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<H1><font color="maroon">Secure Communication with GnuPG on Linux</font></H1>
<H4>By <a href="mailto:kapil@linux4biz.net">Kapil Sharma</a></H4>
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<p><b><font size=+2>Overview</font></b>
<p>GnuPG is a tool for secure communication and data storage. It can be
used to encrypt data and to create digital signatures. GnuPG is a complete
and free replacement for PGP. Because it does not use the patented IDEA
algorithm, it can be used without any restrictions. GnuPG uses public-key
cryptography so that users may communicate securely. In a public-key system,
each user has a pair of keys consisting of a private key and a public key.
A user's private key is kept secret; it need never be revealed. The public
key may be given to anyone with whom the user wants to communicate.
<p><b><font size=+2>Features</font></b>
<ul>
<li>
Full replacement of PGP.</li>
<li>
Does not use any patented algorithms.</li>
<li>
GPLed, written from scratch.</li>
<li>
Can be used as a filter program.</li>
<li>
Full OpenPGP implementation.</li>
<li>
Better functionality than PGP and some security
enhancements over PGP 2.</li>
<li>
Decrypts and verifies PGP 5.x messages.</li>
<li>
Supports ElGamal (signature and encryption), DSA,
3DES, Blowfish, Twofish, CAST5, MD5, SHA-1, RIPE-MD-160 and TIGER.</li>
<li>
Easy implementation of new algorithms using extension
modules.</li>
<li>
User ID is forced to be in a standard format.</li>
<li>
Supports key and signature expiration dates.</li>
<li>
English, Danish, Dutch, Esperanto, French, German,
Japanese, Italian, Polish, Portuguese (Brazilian), Portuguese (Portuguese),
Russian, Spanish and Swedish language support.</li>
<li>
Online help system.</li>
<li>
Optional anonymous message receivers.</li>
<li>
Integrated support for HKP keyservers (wwwkeys.pgp.net).</li>
<li>
Has lots of GUI frontend</li>
</ul>
<p><br>You can find all the software related to GnuPG at <a href="http://www.gnupg.org/download.html">http://www.gnupg.org/download.html</a>
<p><b><font size=+2>Installation</font></b>
<p>Copy the gnupg source file to ./usr/local/ directory or wherever you
want to install it and then cd to that directory.
<br>[root@dragon local] tar xvzf gnupg-1.0.4.tar.gz
<br>[root@dragon local]# cd gnupg-1.0.4
<br>[root@dragon gnupg-1.0.4]# ./configure
<br>[root@dragon gnupg-1.0.4]# make
<br>This will compile all source files into executable binaries.
<br>[root@dragon gnupg-1.0.4]# make check
<br>It will run any self-tests that come with the package.
<br>[root@dragon gnupg-1.0.4]# make install
<br>It will install the binaries and any supporting files into appropriate
locations.
<br>[root@dragon gnupg-1.0.4]# strip /usr/bin/gpg
<br>The "strip" command will reduce the size of the "gpg" binary for better
performance.
<br>
<p><b><font size=+2>Common Commands</font></b>
<p><b>1: Generating a new keypair</b>
<br>We must create a new key-pair (public and private) for the first time.
The command line option --gen-key is used to create a new primary keypair.
<p>Step 1
<br>[root@dragon /]# gpg --gen-key
<br>gpg (GnuPG) 1.0.2; Copyright (C) 2000 Free Software Foundation, Inc.
<br>This program comes with ABSOLUTELY NO WARRANTY.
<br>This is free software, and you are welcome to redistribute it
<br>under certain conditions. See the file COPYING for details.
<p>gpg: /root/.gnupg: directory created
<br>gpg: /root/.gnupg/options: new options file created
<br>gpg: you have to start GnuPG again, so it can read the new options
file
<p>Step 2
<br>Start GnuPG again with the following command:
<br>[root@dragon /]# gpg --gen-key
<br>gpg (GnuPG) 1.0.2; Copyright (C) 2000 Free Software Foundation, Inc.
<br>This program comes with ABSOLUTELY NO WARRANTY.
<br>This is free software, and you are welcome to redistribute it
<br>under certain conditions. See the file COPYING for details.
