-\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.
\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.
-
-\# 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.
+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 \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 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 \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 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
\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.
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.
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}.
-
-
projects / u / mdw / putty / blobdiff