security-and-privacy/gnupg
security-and-privacy/gnupg
1
0
Fork 0
mirror of git://git.gnupg.org/gnupg.git synced 2024-11-09 21:28:51 +01:00
Code Activity
90932bdad6
gnupg/doc/faq.org
Yuri Chornoivan 24cf0606b4 Clean up word replication. 
--

This fixes extra word repetitions (like "the the" or "is is") in the
code and docs.

Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>
2017-02-21 13:11:46 -05:00

63 KiB
Raw Blame History

GnuPG Frequently Asked Questions

<a href="/"><img src="https://gnupg.org/share/logo-gnupg-light-purple-bg.png" class="logo-link" /></a>

WARNING: This FAQ is heavily outdated. Mentioned versions of GnuPG have reached end of life many years ago. Almost all bugs and problems have been fixed in the now current versions of GnuPG. We will try to update this FAQ in the next month. See the section "Changes" for recent updates.

Welcome

Welcome to the GnuPG FAQ. The latest HTML version is available at
http://www.gnupg.org/faq/GnuPG-FAQ.html;
a plain text Gversion at
ftp://ftp.gnupg.org/gcrypt/gnupg/GnuPG-FAQ.txt.

See the end of this file for the release date.

The index is generated automatically, so there may be errors. Not all questions may be in the section they belong to. Suggestions about how to improve the structure of this FAQ are welcome.

Please send additions and corrections to the gnupg-users mailing list. It would be most convenient if you could provide the answer to be included here as well. Your help is very much appreciated!

Please, don't send message like "This should be a FAQ - what's the answer?". If it hasn't been asked before, it isn't a FAQ. In that case you could search in the mailing list archive.

What conventions are used in this FAQ?

Although GnuPG is being developed for several operating systems (often in parallel), the conventions used in this FAQ reflect a UNIX shell environment. For Win32 users, references to a shell prompt ($) should be interpreted as a command prompt (>), directory names separated by a forward slash (/) may need to be converted to a back slash (\), and a tilde (~) represents a user's "home" directory (reference question How do I put my keyring in a different directory? for an example).

Some command-lines presented in this FAQ are too long to properly display in some browsers for the web page version of this file, and have been split into two or more lines. For these commands please remember to enter the entire command-string on one line or the command will error, or at minimum not give the desired results.

Please keep in mind that this FAQ contains information that may not apply to your particular version, as new features and bug fixes are added on a continuing basis (reference the NEWS file included with the source or package for noteworthy changes between versions). One item to note is that starting with GnuPG version 1.1.92 the file containing user options and settings has been renamed from "options" to "gpg.conf". Information in the FAQ that relates to the options file may be interchangeable with the newer gpg.conf file in many instances. See question GnuPG no longer installs a ~/.gnupg/options file. Is it missing? for details.

General Questions

What is GnuPG?

GnuPG stands for GNU Privacy Guard and 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 RFC-4880. As such, it is aimed to be compatible with PGP from PGP Corp. and other OpenPGP tools

Is GnuPG compatible with PGP?

In general, yes. GnuPG and newer PGP releases should be implementing the OpenPGP standard. But there are some interoperability problems. See question How can I encrypt a message with GnuPG so that PGP is able to decrypt it? for details.

Is GnuPG free to use for personal or commercial use?

Yes. GnuPG is part of the GNU family of tools and applications built and provided in accordance with the Free Software Foundation (FSF) General Public License (GPL). Therefore the software is free to copy, use, modify and distribute in accordance with that license. Please read the file titled COPYING that accompanies the application for more information.

Sources of Information

Where can I find more information on GnuPG?

On-line resources:

  • The documentation page is located at gnupgweb:documentation/. Also, have a look at the HOWTOs and the GNU Privacy Handbook (GPH, available in English, Spanish and Russian). The latter provides a detailed user's guide to GnuPG. You'll also find a document about how to convert from PGP 2.x to GnuPG.
  • At gnupgweb:documentation/mailing-lists.html you'll find an online archive of the GnuPG mailing lists. Most interesting should be gnupg-users for all user-related issues and gnupg-devel if you want to get in touch with the developers. In addition, searchable archives can be found on MARC, e.g.:
    For gnupg-users : http://marc.theaimsgroup.com/?l=gnupg-users&r=1&w=2
    For gnupg-devel : http://marc.theaimsgroup.com/?l=gnupg-devel&r=1&w=2 Please: Before posting to a list, read this FAQ and the available documentation. In addition, search the list archive — maybe your question has already been discussed. This way you help people focus on topics that have not yet been resolved.

  • The GnuPG source distribution contains a subdirectory:

    ./doc

    where some additional documentation is located (mainly interesting for hackers, not the casual user).

Where do I get GnuPG?

You can download the GNU Privacy Guard from its primary FTP server ftp.gnupg.org or from one of its mirrors.

The current stable version is FIXME. Please upgrade to this version as it includes additional features, functions and security fixes that may not have existed in prior versions.

Installation

Which OSes does GnuPG run on?

It should run on most Unices as well as Windows versions (including Windows NT/2000) and Macintosh OS/X. A list of OSes reported to be OK is presented at:

gnupgweb:download/supported_systems.html

Which random data gatherer should I use?

"Good" random numbers are crucial for the security of your encryption. Different operating systems provide a variety of more or less quality random data. Linux and *BSD provide kernel generated random data through /dev/random - this should be the preferred choice on these systems. Also Solaris users with the SUNWski package installed have a /dev/random. In these cases, use the configure option:

--enable-static-rnd=linux

In addition, there's also the kernel random device by Andi Maier http://www.cosy.sbg.ac.at/~andi/SUNrand/, but it's still beta. Use at your own risk!

