\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 are \i{RSA} and \i{ECDSA}, but others exist, notably \i{DSA}
(otherwise known as \i{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

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, DSA, ECDSA, and EdDSA keys.

When you run PuTTYgen you will see a window where you have two main
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 OpenSSH 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

Before generating a key pair using PuTTYgen, you need to select
which type of key you need.

The current version of the SSH protocol, SSH-2, supports several
different key types, although specific servers may not support all of
them. PuTTYgen can generate:

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

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

\b An \i{ECDSA} (\i{elliptic curve} DSA) key for use with the
SSH-2 protocol.

\b An \i{EdDSA} key (Edwards-curve DSA, another elliptic curve
algorithm) for use with the SSH-2 protocol.

PuTTYgen can also generate an RSA key suitable for use with the old
SSH-1 protocol (which only supports RSA); for this, you need to select
the \q{SSH-1 (RSA)} option. Since the SSH-1 protocol is no longer
considered secure, it's rare to need this option.

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

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

\b For RSA and DSA, 2048 bits should currently be sufficient for most
purposes.

\b For ECDSA, only 256, 384, and 521 bits are supported. (ECDSA offers
equivalent security to RSA with smaller key sizes.)

\b For EdDSA, the only valid sizes are 255 bits (these keys are also
known as \q{\i{Ed25519}} and are commonly used) and 448 bits
(\q{\i{Ed448}}, which is much less common at the time of writing).
(256 is also accepted for backward compatibility, but the effect is
the same as 255.)

\S{puttygen-primes} Selecting the \i{prime generation method}

On the \q{Key} menu, you can also optionally change the method for
generating the prime numbers used in the generated key. This is used
for RSA and DSA keys only. (The other key types don't require
generating prime numbers at all.)

The prime-generation method does not affect compatibility: a key
generated with any of these methods will still work with all the same
SSH servers.

If you don't care about this, it's entirely sensible to leave it on the
default setting.

The available methods are:

\b Use \i{probable primes} (fast)

\b Use \i{proven primes} (slower)

\b Use proven primes with even distribution (slowest)

The \q{probable primes} method sounds unsafe, but it's the most
commonly used prime-generation strategy. There is in theory a
possibility that it might accidentally generate a number that isn't
prime, but the software does enough checking to make that probability
vanishingly small (less than 1 in 2^80, or 1 in 10^24). So, in
practice, nobody worries about it very much.

The other methods cause PuTTYgen to use numbers that it is \e{sure}
are prime, because it generates the output number together with a
proof of its primality. This takes more effort, but it eliminates that
theoretical risk in the probabilistic method.

There in one way in which PuTTYgen's \q{proven primes} method is not
strictly better than its \q{probable primes} method. If you use
PuTTYgen to generate an RSA key on a computer that is potentially
susceptible to timing- or cache-based \i{side-channel attacks}, such
as a shared computer, the \q{probable primes} method is designed to
resist such attacks, whereas the \q{proven primes} methods are not.
(This is only a concern for RSA keys; for other key types, primes
are either not secret or not involved.)

You might choose to switch from probable to proven primes if you have
a local security standard that demands it, or if you don't trust the
probabilistic argument for the safety of the usual method.

For RSA keys, there's also an option on the \q{Key} menu to use
\i{\q{strong} primes} as the prime factors of the public key. A \q{strong}
prime is a prime number chosen to have a particular structure that
makes certain factoring algorithms more difficult to apply, so some
security standards recommend their use. However, the most modern
factoring algorithms are unaffected, so this option is probably not
worth turning on \e{unless} you have a local standard that recommends
it.

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

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

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; it is supposed to
be more manageable for human beings than the public key itself.

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.

By default, PuTTYgen will display fingerprints in the \q{SHA256}
format. If you need to see the fingerprint in the older \q{MD5} format
(which looks like \c{aa:bb:cc:...}), you can choose
\q{Show fingerprint as MD5} from the \q{Key} menu, but bear in mind
that this is less cryptographically secure; it may be feasible for
an attacker to create a key with the same fingerprint as yours.

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

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

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

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}).

(You can optionally change some details of the PPK format for your saved
key files; see \k{puttygen-save-params}. But The defaults should be
fine for most purposes.)

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

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 OpenSSH
\i{authorized_keys file}}

The \i{OpenSSH} server, among others, requires your public key to be
given to it in a one-line format before it will accept authentication
with your private key. (SSH-1 servers also used this method.)

The \q{Public key for pasting into OpenSSH 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-save-params} Parameters for saving key files

Selecting \q{Parameters for saving key files...} from the \q{Key} menu
lets you adjust some aspects of PPK-format private key files stored on
disk. None of these options affect compatibility with SSH servers.

In most cases, it's entirely sensible to leave all of these at their
default settings.