<p>gpg:/root/.gnupg/secring.gpg: keyring created
<br>gpg: /root/.gnupg/pubring.gpg: keyring created
<br>Please select what kind of key you want:
<br> (1) DSA and ElGamal (default)
<br> (2) DSA (sign only)
<br> (4) ElGamal (sign and encrypt)
<br>Your selection? 1
<br>DSA keypair will have 1024 bits.
<br>About to generate a new ELG-E keypair.
<br>
minimum keysize is 768 bits
<br>
default keysize is 1024 bits
<br> highest suggested keysize is 2048 bits
<br>What keysize do you want? (1024) 2048
<br>Do you really need such a large keysize? y
<br>Requested keysize is 2048 bits
<br>Please specify how long the key should be valid.
<br> 0 = key does not expire
<br> <n> = key expires in n days
<br> <n> w = key expires in n weeks
<br> <n> m = key expires in n months
<br> <n> y = key expires in n years
<br>Key is valid for? (0) 0
<br>Key does not expire at all
<br>Is this correct (y/n)? y
<p>You need a User-ID to identify your key; the software constructs the
user id
<br>from Real Name, Comment and Email Address in this form:
<br> "Heinrich Heine (Der Dichter) <heinrichh@duesseldorf.de>"
<p>Real name: Kapil sharma
<br>Email address: kapil@linux4biz.net
<br>Comment: Unix/Linux consultant
<br>You selected this USER-ID:
<br> "Kapil Sharma (Unix/Linux consultant) <kapil@linux4biz.net> "
<p>Change (N)ame, (C)omment, (E)mail or (O)kay/(Q)uit? o
<br>You need a Passphrase to protect your secret key.
<p>Enter passphrase: [enter a passphrase]
<p>We need to generate a lot of random bytes. It is a good idea to perform
<br>some other action (type on the keyboard, move the mouse, utilize the
<br>disks) during the prime generation; this gives the random number
<br>generator a better chance to gain enough entropy.
<br> .++++++++++..+++++++++++++++..+++++....+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++.+++++.+++++.++++++++++..+++++.++++++++++....++++++++++..+++++>++++++++++.+++++^^^
<br>public and secret key created and signed.
<p>Now I will explain about the various inputs asked during the generation
of the keypairs.
<ul>
<li>
Please select what kind of key you want:</li>
<br>(1) DSA and ElGamal (default)
<br>(2) DSA (sign only)
<br>(4) ElGamal (sign and encrypt)
<br>Your selection?</ul>
GnuPG is capable of creating different kind of keypairs. There are
three options.
<br> A DSA keypair is the primary keypair usable only for making signatures.
An ElGamal subordinate keypair is also created for encryption. Option 2
is similar but creates only a DSA keypair. Option
<br> 4[1] creates a single ElGamal keypair usable for both making
signatures and performing encryption. <b><i>For most users the default
option is fine.</i></b>
<br>
<ul>
<li>
You must also choose a key size. The size of a DSA key must be between
512 and 1024 bits, and an ElGamal key may be of any size.</li>
</ul>
About to generate a new ELG-E keypair.
<br>
minimum keysize is 768 bits
<br>
default keysize is 1024 bits
<br>
highest suggested keysize is 2048 bits
<br>
What keysize do you want? (1024)
<p> There are advantages and disadvantages of choosing a longer key.
The advantages are: 1) The longer the key the more secure it is against
brute-force attacks
<br> The disadvantages are: 1) encryption and decryption will be slower
as the key size is increased 2) a larger keysize may affect signature length
<p> <b>The default keysize is adequate for almost all purpose and
the keysize can never be changed after selection.</b>
<br>
<ul>
<li>
Finally, you must choose an expiration date. If Option 1 was chosen, the
expiration date will be used for both the ElGamal and DSA keypairs</li>
<br> Please specify how long the key should be valid
<br> 0 = key does not expire
<br> <n> = key expires in n days
<br><n> w = key expires in n weeks
<br><n> m = key expires in n months
<br><n> y = key expires in n years
<br>Key is valid for? (0)</ul>
For most users a key that does not expire is adequate. The expiration time
should be chosen with care, however, since although it is possible to change
the expiration date after the key is created,
<br>it may be difficult to communicate a change to users who have your
public key.
