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GnuPG - The GNU Privacy Guard
-------------------------------
Version 1.0
GnuPG is GNU's tool for secure communication and data storage.
It can be used to encrypt data and to create digital signatures.
It includes an advanced key management facility and is compliant
with the proposed OpenPGP Internet standard as described in RFC2440.
GnuPG works best on GNU/Linux or *BSD systems. Most other Unices
are also supported but are not as well tested as the Free Unices.
See http://www.gnupg.org/gnupg.html#supsys for a list of systems
which are known to work.
See the file COPYING for copyright and warranty information.
Because GnuPG does not use use any patented algorithm it cannot be
compatible with PGP2 versions. PGP 2.x uses only IDEA (which is
patented worldwide) and RSA (which is patented in the United States
until Sep 20, 2000).
The default algorithms are DSA and ElGamal. ElGamal for signing
is still available, but because of the larger size of such
signatures it is deprecated (Please note that the GnuPG
implementation of ElGamal signatures is *not* insecure). Symmetric
algorithms are: 3DES, Blowfish, CAST5 and Twofish (GnuPG does not
yet create Twofish encrypted messages because there is no agreement
in the OpenPGP WG on how to use it together with a MDC algorithm)
Digest algorithms available are MD5, RIPEMD160 and SHA1.
Installation
------------
Please read the file INSTALL!
Here is a quick summary:
1) Check that you have unmodified sources. The below on how to do this.
Don't skip it - this is an important step!
2) Unpack the TAR. With GNU tar you can do it this way:
"tar xzvf gnupg-x.y.z.tar.gz"
3) "cd gnupg-x.y.z"
4) "./configure"
5) "make"
6) "make install"
7) You end up with a "gpg" binary in /usr/local/bin.
Note: Because some old programs rely on the existence of a
binary named "gpgm" (which was build by some Beta versions
of GnuPG); you may want to install a symbolic link to it:
"cd /usr/local/bin; ln -s gpg gpgm"
8) To avoid swapping out of sensitive data, you can install "gpg" as
suid root. If you don't do so, you may want to add the option
"no-secmem-warning" to ~/.gnupg/options
How to Verify the Source
------------------------
In order to check that the version of GnuPG which you are going to
install is an original and unmodified one, you can do it in one of
the following ways:
a) If you already have a trusted Version of GnuPG installed, you
can simply check the supplied signature:
$ gpg --verify gnupg-x.y.z.tar.gz.asc
This checks that the detached signature gnupg-x.y.z.tar.gz.asc
is indeed a a signature of gnupg-x.y.z.tar.gz. The key used to
create this signature is:
"pub 1024D/57548DCD 1998-07-07 Werner Koch (gnupg sig) <dd9jn@gnu.org>"
If you do not have this key, you can get it from the source in
the file g10/pubring.asc (use "gpg --import g10/pubring.gpg" to
add it to the keyring) or from any keyserver. You have to make
sure that this is really the key and not a faked one. You can do
this by comparing the output of:
$ gpg --fingerprint 0x57548DCD
with the elsewhere published fingerprint, or - if you are able to
_positively_ verify the signature of this README file - with
this fingerprint: "6BD9 050F D8FC 941B 4341 2DCC 68B7 AB89 5754 8DCD"
Please note, that you have to use an old version of GnuPG to
do all this stuff. *Never* use the version which you are going
to check!
b) If you have a trusted Version of PGP 2 or 5 installed, you
can check the supplied PGP 2 signature:
$ pgp gnupg-x.y.z.tar.gz.sig gnupg-x.y.z.tar.gz
This checks that the detached signature gnupg-x.y.z.tar.gz.sig
is indeed a a signature of gnupg-x.y.z.tar.gz. Please note,
that this signature has been created with a RSA signature and
you probably can't use this method (due to legal reasons) when
you are in the U.S. The key used to create this signature is
the same as the one used to sign this README file. It should be
available at the keyservers and is also included in the source
of GnuPG in g10/pubring.asc.