On other systems, the Entropy Gathering Daemon (EGD) is a good choice. It is a perl-daemon that monitors system activity and hashes it into random data. See the download page gnupgweb:download/ to obtain EGD. Use:

--enable-static-rnd=egd

here.

If the above options do not work, you can use the random number generator "unix". This is very slow and should be avoided. The random quality isn't very good so don't use it on sensitive data.

How do I include support for RSA and IDEA?

RSA is included as of GnuPG version 1.0.3.

The official GnuPG distribution does not contain IDEA due to a patent restriction. The patent does not expire before 2007 so don't expect official support before then.

However, there is an unofficial module to include it even in earlier versions of GnuPG. It's available from ftp://ftp.gnupg.dk/pub/contrib-dk/. Look for:

idea.c.gz        (c module)
idea.c.gz.sig    (signature file)

ideadll.zip      (c module and win32 dll)
ideadll.zip.sig  (signature file)

Compilation directives are in the headers of these files. You will then need to add the following line to your ~/.gnupg/gpg.conf or ~/.gnupg/options file:

load-extension idea

Usage

What is the recommended key size?

1024 bit for DSA signatures; even for plain Elgamal signatures. This is sufficient as the size of the hash is probably the weakest link if the key size is larger than 1024 bits. Encryption keys may have greater sizes, but you should then check the fingerprint of this key:

$ gpg --fingerprint <user ID>

As for the key algorithms, you should stick with the default (i.e., DSA signature and Elgamal encryption). An Elgamal signing key has the following disadvantages: the signature is larger, it is hard to create such a key useful for signatures which can withstand some real world attacks, you don't get any extra security compared to DSA, and there might be compatibility problems with certain PGP versions. It has only been introduced because at the time it was not clear whether there was a patent on DSA.

Why does it sometimes take so long to create keys?

The problem here is that we need a lot of random bytes and for that we (on Linux the /dev/random device) must collect some random data. It is really not easy to fill the Linux internal entropy buffer; I talked to Ted Ts'o and he commented that the best way to fill the buffer is to play with your keyboard. Good security has its price. What I do is to hit several times on the shift, control, alternate, and caps lock keys, because these keys do not produce output to the screen. This way you get your keys really fast (it's the same thing PGP2 does).

Another problem might be another program which eats up your random bytes (a program (look at your daemons) that reads from /dev/random).

And it really takes long when I work on a remote system. Why?

Don't do this at all! You should never create keys or even use GnuPG on a remote system because you normally have no physical control over your secret key ring (which is in most cases vulnerable to advanced dictionary attacks) - I strongly encourage everyone to only create keys on a local computer (a disconnected laptop is probably the best choice) and if you need it on your connected box (I know, we all do this) be sure to have a strong password for both your account and for your secret key, and that you can trust your system administrator.

When I check GnuPG on a remote system via ssh (I have no Alpha here) ;-) I have the same problem. It takes a very long time to create the keys, so I use a special option, –quick-random, to generate insecure keys which are only good for some tests.

What is the difference between options and commands?

If you do a 'gpg –help', you will get two separate lists. The first is a list of commands. The second is a list of options. Whenever you run GPG, you must pick exactly one command (with one exception, see below). You may pick one or more options. The command should, just by convention, come at the end of the argument list, after all the options. If the command takes a file (all the basic ones do), the filename comes at the very end. So the basic way to run gpg is:

$ gpg [--option something] [--option2] [--option3 something] --command file

Some options take arguments. For example, the –output option (which can be abbreviated as -o) is an option that takes a filename. The option's argument must follow immediately after the option itself, otherwise gpg doesn't know which option the argument is supposed to paired with. As an option, –output and its filename must come before the command. The –recipient (-r) option takes a name or keyID to encrypt the message to, which must come right after the -r option. The –encrypt (or -e) command comes after all the options and is followed by the file you wish to encrypt. Therefore in this example the command-line issued would be:

$ gpg -r alice -o secret.txt -e test.txt

If you write the options out in full, it is easier to read:

$ gpg --recipient alice --output secret.txt --encrypt test.txt

If you're encrypting to a file with the extension ".txt", then you'd probably expect to see ASCII-armored text in the file (not binary), so you need to add the –armor (-a) option, which doesn't take any arguments:

$ gpg --armor --recipient alice --output secret.txt --encrypt test.txt

If you imagine square brackets around the optional parts, it becomes a bit clearer:

$ gpg [--armor] [--recipient alice] [--output secret.txt] --encrypt test.txt

The optional parts can be rearranged any way you want:

$ gpg --output secret.txt --recipient alice --armor --encrypt test.txt

If your filename begins with a hyphen (e.g. "-a.txt"), GnuPG assumes this is an option and may complain. To avoid this you have to either use ./-a.txt, or stop the option and command processing with two hyphens: -- -a.txt.

The exception to using only one command: signing and encrypting at the same time. For this you can combine both commands, such as in:

$ gpg [--options] --sign --encrypt foo.txt

I can't delete a user ID on my secret keyring because it has already been deleted on my public keyring. What can I do?

Because you can only select from the public key ring, there is no direct way to do this. However it is not very complicated to do anyway. Create a new user ID with exactly the same name and you will see that there are now two identical user IDs on the secret ring. Now select this user ID and delete it. Both user IDs will be removed from the secret ring.

I can't delete my secret key because the public key disappeared. What can I do?

To select a key a search is always done on the public keyring, therefore it is not possible to select a secret key without having the public key. Normally it should never happen that the public key got lost but the secret key is still available. The reality is different, so GnuPG implements a special way to deal with it: Simply use the long keyID to specify the key to delete, which can be obtained by using the –with-colons options (it is the fifth field in the lines beginning with "sec").