<br>
<ul>
<li>
You must provide a user ID in addition to the key parameters. The user
ID is used to associate the key being created with a real person.</li>
</ul>
You need a User-ID to identify your key; the software constructs the user
id
<br>
from Real Name, Comment and Email Address in this form:
<br>
"Kapil Sharma (Linux consultant) <kapil@linux4biz.net> "
<p>
Real name: <i>Enter you name here</i>
<br>
Email address: <i>Enter you email address</i>
<br><i> </i>Comment:
<i>Enter
any comment here</i>
<br>
<br>
<ul>
<li>
GnuPG needs a passphrase to protect the primary and subordinate private
keys that you keep in your possession.</li>
<br>You need a Passphrase to protect your secret key.</ul>
Enter passphrase:
<p>There is no limit on the length of a passphrase, and it should be carefully
chosen. From the perspective of security, the passphrase to unlock the
private key is one of the weakest points in GnuPG
<br>(and other public-key encryption systems as well) since it is the only
protection you have if another individual gets your private key. Ideally,
the passphrase should not use words from a
<br>dictionary and should mix the case of alphabetic characters as well
as use non-alphabetic characters. A good passphrase is crucial to the secure
use of GnuPG.
<br>
<p><b>2: Generating a revocation certificate</b>
<p>After your keypair is created you should immediately generate a revocation
certificate for the primary public key using the option --gen-revoke. If
you forget your passphrase or if your private
<br>key is compromised or lost, this revocation certificate may be published
to notify others that the public key should no longer be used.
<p> [root@dragon /]# gpg --output revoke.asc --gen-revoke mykey
<p>Here mykey must be a key specifier, either the key ID of your primary
keypair or any part of a user ID that identifies your keypair. The generated
certificate will be left in the file
<br>revoke.asc. The certificate should not be stored where others can access
it since anybody can publish the revocation certificate and render the
corresponding public key
<br>useless.
<br>
<br>
<p><b>3: Listing Keys</b>
<p> To list the keys on your public keyring use the command-line option
--list-keys.
<p>[root@dragon /]# gpg --list-keys
<br>/root/.gnupg/pubring.gpg
<br>------------------------
<br>pub 1024D/020C9884 2000-11-09 Kapil Sharma (Unix/Linux consultant)
<kapil@linux4biz.net>
<br>sub 2048g/555286CA 2000-11-09
<p><b>4: Exporting a public key</b>
<p>You can export your public key to use it on your homepage or on a available
key server on the Internet or any other method. To send your public key
to a correspondent you must first export it. The command-line option --export
is used to do this. It takes an additional argument identifying the public
key to export.
<br>
<ul>
<li>
To export your public key in binary format, use the following command:</li>
<br> [root@dragon /]# gpg --output kapil.gpg --export kapil@linux4biz.net
<li>
To export your public key in ASCII armored output, use the following command:</li>
<br> [root@dragon /]# gpg --export-armor> kapil-key.asc
<br>Here "--export" is for extracting your Public-key from your pubring
encrypted file , "-armor" is to create ASCII armored output that you can
mail, publish it on a web page and "> kapil-key.asc" is to put the result
in a file.
<li>
To export your public key in ASCII armored output and to view it , use
the following command:</li>
<br> [root@dragon /]# gpg --export-armor
<br>-----BEGIN PGP PUBLIC KEY BLOCK-----
<br>Version: GnuPG v1.0.2 (GNU/Linux)
<br>Comment: For info see http://www.gnupg.org
<p>[...]
<br>-----END PGP PUBLIC KEY BLOCK-----
<br> </ul>
<b>5: Importing a public key</b>
<br>Once your own keypair is created, you can put it into your public keyring
database of all keys from trusted third party in order to be able to use
the keys for future encryption and authentication communication. A public
key may be added to your public keyring with the --import option.
<p> [root@dragon /]# gpg --import <filename>
<br>Here "filename" is the name of the exported public key.