"pub 768R/0C9857A5 1995-09-30 Werner Koch <werner.koch@guug.de>"
The fingerprint of this key is published in printed form in the
"Global Trust Register for 1998", ISBN 0-9532397-0-5.
c) If you don't have any of the above programs, you have to verify
the MD5 checksum:
$ md5sum gnupg-x.y.z.tar.gz
This should yield an output similar_to this:
fd9351b26b3189c1d577f0970f9dcadc gnupg-x.y.z.tar.gz
Now check that this checksum is _exactly_ the same as the one
published via the announcement list and probably via Usenet.
Documentation
-------------
The manual will be distributed separate under the name "gnupgdoc".
An online version of the latest manual draft is available at the
GnuPG web pages:
http://www.gnupg.org/gph/
A couple of HOWTO documents are available online; for a listing see:
http://www.gnupg.org/docs.html#howtos
A man page with a description of all commands and options gets installed
along with the program.
Introduction
------------
Here is a brief overview on how to use GnuPG - it is strongly suggested
that you read the manual and other information about the use of
cryptography. GnuPG is only a tool, secure usage requires that
YOU KNOW WHAT YOU ARE DOING.
If you already have a DSA key from PGP 5 (they call them DH/ElGamal)
you can simply copy the pgp keyrings over the GnuPG keyrings after
running gpg once to create the correct directory.
The normal way to create a key is
gpg --gen-key
This asks some questions and then starts key generation. To create
good random numbers for the key parameters, GnuPG needs to gather
enough noise (entropy) from your system. If you see no progress
during key generation you should start some other activities such
as mouse moves or hitting on the CTRL and SHIFT keys.
Generate a key ONLY on a machine where you have direct physical
access - don't do it over the network or on a machine used also
by others - especially if you have no access to the root account.
When you are asked for a passphrase use a good one which you can
easy remember. Don't make the passphrase too long because you have
to type it for every decryption or signing; but, - AND THIS IS VERY
IMPORTANT - use a good one that is not easily to guess because the
security of the whole system relies on your secret key and the
passphrase that protects it when someone gains access to your secret
keyring. A good way to select a passphrase is to figure out a short
nonsense sentence which makes some sense for you and modify it by
inserting extra spaces, non-letters and changing the case of some
characters - this is really easy to remember especially if you
associate some pictures with it.
Next, you should create a revocation certificate in case someone
gets knowledge of your secret key or you forgot your passphrase
gpg --gen-revoke your_user_id
Run this command and store the revocation certificate away. The output
is always ASCII armored, so that you can print it and (hopefully
never) re-create it if your electronic media fails.
Now you can use your key to create digital signatures
gpg -s file
This creates a file "file.gpg" which is compressed and has a
signature attached.
gpg -sa file
Same as above, but creates a file "file.asc" which is ASCII armored
and and ready for sending by mail. It is better to use your
mailers features to create signatures (The mailer uses GnuPG to do
this) because the mailer has the ability to MIME encode such
signatures - but this is not a security issue.
gpg -s -o out file
Creates a signature of "file", but writes the output to the file
"out".
Everyone who knows your public key (you can and should publish
your key by putting it on a key server, a web page or in your .plan
file) is now able to check whether you really signed this text
gpg --verify file
GnuPG now checks whether the signature is valid and prints an
appropriate message. If the signature is good, you know at least
that the person (or machine) has access to the secret key which
corresponds to the published public key.
If you run gpg without an option it will verify the signature and
create a new file that is identical to the original. gpg can also
run as a filter, so that you can pipe data to verify trough it
cat signed-file | gpg | wc -l
which will check the signature of signed-file and then display the
number of lines in the original file.
To send a message encrypted to someone you can use
gpg -e -r heine file
This encrypts "file" with the public key of the user "heine" and
writes it to "file.gpg"
echo "hello" | gpg -ea -r heine | mail heine
Ditto, but encrypts "hello\n" and mails it as ASCII armored message
to the user with the mail address heine.
gpg -se -r heine file
This encrypts "file" with the public key of "heine" and writes it
to "file.gpg" after signing it with your user id.
gpg -se -r heine -u Suttner file
Ditto, but sign the file with your alternative user id "Suttner"
GnuPG has some options to help you publish public keys. This is
called "exporting" a key, thus
gpg --export >all-my-keys
exports all the keys in the keyring and writes them (in a binary
format) to "all-my-keys". You may then mail "all-my-keys" as an
MIME attachment to someone else or put it on an FTP server. To
export only some user IDs, you give them as arguments on the command
line.