If you've lost your public key and need to recreate it instead for continued use with your secret key, you may be able to use gpgsplit as detailed in question I still have my secret key, but lost my public key. What can I do?.

What are trust, validity and ownertrust?

With GnuPG, the term "ownertrust" is used instead of "trust" to help clarify that this is the value you have assigned to a key to express how much you trust the owner of this key to correctly sign (and thereby introduce) other keys. The "validity", or calculated trust, is a value which indicates how much GnuPG considers a key as being valid (that it really belongs to the one who claims to be the owner of the key). For more information on trust values see the chapter "The Web of Trust" in The GNU Privacy Handbook.

How do I sign a patch file?

Use "gpg –clearsign –not-dash-escaped …". The problem with –clearsign is that all lines starting with a dash are quoted with "- "; obviously diff produces many lines starting with a dash and these are then quoted and that is not good for a patch ;-). To use a patch file without removing the cleartext signature, the special option –not-dash-escaped may be used to suppress generation of these escape sequences. You should not mail such a patch because spaces and line endings are also subject to the signature and a mailer may not preserve these. If you want to mail a file you can simply sign it using your MUA (Mail User Agent).

Where is the "encrypt-to-self" option?

Use "–encrypt-to your_keyID". You can use more than one of these options. To temporarily override the use of this additional key, you can use the option "–no-encrypt-to".

How can I get rid of the Version and Comment headers in armored messages?

Use

--no-version --comment ''

Note that the left over blank line is required by the protocol.

What does the "You are using the xxxx character set." mean?

This note is printed when UTF-8 mapping has to be done. Make sure that the displayed character set is the one you have activated on your system. Since "iso-8859-1" is the character set most used, this is the default. You can change the charset with the option --charset. It is important that your active character set matches the one displayed — if not, restrict yourself to plain 7 bit ASCII and no mapping has to be done.

How can I get list of key IDs used to encrypt a message? 

$ gpg --batch --decrypt --list-only --status-fd 1 2>/dev/null | \
  awk '/^\[GNUPG:\] ENC_TO / { print $3 }'

Why can't I decrypt files encrypted as symmetrical-only (-c) with a version of GnuPG prior to 1.0.1.

There was a bug in GnuPG versions prior to 1.0.1 which affected files only if 3DES or Twofish was used for symmetric-only encryption (this has never been the default). The bug has been fixed, but to enable decryption of old files you should run gpg with the option --emulate-3des-s2k-bug, decrypt the file and encrypt it again without this option.

NOTE: This option was removed in GnuPG development version 1.1.0 and later updates, so you will need to use a version between 1.0.1 and 1.0.7 to re-encrypt any affected files.

How can I use GnuPG in an automated environment?

You should use the option --batch and don't use passphrases as there is usually no way to store it more securely than on the secret keyring itself. The suggested way to create keys for an automated environment is:

On a secure machine:

  1. If you want to do automatic signing, create a signing subkey for your key. Use the interactive key editing menu by issuing the command

    gpg --edit-key keyID

    enter "addkey" and select the DSA key type).

  2. Make sure that you use a passphrase (needed by the current implementation).
  3.  gpg --export-secret-subkeys --no-comment foo >secring.auto
  4. Copy secring.auto and the public keyring to a test directory.
  5. Change to this directory.

  6. Run the command

    gpg --homedir . --edit foo

    and use the sub-command passwd to remove the passphrase from the subkeys. You may also want to remove all unused subkeys.

  7. Copy secring.auto to a floppy and carry it to the target box.

On the target machine:

  1. Install secring.auto as the secret keyring.
  2. Now you can start your new service. It's also a good idea to install an intrusion detection system so that you hopefully get a notice of an successful intrusion, so that you in turn can revoke all the subkeys installed on that machine and install new subkeys.

Which email-client can I use with GnuPG?

Using GnuPG to encrypt email is one of the most popular uses. Several mail clients or mail user agents (MUAs) support GnuPG to varying degrees. Simplifying a bit, there are two ways mail can be encrypted with GnuPG: the "old style" ASCII armor (i.e. cleartext encryption), and RFC 2015 style (previously PGP/MIME, now OpenPGP). The latter has full MIME support. Some MUAs support only one of them, so whichever you actually use depends on your needs as well as the capabilities of your addressee. As well, support may be native to the MUA, or provided via "plug-ins" or external tools.

The following list is not exhaustive:

MUA OpenPGP ASCII How? (N,P,T)
Calypso N Y P (Unixmail)
Elm N Y T (mailpgp,morepgp)
Elm ME+ N Y N
Emacs/Gnus Y Y T (Mailcrypt,gpg.el)
Emacs/Mew Y Y N
Emacs/VM N Y T (Mailcrypt)
Evolution Y Y N
Exmh Y Y N
GNUMail.app Y Y P (PGPBundle)
GPGMail Y Y N
KMail (<=1.4.x) N Y N
KMail (1.5.x) Y(P) Y(N) P/N
Mozilla Y Y P (Enigmail)
Mulberry Y Y P
Mutt Y Y N
Sylpheed Y Y N
Claws-mail Y Y N
TkRat Y Y N
XEmacs/Gnus Y Y T (Mailcrypt)
XEmacs/Mew Y Y N
XEmacs/VM N Y T (Mailcrypt)
XFmail Y Y N

( N - Native, P - Plug-in, T - External Tool)

The following table lists proprietary MUAs. The GNU Project suggests against the use of these programs, but they are listed for interoperability reasons for your convenience.

