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GnuPG - The GNU Privacy Guard
-------------------------------
Version 0.9
GnuPG is now in Beta test and you should report all bugs to the
mailing list (see below). The 0.9.x versions are mainly released
to fix all remaining serious bugs. As soon as version 1.0 is out,
development will continue with a 1.1 series and bug fixes for the
1.0 version are released as needed.
GnuPG works best on GNU/Linux or *BSD. Other Unices are
also supported but not as good tested as those Freenix ones.
Please verify the tar file; there is a PGP2 and a GnuPG/PGP5
signature available. My PGP2 key is well known and published in
the "Global Trust Register for 1998", ISBN 0-9532397-0-5.
I have included my pubring as "g10/pubring.asc", which contains
the key used to make GnuPG signatures:
"pub 1024D/57548DCD 1998-07-07 Werner Koch (gnupg sig) <dd9jn@gnu.org>"
"Key fingerprint = 6BD9 050F D8FC 941B 4341 2DCC 68B7 AB89 5754 8DCD"
My new DSA key is:
"pub 1024D/621CC013 1998-07-07 Werner Koch <werner.koch@guug.de>"
"Key fingerprint = ECAF 7590 EB34 43B5 C7CF 3ACB 6C7E E1B8 621C C013"
You may want add it to your GnuPG pubring and use it in the future to
verify new releases. Because you verified this README file and
_checked_that_it_is_really_my PGP2 key 0C9857A5, you can be sure
that the above fingerprints are correct.
Please subscribe to g10@net.lut.ac.uk by sending a mail with
the word "subscribe" in the body to "g10-request@net.lut.ac.uk".
This mailing list is a closed one (only subscribers are allowed
to post) to avoid misuse by folks who don't know the Netiquette
and trash you mailspool with commercial junk.
See the file COPYING for copyright and warranty information.
GnuPG is in compliance with RFC2440 (OpenPGP), see doc/OpenPGP for
details.
Due to the fact that GnuPG does not use use any patented algorithm,
it cannot be compatible with PGP2 versions; PGP 2.x does only use
IDEA (which is patented worldwide) and RSA (which is patented in
the United States until Sep 20, 2000).
The default algorithms are now DSA and ElGamal. ElGamal for signing
is still available, but due to the larger size of such signatures it
is depreciated (Please note that the GnuPG implementation of ElGamal
signatures is *not* insecure). Symmetric algorithms are: 3DES,
Blowfish and CAST5 (Twofish will come soon), available digest
algorithms are MD5, RIPEMD160, SHA1 and TIGER/192.
Installation
------------
Please read the file INSTALL!
Here is a quick summary:
1) "./configure"
2) "make"
3) "make install"
4) You end up with the binaries "gpg" and "gpgm" in /usr/local/bin.
5) Optional, but suggested: install the binary "gpg" as suid root.
Intro
-----
This is a brief overview how to use GnuPG - it is highly suggested
that you read the manual^H^H^H more information about the use
of cryptography. GnuPG is only the technical tool to do it and
the security highly depends on 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, so that it can 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 which is not easily guessable as the
security of the whole system relies on your secret key and the
passphrase is used to protect this secret key in case someone was
able to get 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.
Then 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 it away; 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 the file.asc which is ascii armored and
and ready for sending by mail. Note: 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 which is identical to the original file. gpg
can also run as a filter, so that you can pipe data to verify
trough it:
cat signed-file | gpg | wc -l
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 this:
gpg -e -r heine file
This encrypts file with the public key of the user "heine" and
writes it to "file.gpg"
echo "hallo" | gpg -ea -r heine | mail heine
Ditto, but encrypts "hallo\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:
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; is called "importing":
gpg --import [filenames]
New keys are appended to your keyring and already existing
keys are updated. Note that GnuPG does not allow keys which
are not self-signed by the user.
Because anyone can claim that the public key belongs to her
we must have some way to check that the 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; this
is a sequence of hex bytes (which you may have noticed in mail
sigs or on business cards) which uniquely identify the public
key - two different keys will always have different fingerprints.
It is easy to compare this fingerprint by phone and I suggest
that you print your fingerprint on the back of your business
card.
If you don't know the owner of the public key you are in trouble;
but wait: A friend of you knows someone who knows someone who
has met the owner of the public key at some computer conference.
So all the persons between you and the public key holder may now
act as introducer to you; this is done by signing the keys and
thereby certifying the other keys. If you then trust all the
introducers to correctly sign other keys, you can be be sure that
the other key really belongs to the one who claims so.
There are 2 steps to validate a target 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).
These information are confidential because they give your
personal opinion on the trustworthy 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 you - decide how far she
understands all the implications of key signatures and you may
want to tell him more about public key cryptography so you
can later change the trust value you assigned.
Okay, here is how GnuPG helps you in 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
which 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 user ID of yours, 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
does only use one and this keeps the public key certificate
small. Because such key signatures are very important you
should make sure that the signators 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 signators sign your public key (the
primary one) together with one od 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).
7 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 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 the use of 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 PGPs
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.
Esoteric commands
-----------------
gpg --list-packets datafile
Use this to list the contents of a data file. If the file is encrypted
you are asked for the passphrase, so that GnuPG is able to look at the
inner structure of a encrypted packet. This command should be able
to list all kinds of rfc2440 messages.
gpgm --list-trustdb
List the contents of the trust DB in a human readable format
gpgm --list-trustdb <usernames>
List the tree of certificates for the given usernames
gpgm --list-trust-path username
List the possible trust paths for the given username. The length
of such a trust path is limited by the option --max-cert-depth
which defaults to 5.
For more options/commands see the man page or use "gpg --help".
Other Notes
-----------
The primary FTP site is "ftp://ftp.gnupg.org/pub/gcrypt/"
The primary WWW page is "http://www.gnupg.org"
See http://www.gnugp.org/mirrors.html for a list of FTP mirrors
and use them if possible.
Please direct bug reports to <gnupg-bugs@gnu.org> or better
post them to the mailing list <g10@net.lut.ac.uk> (this is a
closed list - subscribe before posting, see above (~line 33)).
Please direct questions about GnuPG to the mailing list or
one of the pgp newsgroups; this gives me more time to improve
GnuPG. Commercial support for GnuPG will be availabe soon.
Have fun and remember: Echelon is looking at you kid.