To mail a public key or put it on a web page you have to create
the key in ASCII armored format
gpg --export --armor | mail panther@tiger.int
This will send all your public keys to your friend panther.
If you have received a key from someone else you can put it
into your public keyring. This is called "importing"
gpg --import [filenames]
New keys are appended to your keyring and already existing
keys are updated. Note that GnuPG does not import keys that
are not self-signed.
Because anyone can claim that a public key belongs to her
we must have some way to check that a public key really belongs
to the owner. This can be achieved by comparing the key during
a phone call. Sure, it is not very easy to compare a binary file
by reading the complete hex dump of the file - GnuPG (and nearly
every other program used for management of cryptographic keys)
provides other solutions.
gpg --fingerprint <username>
prints the so called "fingerprint" of the given username which
is a sequence of hex bytes (which you may have noticed in mail
sigs or on business cards) that uniquely identifies the public
key - different keys will always have different fingerprints.
It is easy to compare fingerprints by phone and I suggest
that you print your fingerprint on the back of your business
card. To see the fingerprints of the secondary keys, you can
give the command twice; but this is normally not needed.
If you don't know the owner of the public key you are in trouble.
Suppose however that friend of yours knows someone who knows someone
who has met the owner of the public key at some computer conference.
Suppose that all the people between you and the public key holder
may now act as introducers to you. Introducers signing keys thereby
certify that they know the owner of the keys they sign. If you then
trust all the introducers to have correctly signed other keys, you
can be be sure that the other key really belongs to the one who
claims to own it..
There are 2 steps to validate a key:
1. First check that there is a complete chain
of signed keys from the public key you want to use
and your key and verify each signature.
2. Make sure that you have full trust in the certificates
of all the introduces between the public key holder and
you.
Step 2 is the more complicated part because there is no easy way
for a computer to decide who is trustworthy and who is not. GnuPG
leaves this decision to you and will ask you for a trust value
(here also referenced as the owner-trust of a key) for every key
needed to check the chain of certificates. You may choose from:
a) "I don't know" - then it is not possible to use any
of the chains of certificates, in which this key is used
as an introducer, to validate the target key. Use this if
you don't know the introducer.
b) "I do not trust" - Use this if you know that the introducer
does not do a good job in certifying other keys. The effect
is the same as with a) but for a) you may later want to
change the value because you got new information about this
introducer.
c) "I trust marginally" - Use this if you assume that the
introducer knows what he is doing. Together with some
other marginally trusted keys, GnuPG validates the target
key then as good.
d) "I fully trust" - Use this if you really know that this
introducer does a good job when certifying other keys.
If all the introducer are of this trust value, GnuPG
normally needs only one chain of signatures to validate
a target key okay. (But this may be adjusted with the help
of some options).
This information is confidential because it gives your personal
opinion on the trustworthiness of someone else. Therefore this data
is not stored in the keyring but in the "trustdb"
(~/.gnupg/trustdb.gpg). Do not assign a high trust value just
because the introducer is a friend of yours - decide how well she
understands the implications of key signatures and you may want to
tell her more about public key cryptography so you can later change
the trust value you assigned.
Okay, here is how GnuPG helps you with key management. Most stuff
is done with the --edit-key command
gpg --edit-key <keyid or username>
GnuPG displays some information about the key and then prompts
for a command (enter "help" to see a list of commands and see
the man page for a more detailed explanation). To sign a key
you select the user ID you want to sign by entering the number
that is displayed in the leftmost column (or do nothing if the
key has only one user ID) and then enter the command "sign" and
follow all the prompts. When you are ready, give the command
"save" (or use "quit" to cancel your actions).
If you want to sign the key with another of your user IDs, you
must give an "-u" option on the command line together with the
"--edit-key".