<br>For example:
<br>[root@dragon /]# gpg --import mandrake.asc
<br>gpg: key :9B4A4024: public key imported
<br>gpg: /root/.gnupg/trustdb.gpg: trustdb created
<br>gpg: Total number processed: 1
<br>gpg:
imported: 1
<p>In the above example we imported the Public key file "mandrake.asc"
from the company Mandrake Linux, downloadable from Mandrake Internet site,
into our keyring.
<p><b>6: Validating the key</b>
<br>Once a key is imported it should be validated. A key is validated
by verifying the key's fingerprint and then signing the key to certify
it as a valid key. A key's fingerprint can be quickly viewed with the --fingerprint
command-line option.
<br>[root@dragon /]# gpg --fingerprint <UID>
<br>As a example:
<br>[root@dragon /]# gpg --fingerprint mandrake
<br>pub 1024D/9B4A4024 2000-01-06 MandrakeSoft (MandrakeSoft official
keys) <mandrake@mandrakesoft.com>
<br> Key fingerprint = 63A2 8CBD A7A8 387E 1A53
2C1E 59E7 0DEE 9B4A 4024
<br>sub 1024g/686FF394 2000-01-06
<p>In the above example we verified the fingerprint of mandrake. A key's
fingerprint is verified with the key's owner. This may be done in person
or over the phone or through any other means as long as you can guarantee
that you are communicating with the key's true owner. If the fingerprint
you get is the same as the fingerprint the key's owner gets, then you can
be sure that you have a correct copy of the key.
<p><b>7: Key Signing</b>
<br>After importing and verifying the keys that you have imported into
your public database, you can start signing them. Signing a key certifies
that you know the owner of the keys. You should only sign the keys when
you are 100% sure of the authentication of the key.
<br>
<ul>
<li>
To sign a key for the company Mandrake that we have added on our keyring
above, use the following command:</li>
<br>[root@dragon /]# gpg --sign-key <UID>
<br>As an example:
<br>[root@dragon /]# gpg --sign-key <UID>
<br>pub 1024D/9B4A4024 created: 2000-01-06 expires: never
trust: -/q
<br>sub 1024g/686FF394 created: 2000-01-06 expires: never
<br>(1) MandrakeSoft (MandrakeSoft official keys) <mandrake@mandrakesoft.com>
<br>
<p>pub 1024D/9B4A4024 created: 2000-01-06 expires: never
trust: -/q
<br>
Fingerprint: 63A2 8CBD A7A8 387E 1A53 2C1E 59E7 0DEE 9B4A 4024
<p> MandrakeSoft (MandrakeSoft official keys) <mandrake@mandrakesoft.com>
<p>Are you really sure that you want to sign this key
<br>with your key: "Kapil Sharma (Unix/Linux consultant) <kapil@linux4biz.net> "
<p>Really sign? y
<p>You need a passphrase to unlock the secret key for
<br>user: "Kapil Sharma (Unix/Linux consultant) <kapil@linux4biz.net> "
<br>1024-bit DSA key, ID 020C9884, created 2000-11-09
<p>Enter passphrase:
<br> </ul>
<b>8: Checking Signatures</b>
<br>Once signed you can check the key to list the signatures on it and
see the signature that you have added. Every user ID on the key will have
one or more self-signatures as well as a signature for
<br>each user that has validated the key. We can check the signatures of
the keys by the gpg option "--check-sigs:
<br>As an example:
<br>[root@dragon /]# gpg --check-sigs mandrake
<br>pub 1024D/9B4A4024 2000-01-06 MandrakeSoft (MandrakeSoft official
keys) <mandrake@mandrakesoft.com>
<br>sig! 9B4A4024 2000-01-06
MandrakeSoft (MandrakeSoft official keys) <mandrake@mandrakesoft.com>
<br>sig! 020C9884 2000-11-09
Kapil Sharma (Unix/Linux consultant) <kapil@linux4biz.net>
<br>sub 1024g/686FF394 2000-01-06
<br>sig! 9B4A4024 2000-01-06
MandrakeSoft (MandrakeSoft official keys) <mandrake@mandrakesoft.com>
<p><b>9: Encrypting and decrypting</b>
<br>The procedure for encrypting and decrypting documents is very simple.
