\C{pubkey} Using public keys for SSH authentication
-\H{pubkey-intro} Public key authentication - an introduction
+\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
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 \e{spoofed} (see \k{gs-hostkey}), an attacker can learn
+hacked, or \i\e{spoofed} (see \k{gs-hostkey}), an attacker can learn
your password.
-Public key authentication solves this problem. You generate a \e{key
-pair}, consisting of a public key (which everybody is allowed to
-know) and a private key (which you keep secret and do not give to
-anybody). The private key is able to generate \e{signatures}.
+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.
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 \e{encrypted} when it is
-stored on your local machine, using a passphrase of your choice. In
+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 \e{authentication
+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 Pageant. When you begin a Windows session, you start Pageant
+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
disk. Many people feel this is a good compromise between security
and convenience. See \k{pageant} for further details.
-There is more than one public-key algorithm available. The most
-common is RSA, but others exist, notably DSA (otherwise known as
+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 PuTTYgen, the PuTTY key generator
+\H{pubkey-puttygen} Using \i{PuTTYgen}, the PuTTY key generator
\cfg{winhelp-topic}{puttygen.general}
-PuTTYgen is a key generator. It 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.
+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
which type of key you need. PuTTYgen currently supports three types
of key:
-\b An RSA key for use with the SSH-1 protocol.
+\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 DSA 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
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. 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!
+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
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}
When the key generation is complete, a new set of controls will
appear in the window to indicate this.
-\S{puttygen-fingerprint} The \q{Key fingerprint} box
+\S{puttygen-fingerprint} The \q{\ii{Key fingerprint}} box
\cfg{winhelp-topic}{puttygen.fingerprint}
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. PuTTY allows you to enter a \e{comment} for your key,
+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.
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 passphrase for your key
+\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
-encrypt the key on disk, so you will not be able to use the key
+\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, PuTTY will check that the \q{Key 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.
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 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.
+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
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. DiceWare
+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
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{*.PPK}); it is the one you
+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}).
\cfg{winhelp-topic}{puttygen.savepub}
-The SSH-2 protocol drafts specify a standard format for storing
-public keys on disk. Some SSH servers (such as \cw{ssh.com}'s)
-require a public key in this format in order to accept
+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}.)
for pasting} box. This is the only existing standard for SSH-1
public keys.
-\S{puttygen-pastekey} \q{Public key for pasting into authorized_keys
-file}
+\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 OpenSSH server also requires this for SSH-2.
+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
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. OpenSSH and
-\cw{ssh.com} have different formats, and PuTTY's is different again.
+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 \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{*.PPK}) so that you can use
-it with the PuTTY suite. The passphrase will be unchanged by this
+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 export private keys in OpenSSH format and in
+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
accept your public key for authentication:
\b If your server is using the SSH-1 protocol, you should change
-into the \c{.ssh} directory and open the file \c{authorized_keys}
+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
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 OpenSSH and is using the SSH-2 protocol, you
+\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 \cw{ssh.com}'s product and is using SSH-2, you
+\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 \c{.ssh2} directory on
+\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
~mdw / sgt / putty / blobdiff