\S2{puttygen-save-ppk-version} PPK file version

This defaults to version 3, which is fine for most uses.

You might need to select PPK version 2 if you need your private key
file to be loadable in older versions of PuTTY (0.74 and older), or in
other tools which do not yet support the version 3 format (which was
introduced in 2021).

The version 2 format is less resistant to brute-force decryption, and
doesn't support any of the following options to control that.

\S2{puttygen-save-passphrase-hashing} Options affecting \i{passphrase hashing}

All of the following options only affect keys saved with passphrases.
They control how much work is required to decrypt the key (which
happens every time you type its passphrase). This allows you to trade
off the cost of legitimate use of the key against the resistance of
the encrypted key to password-guessing attacks.

These options only affect PPK version 3.

\dt Key derivation function

\dd The variant of the \i{Argon2} key derivation function to use.
You might change this if you consider your exposure to \i{side-channel
attacks} to be different to the norm.

\dt Memory to use for passphrase hash

\dd The amount of memory needed to decrypt the key, in Kbyte.

\dt Time to use for passphrase hash

\dd Controls how much time is required to attempt decrypting the key.
You can either specify an approximate time in milliseconds (on this
machine), or explicitly specify a number of hash passes (which is what
the time is turned into during encryption).

\dt Parallelism for passphrase hash

\dd Number of parallelisable threads that can be used to decrypt the
key. The default, 1, forces the process to run single-threaded, even
on machines with multiple cores.

\S{puttygen-load} Reloading a private key

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

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 some OpenSSH key
formats contained 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.

For OpenSSH there are two options. Modern OpenSSH actually has two
formats it uses for storing private keys. \q{Export OpenSSH key}
will automatically choose the oldest format supported for the key
type, for maximum backward compatibility with older versions of
OpenSSH; for newer key types like Ed25519, it will use the newer
format as that is the only legal option. If you have some specific
reason for wanting to use OpenSSH's newer format even for RSA, DSA,
or ECDSA keys, you can choose \q{Export OpenSSH key (force new file
format)}.

Most clients for the older SSH-1 protocol 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.
Hence, the export options are not available if you have generated an
SSH-1 key.

\S{puttygen-cli} PuTTYgen command-line configuration

PuTTYgen supports a set of command-line options to configure many of
the same settings you can select in the GUI. This allows you to start
it up with your own preferences ready-selected, which might be useful
if you generate a lot of keys. (For example, you could make a Windows
shortcut that runs PuTTYgen with some command line options, or a batch
file or Powershell script that you could distribute to a whole
organisation containing your local standards.)

The options supported on the command line are:

\dt \cw{\-t} \e{keytype}

\dd Type of key to generate. You can select \c{rsa}, \c{dsa},
\c{ecdsa}, \c{eddsa} or \c{rsa1}. See \k{puttygen-keytype}.

\dt \cw{\-b} \e{bits}

\dd Size of the key to generate, in bits. See \k{puttygen-strength}.

\dt \cw{\-\-primes} \e{method}

\dd Method for generating prime numbers. You can select \c{probable},
\c{proven}, and \c{proven-even}. See \k{puttygen-primes}.

\dt \cw{\-\-strong-rsa}

\dd When generating an RSA key, make sure the prime factors of the key
modulus are \q{strong primes}. See \k{puttygen-primes}.

\dt \cw{\-\-ppk-param} \e{key}\cw{=}\e{value}\cw{,}...

\dd Allows setting all the same details of the PPK save file format
described in \k{puttygen-save-params}.

\lcont{

Aspects to change are specified as a series of \e{key}\cw{=}\e{value} pairs
separated by commas. The \e{key}s are:

\dt \cw{version}

\dd The PPK format version: either \cw{3} or \cw{2}.

\dt \cw{kdf}

\dd The variant of Argon2 to use: \cw{argon2id}, \cw{argon2i}, and
\cw{argon2d}.

\dt \cw{memory}

\dd The amount of memory needed to decrypt the key, in Kbyte.

\dt \cw{time}

\dd Specifies how much time is required to attempt decrypting the key,
in milliseconds.

\dt \cw{passes}

\dd Alternative to \cw{time}: specifies the number of hash passes
required to attempt decrypting the key.

\dt \cw{parallelism}

\dd Number of parallelisable threads that can be used to decrypt the
key.

}

\dt \cw{\-E} \e{fptype}

\dd Algorithm to use when displaying key fingerprints. You can
select \c{sha256} or \c{md5}. See \k{puttygen-fingerprint}.

\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 \i{OpenSSH}, you should change into the
\i\c{.ssh} directory under your home 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 OpenSSH 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.

\lcont{
(In very old versions of OpenSSH, SSH-2 keys had to be put in a
separate file called \c{authorized_keys2}. In all current 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; servers will typically ignore
the keys unless this is done. 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.