MUA OpenPGP ASCII How? (N,P,T)
Apple Mail Y Y P (GPGMail)
Becky2 Y Y P (BkGnuPG)
Eudora Y Y P (EuroraGPG)
Eudora Pro Y Y P (EudoraGPG)
Lotus Notes N Y P
Netscape 4.x N Y P
Netscape 7.x Y Y P (Enigmail)
Novell Groupwise N Y P
Outlook N Y P (G-Data)
Outlook Express N Y P (GPGOE)
Pegasus N Y P (QDPGP,PM-PGP)
Pine N Y T (pgpenvelope,gpg4pine)
Postme N Y P (GPGPPL)
The Bat! N Y P (Ritlabs)

Good overviews of OpenPGP-support can be found at:
http://www.openpgp.fr.st/courrier_en.html
http://www.bretschneidernet.de/tips/secmua.html

Users of Win32 MUAs that lack OpenPGP support may look into using GPGrelay http://gpgrelay.sourceforge.net, a small email-relaying server that uses GnuPG to enable many email clients to send and receive emails that conform to PGP-MIME (RFC 2015).

Can't we have a gpg library?

This has been frequently requested. However, the current viewpoint of the GnuPG maintainers is that this would lead to several security issues and will therefore not be implemented in the foreseeable future. However, for some areas of application gpgme could do the trick. You'll find it at gnupgweb:related_software/gpgme.

I have successfully generated a revocation certificate, but I don't understand how to send it to the key servers.

Most keyservers don't accept a 'bare' revocation certificate. You have to import the certificate into gpg first:

$ gpg --import my-revocation.asc

then send the revoked key to the keyservers:

$ gpg --keyserver certserver.pgp.com --send-keys mykeyid

(or use a keyserver web interface for this).

How do I put my keyring in a different directory?

GnuPG keeps several files in a special homedir directory. These include the options file, pubring.gpg, secring.gpg, trustdb.gpg, and others. GnuPG will always create and use these files. On unices, the homedir is usually ~/.gnupg; on Windows it is name "gnupg" and found below the user's application directory. Run the gpg and pass the option –version to see the name of that directory.

If you want to put your keyrings somewhere else, use the option:

--homedir /my/path/

to make GnuPG create all its files in that directory. Your keyring will be "/my/path/pubring.gpg". This way you can store your secrets on a floppy disk. Don't use "–keyring" as its purpose is to specify additional keyring files.

How do I verify signed packages?

must first have the vendor, organisation, or issuing person's key Before you can verify the signature that accompanies a package, you imported into your public keyring. To prevent GnuPG warning messages the key should also be validated (or locally signed).

You will also need to download the detached signature file along with the package. These files will usually have the same name as the package, with either a binary (.sig) or ASCII armor (.asc) extension.

Once their key has been imported, and the package and accompanying signature files have been downloaded, use:

$ gpg --verify sigfile signed-file

If the signature file has the same base name as the package file, the package can also be verified by specifying just the signature file, as GnuPG will derive the package's file name from the name given (less the .sig or .asc extension). For example, to verify a package named foobar.tar.gz against its detached binary signature file, use:

$ gpg --verify foobar.tar.gz.sig

How do I export a keyring with only selected signatures (keys)?

If you're wanting to create a keyring with only a subset of keys selected from a master keyring (for a club, user group, or company department for example), simply specify the keys you want to export:

$ gpg --armor --export key1 key2 key3 key4 > keys1-4.asc

I still have my secret key, but lost my public key. What can I do?

All OpenPGP secret keys have a copy of the public key inside them, and in a worst-case scenario, you can create yourself a new public key using the secret key.

A tool to convert a secret key into a public one has been included (it's actually a new option for gpgsplit) and is available with GnuPG versions 1.2.1 or later (or can be found in CVS). It works like this:

$ gpgsplit --no-split --secret-to-public secret.gpg >publickey.gpg

One should first try to export the secret key and convert just this one. Using the entire secret keyring should work too. After this has been done, the publickey.gpg file can be imported into GnuPG as usual.

Clearsigned messages sent from my web-mail account have an invalid signature. Why?

Check to make sure the settings for your web-based email account do not use HTML formatting for the pasted clearsigned message. This can alter the message with embedded HTML markup tags or spaces, resulting in an invalid signature. The recipient may be able to copy the signed message block to a text file for verification, or the web email service may allow you to attach the clearsigned message as a file if plaintext messages are not an option.

Compatibility Issues

How can I encrypt a message with GnuPG so that PGP is able to decrypt it?

It depends on the PGP version.

PGP 2.x

You can't do that because PGP 2.x normally uses IDEA which is not supported by GnuPG as it is patented (see How do I include support for RSA and IDEA?), but if you have a modified version of PGP you can try this:

$ gpg --rfc1991 --cipher-algo 3des ...

Please don't pipe the data to encrypt to gpg but provide it using a filename; otherwise, PGP 2 will not be able to handle it.

As for conventional encryption, you can't do this for PGP 2.

PGP 5.x and higher

You need to provide two additional options:

--compress-algo 1 --cipher-algo cast5

You may also use "3des" instead of "cast5", and "blowfish" does not work with all versions of PGP 5. You may also want to put:

compress-algo 1

into your ~/.gnupg/options file — this does not affect normal GnuPG operation.

This applies to conventional encryption as well.

How do I migrate from PGP 2.x to GnuPG?

PGP 2 uses the RSA and IDEA encryption algorithms. Whereas the RSA patent has expired and RSA is included as of GnuPG 1.0.3, the IDEA algorithm is still patented until 2007. Under certain conditions you may use IDEA even today. In that case, you may refer to Question How do I include support for RSA and IDEA? about how to add IDEA support to GnuPG and read gnupgweb:gph/en/pgp2x.html to perform the migration.

