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Add some extra documentation: filled in the Getting Started chapter,

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added an introduction to public key authentication, and made a
couple of changes in intro.but. Transatlantic flights have some uses
after all.

[originally from svn r1146]
This commit is contained in:
Simon Tatham
2001-06-15 19:31:10 +00:00
parent 56aa28467b
commit 35bdd95a56
3 changed files with 192 additions and 18 deletions
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66
doc/pubkey.but
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@ -1,4 +1,4 @@
\versionid $Id: pubkey.but,v 1.2 2001/02/06 09:34:42 owen Exp $
\versionid $Id: pubkey.but,v 1.3 2001/06/15 19:31:10 simon Exp $
\# FIXME: passphrases, examples (e.g what does a key for pasting into
\# authorized_keys look like?), index entries, links.
@ -7,13 +7,57 @@
\H{pubkey-intro} Public key authentication - an introduction
\# Explain the basic principles of public key authentication. Many
\# people don't have the faintest idea what it is or why it's good.
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.
\# Explain the dangers of leaving an unprotected private key around.
\# Explain passphrases, and urge that people NEVER store
\# unpassphrased keys unless they really need to or they can be sure
\# the machine is secure.
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 \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}.
A<EFBFBD>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<54>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 \e{encrypted} when it is
stored on your local machine, using a 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
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
and load your public key into it (typing your passphrase once). For
the rest of your session, you can start PuTTY<54>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.
\H{pubkey-puttygen} PuTTYgen: RSA key generator for PuTTY
@ -28,7 +72,7 @@ existing private key.
\S{pubkey-puttygen-generate} Generate a new key
Before generating a new key you have to chose the strength of the
Before generating a new key you have to choose the strength of the
encryption. With \e{Parameters} you define the strength of the key. The
default of 1024 should be OK for most users.
@ -37,8 +81,8 @@ new key pair. You then have to move the mouse over the blank area in
order to generate random data for the algorithm. Continue until the
progress bar is complete.
As soon as enough random data is available the key is generated. This
may take a little while, especially on slow machines. Once the key is
As soon as enough random data is available the key is generated. This
may take a little while, especially on slow machines. Once the key is
generated, its details appear in the \e{Key} part of the PuTTYgen
window.
@ -85,5 +129,3 @@ From now on you can use the private key for authentication to this
host. Either select the private key in PuTTY's \e{Connection},
\e{SSH} panel: \e{Private key file for authentication} dialog or use
it with Pageant as described in \k{pageant}.