If you want to encrypt a message to mandrake, you encrypt it using mandrake
public key, and then only mandrake can
<br>decrypt that file with his private key. If Mandrake wants to
send you a message, it encrypts it using your public key, and you
decrypt it with your private key.
<p>To encrypt and sign data for the user Mandrake that we have added on
our keyring use the following command (You must have a public key of the
recipient):
<br>[root@dragon /]# gpg -sear <UID of the public key> <file>
<p>As an example:
<br>[root@dragon /]# gpg -sear Mandrake document.txt
<br>You need a passphrase to unlock the secret key for
<br>user: "Kapil Sharma (Unix/Linux consultant) <kapil@linux4biz.net> "
<br>1024-bit DSA key, ID 020C9884, created 2000-11-09
<p>Enter passphrase:
<p>Here "s" is for signing , "e" for encrypting, "a" to create ASCII armored
output (".asc" is ready for sending by mail), "r" to encrypt the user id
name and <file> is the data you want to encrypt
<p> To decrypt data ,use the following command:
<br>[root@dragon /]# gpg -d <file>
<p>As an example:
<br>[root@dragon /]# gpg -d documentforkapil.asc
<br>You need a passphrase to unlock the secret key for
<br>user: "Kapil Sharma (Unix/Linux consultant) <kapil@linux4biz.net> "
<br>1024-bit DSA key, ID 020C9884, created 2000-11-09
<br>Enter passphrase:
<p>Here the parameter "d" is for decrypting the data and <file> is a
data you want to decrypt.
<br>[Note: you must have the public key of the sender of the message/data
that you want to decrypt in your public keyring database.]
<p><b>10: Checking the signature</b>
<br>Once you have extracted your public key and exported it then by using
the --verify option of GnuPG anybody can check whether encrypted data from
you is also signed by you.
<ul>
<li>
To check the signature of encrypted data, use the following command:</li>
<br>[root@dragon /]# gpg --verify <Data>
<br>Here "--verify" option is to check the signature and "<Data> " is
the encrypted data/file you want to verify.</ul>
<p><br><b><font size=+2>Some uses of GnuPG software</font></b>
<p>1: Send encrypted mail messages.
<br>2: Encrypt files and documents
<br>3: Transmit encrypted files and important documents through network
<br>
<p><b><font size=+2>Here is a list of some of the Frontend and software
for GnuPG</font></b>
<p><b>
<a href="http://www.gnupg.org/gpa.html">GPA</a> aims to be the standard
GnuPG graphical frontend. This has a very nice GUI interface.</b>
<br><b>
<a href="http://www.geocities.com/SiliconValley/Chip/3708/gpgp/gpgp-intro.html#this">GnomePGP</a>
is a GNOME desktop tool to control GnuPG.</b>
<br><b>
<a href="http://geheimnis.sourceforge.net/">Geheimniss</a> is a KDE frontend
for GnuPG.</b>
<br><b>
<a href="http://pgp4pine.flatline.de/">pgp4pine</a> is a Pine filter to
handle PGP messages.</b>
<br><b>
<a href="http://www.physto.se/~p99jlu/MagicPGP.html">MagicPGP</a> is yet
another set of scripts to use GnuPG with Pine.</b>
<br><b>
<a href="http://www.megaloman.com/~hany/software/pinepgp/">PinePGP</a>
is also a Pine filter for GnuPG.</b>
<br>
<p><b><font size=+2>More Information</font></b>
<p><a href="http://www.gnupg.org/docs.html">http://www.gnupg.org/docs.html</a>
<p><b><font size=+2>Conclusion</font></b>
<p>Anybody who is cautious about security must use GnuPG. It is one of
the best open-source programs which has all the functions for encryption
and decryption for all your secure data and can be used without any restrictions
since it is under GNU General Public License. It can be used to send encrypted
mail messages, files and documents for security. It can also be used
to transmit files and important documents through network securely.
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<H5 ALIGN=center>
Copyright © 2000, Kapil Sharma.<BR>
Copying license <A HREF="../copying.html">http://www.linuxgazette.com/copying.html</A><BR>
Published in Issue 60 of <i>Linux Gazette</i>, December 2000</H5>
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