Why is PGP 5.x not able to encrypt messages with some keys?

PGP, Inc. refuses to accept Elgamal keys of type 20 even for encryption. They only support type 16 (which is identical at least for decryption). To be more inter-operable, GnuPG (starting with version 0.3.3) now also uses type 16 for the Elgamal subkey which is created if the default key algorithm is chosen. You may add a type 16 Elgamal key to your public key, which is easy as your key signatures are still valid.

Why is PGP 5.x not able to verify my messages?

PGP 5.x does not accept v4 signatures for data material but OpenPGP requests generation of v4 signatures for all kind of data, that's why GnuPG defaults to them. Use the option "–force-v3-sigs" to generate v3 signatures for data.

How do I transfer owner trust values from PGP to GnuPG?

There is a script in the tools directory to help you. After you have imported the PGP keyring you can give this command:

$ lspgpot pgpkeyring | gpg --import-ownertrust

where pgpkeyring is the original keyring and not the GnuPG keyring you might have created in the first step.

PGP does not like my secret key.

Older PGPs probably bail out on some private comment packets used by GnuPG. These packets are fully in compliance with OpenPGP; however PGP is not really OpenPGP aware. A workaround is to export the secret keys with this command:

$ gpg --export-secret-keys --no-comment -a your-KeyID

Another possibility is this: by default, GnuPG encrypts your secret key using the Blowfish symmetric algorithm. Older PGPs will only understand 3DES, CAST5, or IDEA symmetric algorithms. Using the following method you can re-encrypt your secret gpg key with a different algo:

$ gpg --s2k-cipher-algo=CAST5 --s2k-digest-algo=SHA1 \
      --compress-algo=1  --edit-key <username>

Then use passwd to change the password (just change it to the same thing, but it will encrypt the key with CAST5 this time).

Now you can export it and PGP should be able to handle it.

For PGP 6.x the following options work to export a key:

$ gpg --s2k-cipher-algo 3des --compress-algo 1 --rfc1991 \
      --export-secret-keys <KeyID>

GnuPG no longer installs a ~/.gnupg/options file. Is it missing?

No. The ~/.gnupg/options file has been renamed to ~/.gnupg/gpg.conf for new installs as of version 1.1.92. If an existing ~/.gnupg/options file is found during an upgrade it will still be used, but this change was required to have a more consistent naming scheme with forthcoming tools. An existing options file can be renamed to gpg.conf for users upgrading, or receiving the message that the "old default options file" is ignored (occurs if both a gpg.conf and an options file are found).

How do you export GnuPG keys for use with PGP?

This has come up fairly often, so here's the HOWTO:

PGP can (for most key types) use secret keys generated by GnuPG. The problems that come up occasionally are generally because GnuPG supports a few more features from the OpenPGP standard than PGP does. If your secret key has any of those features in use, then PGP will reject the key or you will have problems communicating later. Note that PGP doesn't do Elgamal signing keys at all, so they are not usable with any version.

These instructions should work for GnuPG 1.0.7 and later, and PGP 7.0.3 and later.

Start by editing the key. Most of this line is not really necessary as the default values are correct, but it does not hurt to repeat the values, as this will override them in case you have something else set in your options file.

$ gpg --s2k-cipher-algo cast5 --s2k-digest-algo sha1 --s2k-mode 3 \
      --simple-sk-checksum --edit KeyID

Turn off some features. Set the list of preferred ciphers, hashes, and compression algorithms to things that PGP can handle. (Yes, I know this is an odd list of ciphers, but this is what PGP itself uses, minus IDEA).

> setpref S9 S8 S7 S3 S2 S10 H2 H3 Z1 Z0

Now put the list of preferences onto the key.

> updpref

Finally we must decrypt and re-encrypt the key, making sure that we encrypt with a cipher that PGP likes. We set this up in the –edit line above, so now we just need to change the passphrase to make it take effect. You can use the same passphrase if you like, or take this opportunity to actually change it.

> passwd

Save our work.

> save

Now we can do the usual export:

$ gpg --export KeyID > mypublickey.pgp[H br]
$ gpg --export-secret-key KeyID > mysecretkey.pgp

Thanks to David Shaw for this information!

What are DH/DSS keys?

PGP uses a different name for the former default encryption algorithm Elgamal: They name it DH, which usually stands for the Diffie-Hellman key exchange algorithm. It has been said that this had historic patent and business reasons. It is however exactly the same thing as the Elgamal algorithm.

They also use the acronym DSS (Digital Signature Standard) instead of the DSA (Digital Signature Algorithm). The difference is that DSS requires the use of certain hash algorithms; however OpenPGP allows the use of more than those hash algorithms, thus GPG usually uses the term DSA.

Problems and Error Messages

Why do I get "gpg: Warning: using insecure memory!"

You see this warning if GPG is not able to lock pages against being swapped out to disk.

However, on most modern system you should not see this message anymore because these systems allow any process to prevent a small number of memory pages from being swapped out to disk (using the mlock system call). Other (mostly older) systems don't allow this unless you install GPG as setuid(root).

Locking pages against being swapped out is not necessary if your system uses an encrypted swap partition. In fact that is the best way to protect sensitive data from ending up on a disk. If your system allows for encrypted swap partitions, please make use of that feature. Note that GPG does not know about encrypted swap partitions and might print the warning; thus you should disabled the warning if your swap partition is encrypted. You may also want to disable this warning if you can't or don't want to install GnuPG setuid(root). To disable the warning you put a line

no-secmem-warning

into your ~/.gnupg/gpg.conf file.

