on configuring public-key authentication once you have generated a
key.

If you use this option with an SSH-1 key, the file PuTTYgen saves
will contain exactly the same text that appears in the \q{Public key
for pasting} box. This is the only existing standard for SSH-1
public keys.

\S{puttygen-pastekey} \q{Public key for pasting into \i{authorized_keys
file}}

\cfg{winhelp-topic}{puttygen.pastekey}

All SSH-1 servers require your public key to be given to it in a
one-line format before it will accept authentication with your
private key. The \i{OpenSSH} server also requires this for SSH-2.

The \q{Public key for pasting into authorized_keys file} gives the
public-key data in the correct one-line format. Typically you will
want to select the entire contents of the box using the mouse, press
Ctrl+C to copy it to the clipboard, and then paste the data into a
PuTTY session which is already connected to the server.

See \k{pubkey-gettingready} for general instructions on configuring
public-key authentication once you have generated a key.

\S{puttygen-load} Reloading a private key

\cfg{winhelp-topic}{puttygen.load}

PuTTYgen allows you to load an existing private key file into
memory. If you do this, you can then change the passphrase and
comment before saving it again; you can also make extra copies of
the public key.

To load an existing key, press the \q{Load} button. PuTTYgen will
put up a dialog box where you can browse around the file system and
find your key file. Once you select the file, PuTTYgen will ask you
for a passphrase (if necessary) and will then display the key
details in the same way as if it had just generated the key.

If you use the Load command to load a foreign key format, it will
work, but you will see a message box warning you that the key you
have loaded is not a PuTTY native key. See \k{puttygen-conversions}
for information about importing foreign key formats.

\S{puttygen-conversions} Dealing with private keys in other formats

\cfg{winhelp-topic}{puttygen.conversions}

Most SSH-1 clients use a standard format for storing private keys on
disk. PuTTY uses this format as well; so if you have generated an
SSH-1 private key using OpenSSH or \cw{ssh.com}'s client, you can use
it with PuTTY, and vice versa.

However, SSH-2 private keys have no standard format. \I{OpenSSH private
key format}OpenSSH and \I{ssh.com private key format}\cw{ssh.com} have
different formats, and PuTTY's is different again.
So a key generated with one client cannot immediately be used with
another.

Using the \I{importing keys}\q{Import} command from the \q{Conversions}
menu, PuTTYgen can load SSH-2 private keys in OpenSSH's format and
\cw{ssh.com}'s format. Once you have loaded one of these key types, you
can then save it back out as a PuTTY-format key (\c{*.\i{PPK}}) so that
you can use it with the PuTTY suite. The passphrase will be unchanged by this
process (unless you deliberately change it). You may want to change
the key comment before you save the key, since OpenSSH's SSH-2 key
format contains no space for a comment and \cw{ssh.com}'s default
comment format is long and verbose.

PuTTYgen can also \i{export private keys} in OpenSSH format and in
\cw{ssh.com} format. To do so, select one of the \q{Export} options
from the \q{Conversions} menu. Exporting a key works exactly like
saving it (see \k{puttygen-savepriv}) - you need to have typed your
passphrase in beforehand, and you will be warned if you are about to
save a key without a passphrase.

Note that since only SSH-2 keys come in different formats, the export
options are not available if you have generated an SSH-1 key.

\H{pubkey-gettingready} Getting ready for public key authentication

Connect to your SSH server using PuTTY with the SSH protocol. When the
connection succeeds you will be prompted for your user name and
password to login. Once logged in, you must configure the server to
accept your public key for authentication:

\b If your server is using the SSH-1 protocol, you should change
into the \i\c{.ssh} directory and open the file \i\c{authorized_keys}
with your favourite editor. (You may have to create this file if
this is the first key you have put in it). Then switch to the
PuTTYgen window, select all of the text in the \q{Public key for
pasting into authorized_keys file} box (see \k{puttygen-pastekey}),
and copy it to the clipboard (\c{Ctrl+C}). Then, switch back to the
PuTTY window and insert the data into the open file, making sure it
ends up all on one line. Save the file.

\b If your server is \i{OpenSSH} and is using the SSH-2 protocol, you
should follow the same instructions, except that in earlier versions
of OpenSSH 2 the file might be called \c{authorized_keys2}. (In
modern versions the same \c{authorized_keys} file is used for both
SSH-1 and SSH-2 keys.)

\b If your server is \i\cw{ssh.com}'s product and is using SSH-2, you
need to save a \e{public} key file from PuTTYgen (see
\k{puttygen-savepub}), and copy that into the \i\c{.ssh2} directory on
the server. Then you should go into that \c{.ssh2} directory, and edit
(or create) a file called \c{authorization}. In this file you should
put a line like \c{Key mykey.pub}, with \c{mykey.pub} replaced by the
name of your key file.

\b For other SSH server software, you should refer to the manual for
that server.

You may also need to ensure that your home directory, your \c{.ssh}
directory, and any other files involved (such as
\c{authorized_keys}, \c{authorized_keys2} or \c{authorization}) are
not group-writable or world-writable. You can typically do this by
using a command such as

\c chmod go-w $HOME $HOME/.ssh $HOME/.ssh/authorized_keys

Your server should now be configured to accept authentication using
your private key. Now you need to configure PuTTY to \e{attempt}
authentication using your private key. You can do this in any of
three ways:

\b Select the private key in PuTTY's configuration. See
\k{config-ssh-privkey} for details.

\b Specify the key file on the command line with the \c{-i} option.
See \k{using-cmdline-identity} for details.

\b Load the private key into Pageant (see \k{pageant}). In this case
PuTTY will automatically try to use it for authentication if it can.
