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<H2><A NAME="GPG-Minihowto-Concept"></A> <A NAME="s1">1.</A>Concepts</H2>
<H2><A NAME="ss1.1">1.1</A> Public Key Encryption
</H2>
<P>Classic methods for encryption only use one key for encryption. The sender
encrypts the message with this key. To be able to decrypt this the receiver
needs to have this very same key. This key must have been given to the
receiver in a way, that others won't have had the opportunity to obtain this
key. If somebody else does have the key, this method of encryption is useless.</P>
<P>The use of so-called Public Keys can solve this problem. Public Keys is a
concept where two keys are involved. One key is a Public Key that can be
spread through all sorts of media and may be obtained by anyone. The other
key is the Private Key. This key is secret and cannot be spread. This key is
only available to the owner. When the system is well implemented the secret
key cannot be derived from the public key. Now the sender will crypt the
message with the public key belonging to the receiver.
Then decryption will be done with the secret key of the
receiver.</P>
<P>Crucial in this concept is that the secret key remains a secret and should
not be given away or become available to anyone else but the owner of this
key. YOU CANNOT SEND THIS KEY OVER THE INTERNET. Also it is very unwise to
use GnuPG over <CODE>telnet</CODE> (you might consider never to use telnet based
on the high security risks).</P>
<H2><A NAME="ss1.2">1.2</A> Digital Signatures
</H2>
<P>In order to prove that a message was really sent by the alleged sender the
concept of Digital Signatures was invented. As the name says a message is
digitally signed by the sender. By using this signature you can check the
authenticity of a message. Using this will reduce the risk for Trojan horses
(a message that claims to be a patch to a certain problem but actually contains
a virus or does something bad with data on your computer). Also information or
data can be verified as coming from a legitimate source and thus be regarded
as real.</P>
<P>A digital signature is made through a combination of the secret key and the
text. Using the senders public key the message can be verified. Not only will
be checked if the correct sender is involved, also the content will be checked.
So you know that the message comes from the sender and has not been changed
during the transportation process.</P>
<H2><A NAME="ss1.3">1.3</A> Web of trust
</H2>
<P>A weak point of the Public key algorithms is the spreading of the public keys.
A user could bring a public key with false user ID in circulation. If with
this particular key messages are made, the intruder can decode and read the
messages. If the intruder passes it on then still with a genuine public key coded to the
actual recipient, this attack is not noticeable.</P>
<P>The PGP solution (and because of that automatically the GnuPG solution)
exists in signing codes. A public key can be signed by other people. This
signature acknowledges that the key used by the UID (User Identification)
actually belongs to the person it claims to be. It is then up to the user of
GnuPG how far the trust in the signature goes. You can consider a key as
trustworthy when you trust the sender of the key and you know for sure that
the key really belongs to that person. Only when you can trust the key of the
signer, you can trust the signature. To be absolutely positive that the key is
correct you have to compare the finger print over reliable channels before
giving absolute trust.</P>
<H2><A NAME="ss1.4">1.4</A> Boundaries to security
</H2>
<P>If you have data that you would like to remain confidential, there is more to
it than just determining which encoding algorithm to use. You should be thinking
about your system security in general. Basically we consider PGP to be secure
and as I write this documents no incidents of PGP being cracked are known to me.
But that doesn't mean that all encoding must be safe then (for instance the NSA
wouldn't notify me if they cracked PGP somehow, neither would other people who
crack for real malicious grounds). But even if the PGP is fully 'unhackable',
other means can be used to attack the security. Early February 1999 a Trojan Horse had
been found that searched for secret PGP keys on the hard disk and FTP-ed them
away. If the password has been chosen badly the secret key can easily be
cracked.</P>
<P>Another technical possibility (although more difficult) is a Trojan Horse that
broadcasts keyboard entries. Also possible (but very difficult) is to pass
the content of a screen along. Then no cracking of scrambled messages needs to
be done. For all these risks there need to be a good, well-thought security plan
that is actually deployed.</P>
<P>It is not a goal to create paranoia among people, but to point out that a lot
needs to be done to be more secure. The most important thing is to realize that
encryption is just one step to security and is not a total solution. Trojan
horses as they appeared in the Melissa virus in March 1999 proved that many
companies are not prepared for that.</P>
<hr>
<p>
Disclaimer: this chapter originates from the <a href="http://www.gnupg.org/documentation/howtos.html">GnuPG MiniHOWTO</a>
</p>
<p>
Copyright © 1999 Brenno J.S.A.A.F. de Winter (English version) Copyright © 1999
Michael Fischer v. Mollard (original German version) Copyright © 2002 Arjen Baart
(Dutch version) Copyright © 2004 Baris Cicek (Turkish version)
</p><p>
This document is free documentation you can redistribute it and/or modify it under
the terms of the GNU Library General Public License as published by the Free
Software Foundation; either version 2 of the License, or (at your option) any
later version.
</p><p>
This library is distributed in the hope that it will be useful, but WITHOUT ANY
WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A
PARTICULAR PURPOSE. See the GNU Library General Public License for more details.
</p><p>
You should have received a copy of the GNU Library General Public License along
with this library; if not, write to the Free Software Foundation, Inc., 59 Temple
Place - Suite 330, Boston, MA 02111-1307, USA.
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