What follows is a short description on how to install GPG setuid(root); for those who need this.

On some systems this program should be installed as setuid(root). This is necessary to lock memory pages. Locking memory pages prevents the operating system from writing them to disk and thereby keeping your secret keys really secret. If you get no warning message about insecure memory your operating system supports locking without being root. The program drops root privileges as soon as locked memory is allocated.

To setuid(root) permissions on the gpg binary you can either use:

$ chmod u+s /path/to/gpg

or

$ chmod 4755 /path/to/gpg

Some refrain from using setuid(root) unless absolutely required for security reasons. Please check with your system administrator if you are not able to make these determinations yourself.

On UnixWare 2.x and 7.x you should install GnuPG with the 'plock' privilege to get the same effect:

$ filepriv -f plock /path/to/gpg

On some systems (e.g., Windows) GnuPG does not lock memory pages and older GnuPG versions (<=1.0.4) issue the warning:

gpg: Please note that you don't have secure memory

This warning can't be switched off by the above option because it was thought to be too serious an issue. However, it confused users too much, so the warning was eventually removed.

Large File Support doesn't work

LFS works correctly in post-1.0.4 versions. If configure doesn't detect it, try a different (i.e., better) compiler. egcs 1.1.2 works fine, other gccs sometimes don't. BTW, several compilation problems of GnuPG 1.0.3 and 1.0.4 on HP-UX and Solaris were due to broken LFS support.

In the edit menu the trust values are not displayed correctly after signing uids. Why?

This happens because some information is stored immediately in the trustdb, but the actual trust calculation can be done after the save command. This is a "not easy to fix" design bug which will be addressed in some future release.

What does "skipping pubkey 1: already loaded" mean?

As of GnuPG 1.0.3, the RSA algorithm is included. If you still have a "load-extension rsa" in your options file, the above message occurs. Just remove the load command from the options file.

GnuPG 1.0.4 doesn't create ~/.gnupg …

That's a known bug, already fixed in newer versions.

An Elgamal signature does not verify anymore since version 1.0.2

Use the option –emulate-md-encode-bug.

Old versions of GnuPG can't verify Elgamal signatures

Update to GnuPG 1.0.2 or newer.

When I use –clearsign, the plain text has sometimes extra dashes in it - why?

This is called dash-escaped text and is required by OpenPGP. It always happens when a line starts with a dash ("-") and is needed to make the lines that structure signature and text (i.e., "—–BEGIN PGP SIGNATURE—–") to be the only lines that start with two dashes.

If you use GnuPG to process those messages, the extra dashes are removed. Good mail clients remove those extra dashes when displaying such a message.

What is the thing with "can't handle multiple signatures"?

Due to different message formats GnuPG is not always able to split a file with multiple signatures unambiguously into its parts. This error message informs you that there is something wrong with the input.

The only way to have multiple signatures in a file is by using the OpenPGP format with one-pass-signature packets (which is GnuPG's default) or the cleartext signed format.

If I submit a key to a keyserver, nothing happens

You are most likely using GnuPG 1.0.2 or older on Windows. That's feature isn't yet implemented, but it's a bug not to say it. Newer versions issue a warning. Upgrade to 1.4.5 or newer.

I get "gpg: waiting for lock …"

A previous instance of gpg has most likely exited abnormally and left a lock file. Go to ~/.gnupg and look for .*.lock files and remove them.

Older gpg binaries (e.g., 1.0) have problems with keys from newer gpg binaries

As of 1.0.3, keys generated with gpg are created with preferences to TWOFISH (and AES since 1.0.4) and that also means that they have the capability to use the new MDC encryption method. This will go into OpenPGP soon, and is also suppoted by PGP 7. This new method avoids a (not so new) attack on all email encryption systems.

This in turn means that pre-1.0.3 gpg binaries have problems with newer keys. Because of security and bug fixes, you should keep your GnuPG installation in a recent state anyway. As a workaround, you can force gpg to use a previous default cipher algo by putting:

cipher-algo cast5

into your options file.

With 1.0.4, I get "this cipher algorithm is deprecated …"

If you just generated a new key and get this message while encrypting, you've witnessed a bug in 1.0.4. It uses the new AES cipher Rijndael that is incorrectly being referred as "deprecated". Ignore this warning, more recent versions of gpg are corrected.

Some dates are displayed as ????-??-??. Why?

Due to constraints in most libc implementations, dates beyond 2038-01-19 can't be displayed correctly. 64-bit OSes are not affected by this problem. To avoid printing wrong dates, GnuPG instead prints some question marks. To see the correct value, you can use the options –with-colons and –fixed-list-mode.

I still have a problem. How do I report a bug?

Are you sure that it's not been mentioned somewhere on the mailing lists? Did you have a look at the bug list (you'll find a link to the list of reported bugs on the documentation page). If you're not sure about it being a bug, you can send mail to the gnupg-devel list. Otherwise, use the bug tracking system bugs.gnupg.org.

Why doesn't GnuPG support X.509 certificates?

That is only the case for GnuPG version 1.x. GnuPG 2.x fully supports X.509 and S/MIME using the gpgsm tool.

Why do national characters in my user ID look funny?

According to OpenPGP, GnuPG encodes user ID strings (and other things) using UTF-8. In this encoding of Unicode, most national characters get encoded as two- or three-byte sequences. For example, &aring; (0xE5 in ISO-8859-1) becomes &Atilde;&yen; (0xC3, 0xA5). This might also be the reason why keyservers can't find your key.

I get 'sed' errors when running ./configure on Mac OS X …

This problem has been fixed for all modern GnuPG versions. (By using an autoconf 2.50 generated configure script).