Normally you want to sign only one user ID because GnuPG
uses only one and this keeps the public key certificate
small. Because such key signatures are very important you
should make sure that the signatories of your key sign a user ID
which is very likely to stay for a long time - choose one with an
email address you have full control of or do not enter an email
address at all. In future GnuPG will have a way to tell which
user ID is the one with an email address you prefer - because
you have no signatures on this email address it is easy to change
this address. Remember, your signatories sign your public key (the
primary one) together with one of your user IDs - so it is not possible
to change the user ID later without voiding all the signatures.
Tip: If you hear about a key signing party on a computer conference
join it because this is a very convenient way to get your key
certified (But remember that signatures have nothing to to with the
trust you assign to a key).
8 Ways to Specify a User ID
--------------------------
There are several ways to specify a user ID, here are some examples.
* Only by the short keyid (prepend a zero if it begins with A..F):
"234567C4"
"0F34E556E"
"01347A56A"
"0xAB123456
* By a complete keyid:
"234AABBCC34567C4"
"0F323456784E56EAB"
"01AB3FED1347A5612"
"0x234AABBCC34567C4"
* By a fingerprint:
"1234343434343434C434343434343434"
"123434343434343C3434343434343734349A3434"
"0E12343434343434343434EAB3484343434343434"
The first one is MD5 the others are ripemd160 or sha1.
* By an exact string:
"=Heinrich Heine <heinrichh@uni-duesseldorf.de>"
* By an email address:
"<heinrichh@uni-duesseldorf.de>"
* By word match
"+Heinrich Heine duesseldorf"
All words must match exactly (not case sensitive) and appear in
any order in the user ID. Words are any sequences of letters,
digits, the underscore and characters with bit 7 set.
* By the Local ID (from the trust DB):
"#34"
This may be used by a MUA to specify an exact key after selecting
a key from GnuPG (by using a special option or an extra utility)
* Or by the usual substring:
"Heine"
"*Heine"
The '*' indicates substring search explicitly.
Batch mode
----------
If you use the option "--batch", GnuPG runs in non-interactive mode and
never prompts for input data. This does not even allow entering the
passphrase. Until we have a better solution (something like ssh-agent),
you can use the option "--passphrase-fd n", which works like PGP's
PGPPASSFD.
Batch mode also causes GnuPG to terminate as soon as a BAD signature is
detected.
Exit status
-----------
GnuPG returns with an exit status of 1 if in batch mode and a bad signature
has been detected or 2 or higher for all other errors. You should parse
stderr or, better, the output of the fd specified with --status-fd to get
detailed information about the errors.
How to Get More Information
---------------------------
The primary WWW page is "http://www.gnupg.org"
The primary FTP site is "ftp://ftp.gnupg.org/pub/gcrypt/"
See http://www.gnupg.org/mirrors.html for a list of mirrors
and use them if possible. You may also find GnuPG mirrored on
some of the regular GNU mirrors.
We have some mailing lists dedicated to GnuPG:
gnupg-announce@gnupg.org For important announcements like
new versions and such stuff.
This is a moderated list and has
very low traffic.
gnupg-users@gnupg.org For general user discussion and
help.
gnupg-i18n@gnupg.org Discussion about internationalization
issues.
gnupg-devel@gnupg.org GnuPG developers main forum.
You subscribe to one of the list by sending mail with a subject
of "subscribe" to x-request@gnupg.org, where x is the name of the
mailing list (gnupg-announce, gnupg-users, etc.). An archive of
the mailing lists is available at http://lists.gnupg.org .
The gnupg.org domain is hosted in Germany to avoid possible legal
problems (technical advices may count as a violation of ITAR).
Please direct bug reports to <gnupg-bugs@gnu.org> or post
them direct to the mailing list <gnupg-devel@gnupg.org>.
Please direct questions about GnuPG to the users mailing list or
one of the pgp newsgroups; please do not direct questions to one
of the authors directly as we are busy working on improvements
and bug fixes. Both mailing lists are watched by the authors
and we try to answer questions when time allows us to do so.
Commercial grade support for GnuPG is available; please see
the GNU service directory or search other resources.
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