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GnuPG - The GNU Privacy Guard ------------------------------- Version 1.0 Copyright 1998, 1999, 2000, 2001 Free Software Foundation, Inc. This file is free software; as a special exception the author gives unlimited permission to copy and/or distribute it, with or without modifications, as long as this notice is preserved. This file is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY, to the extent permitted by law; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. Intro ----- 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 IDEA (which is patented worldwide). 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: AES, 3DES, Blowfish, CAST5 and Twofish 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 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 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 "gph". An online version of the latest manual draft is available at the GnuPG web pages: http://www.gnupg.org/gph/ A list of frequently asked questions is available in GnuPG's distibution in the file doc/FAQ and online as: http://www.gnupg.org/faq.html 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/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.