Why does GnuPG 1.0.6 bail out on keyrings used with 1.0.7?

There is a small bug in 1.0.6 which didn't parse trust packets correctly. You may want to apply this patch if you can't upgrade: http://www.gnupg.org/developer/gpg-woody-fix.txt.

I upgraded to GnuPG version 1.0.7 and now it takes longer to load my keyrings. What can I do?

The way signature states are stored has changed so that v3 signatures can be supported. You can use the new –rebuild-keydb-caches migration command, which was built into this release and increases the speed of many operations for existing keyrings.

Doesn't a fully trusted user ID on a key prevent warning messages when encrypting to other IDs on the key?

No. That was actually a key validity bug in GnuPG 1.2.1 and earlier versions. As part of the development of GnuPG 1.2.2, a bug was discovered in the key validation code. This bug causes keys with more than one user ID to give all user IDs on the key the amount of validity given to the most-valid key. The bug has been fixed in GnuPG release 1.2.2, and upgrading is the recommended fix for this problem. More information and a patch for a some pre-1.2.2 versions of GnuPG can be found at:

http://lists.gnupg.org/pipermail/gnupg-announce/2003q2/000268.html.

I just compiled GnuPG from source on my GNU/Linux RPM-based system and it's not working. Why?

Many GNU/Linux distributions that are RPM-based will install a version of GnuPG as part of its standard installation, placing the binaries in the /usr/bin directory. Later, compiling and installing GnuPG from source other than from a source RPM won't normally overwrite these files, as the default location for placement of GnuPG binaries is in /usr/local/bin unless the '–prefix' switch is used during compile to specify an alternate location. Since the /usr/bin directory more than likely appears in your path before /usr/local/bin, the older RPM-version binaries will continue to be used when called since they were not replaced.

To resolve this, uninstall the RPM-based version with 'rpm -e gnupg' before installing the binaries compiled from source. If dependency errors are displayed when attempting to uninstall the RPM (such as when Red Hat's up2date is also installed, which uses GnuPG), uninstall the RPM with 'rpm -e gnupg –nodeps' to force the uninstall. Any dependent files should be automatically replaced during the install of the compiled version. If the default /usr/local/bin directory is used, some packages such as SuSE's Yast Online Update may need to be configured to look for GnuPG binaries in the /usr/local/bin directory, or symlinks can be created in /usr/bin that point to the binaries located in /usr/local/bin.

Advanced Topics

How does this whole thing work?

To generate a secret/public keypair, run:

$ gpg --generate-key

and choose the default values.

Data that is encrypted with a public key can only be decrypted by the matching secret key. The secret key is protected by a password, the public key is not.

So to send your friend a message, you would encrypt your message with his public key, and he would only be able to decrypt it by having the secret key and putting in the password to use his secret key.

GnuPG is also useful for signing things. Files that are encrypted with the secret key can be decrypted with the public key. To sign something, a hash is taken of the data, and then the hash is in some form encoded with the secret key. If someone has your public key, they can verify that it is from you and that it hasn't changed by checking the encoded form of the hash with the public key.

A keyring is just a large file that stores keys. You have a public keyring where you store yours and your friend's public keys. You have a secret keyring that you keep your secret key on, and should be very careful with. Never ever give anyone else access to it and use a good passphrase to protect the data in it.

You can 'conventionally' encrypt something by using the option 'gpg -c'. It is encrypted using a passphrase, and does not use public and secret keys. If the person you send the data to knows that passphrase, they can decrypt it. This is usually most useful for encrypting things to yourself, although you can encrypt things to your own public key in the same way. It should be used for communication with partners you know and where it is easy to exchange the passphrases (e.g. with your boy friend or your wife). The advantage is that you can change the passphrase from time to time and decrease the risk, that many old messages may be decrypted by people who accidentally got your passphrase.

You can add and copy keys to and from your keyring with the 'gpg –import' and 'gpg –export' command. 'gpg –export-secret-keys' will export secret keys. This is normally not useful, but you can generate the key on one machine then move it to another machine.

Keys can be signed under the 'gpg –edit-key' option. When you sign a key, you are saying that you are certain that the key belongs to the person it says it comes from. You should be very sure that is really that person: You should verify the key fingerprint with:

$ gpg --fingerprint KeyID

over the phone (if you really know the voice of the other person), at a key signing party (which are often held at computer conferences), or at a meeting of your local GNU/Linux User Group.

Hmm, what else. You may use the option '-o filename' to force output to this filename (use '-' to force output to stdout). '-r' just lets you specify the recipient (which public key you encrypt with) on the command line instead of typing it interactively.

Oh yeah, this is important. By default all data is encrypted in some weird binary format. If you want to have things appear in ASCII text that is readable, just add the '-a' option. But the preferred method is to use a MIME aware mail reader (Mutt, Pine and many more).

There is a small security glitch in the OpenPGP (and therefore GnuPG) system; to avoid this you should always sign and encrypt a message instead of only encrypting it.

Why are some signatures with an ELG-E key valid?

These are Elgamal keys generated by GnuPG in v3 (RFC 1991) packets. The OpenPGP draft later changed the algorithm identifier for Elgamal keys which are usable for signatures and encryption from 16 to 20. GnuPG now uses 20 when it generates new Elgamal keys but still accepts 16 (which is according to OpenPGP "encryption only") if this key is in a v3 packet. GnuPG is the only program which had used these v3 Elgamal keys - so this assumption is quite safe.

How does the whole trust thing work?

It works more or less like PGP. The difference is that the trust is computed at the time it is needed. This is one of the reasons for the trustdb which holds a list of valid key signatures. If you are not running in batch mode you will be asked to assign a trust parameter (ownertrust) to a key.

You can see the validity (calculated trust value) using this command.

$ gpg --list-keys --with-colons

If the first field is "pub" or "uid", the second field shows you the trust:

  o = Unknown (this key is new to the system)
  e = The key has expired
  q = Undefined (no value assigned)
  n = Don't trust this key at all
  m = There is marginal trust in this key
  f = The key is full trusted
  u = The key is ultimately trusted; this is only used
      for keys for which the secret key is also available.
  r = The key has been revoked
  d = The key has been disabled

The value in the "pub" record is the best one of all "uid" records. You can get a list of the assigned trust values (how much you trust the owner to correctly sign another person's key) with:

$ gpg --export-ownertrust

The first field is the fingerprint of the primary key, the second field is the assigned value:

 - = No ownertrust value yet assigned or calculated.
 n = Never trust this keyholder to correctly verify others signatures.
 m = Have marginal trust in the keyholders capability to sign other
     keys.
 f = Assume that the key holder really knows how to sign keys.
 u = No need to trust ourself because we have the secret key.

Keep these values confidential because they express your opinions about others. PGP stores this information with the keyring thus it is not a good idea to publish a PGP keyring instead of exporting the keyring. GnuPG stores the trust in the trustdb.gpg file so it is okay to give a gpg keyring away (but we have a –export command too).

What kind of output is this: "key C26EE891.298, uid 09FB: …."?

This is the internal representation of a user ID in the trustdb. "C26EE891" is the keyid, "298" is the local ID (a record number in the trustdb) and "09FB" is the last two bytes of a ripe-md-160 hash of the user ID for this key.

How do I interpret some of the informational outputs?

While checking the validity of a key, GnuPG sometimes prints some information which is prefixed with information about the checked item.

"key 12345678.3456"

This is about the key with key ID 12345678 and the internal number 3456, which is the record number of the so called directory record in the trustdb.

"uid 12345678.3456/ACDE"

This is about the user ID for the same key. To identify the user ID the last two bytes of a ripe-md-160 over the user ID ring is printed.

"sig 12345678.3456/ACDE/9A8B7C6D"

This is about the signature with key ID 9A8B7C6D for the above key and user ID, if it is a signature which is direct on a key, the user ID part is empty (..//..).

Are the header lines of a cleartext signature part of the signed material?

No. For example you can add or remove "Comment:" lines. They have a purpose like the mail header lines. However a "Hash:" line is needed for OpenPGP signatures to tell the parser which hash algorithm to use.

What is the list of preferred algorithms?

The list of preferred algorithms is a list of cipher, hash and compression algorithms stored in the self-signature of a key during key generation. When you encrypt a document, GnuPG uses this list (which is then part of a public key) to determine which algorithms to use. Basically it tells other people what algorithms the recipient is able to handle and provides an order of preference.

How do I change the list of preferred algorithms?

In version 1.0.7 or later, you can use the edit menu and set the new list of preference using the command "setpref"; the format of this command resembles the output of the command "pref". The preference is not changed immediately but the set preference will be used when a new user ID is created. If you want to update the preferences for existing user IDs, select those user IDs (or select none to update all) and enter the command "updpref". Note that the timestamp of the self-signature is increased by one second when running this command.

How can I import all the missing signer keys?

If you imported a key and you want to also import all the signer's keys, you can do this with this command:

 gpg --check-sigs --with-colon KEYID \
  | awk -F: '$1 == "sig" && $2 == "?"  { print $5 }' \
  | sort | uniq | xargs echo gpg --recv-keys

Note that the invocation of sort is also required to wait for the of the listing before starting the import.

Bug reporting and hacking

Copyright assignments

Like most core GNU projects, GnuPG requires the signing of a copyright assignment to the FSF. Without such an assignment we may only accept trivial patches. As a rule of thumb the sum of all changed lines by one contributor may not exceed about 15 lines. Exceptions are typo corrections and translations. See http://www.gnu.org/prep/maintain/html_node/Copyright-Papers.html for details.

U.S. export restrictions

GnuPG has originally been developed in Germany because we have been able to do that without being affected by the US export restrictions. We had to reject any contributions from US citizens or from people living in the US. That changed by end of 2000 when the export restrictions were basically dropped for all kind of freely available software. However there are still some requirements in the US. Quoting David Shaw: mail

For each release of GPG that I contributed to, I sent an email containing a pointer to the new source code to the Commerce Department. The rules changed slightly in 2004, so that you could send a single email and then be done until the information in that email changed, so I just sent "www.gnupg.org" and haven't bothered with the email since.

The rules: http://www.bis.doc.gov/encryption/pubavailencsourcecodenofify.html The 2004 rule change: http://edocket.access.gpo.gov/2004/04-26992.htm

Acknowledgements

Many thanks to Nils Ellmenreich for maintaining this FAQ file for such a long time, David D. Scribner for continuing maintenance, Werner Koch for the original FAQ file, and to all posters to gnupg-users and gnupg-devel. They all provided most of the answers. Converted to org-mode and removed from the tarballs in October 2010.

Copyright (C) 2000, 2001, 2002, 2003, 2010 Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02111, USA

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.

Changes

  • 2010-11-14: Update "gpg: Warning: using insecure memory!"

COMMENT HTML style specifications 

  (defun org-faq-make-target ()
    "Make hard target for current headline."
    (interactive)
    (if (not (org-on-heading-p))
        (error "Not on a headline"))
    (let ((h (org-trim (org-get-heading 'no-tags))))
      (if (string-match "[ \t]*\\?\\'" h)
          (setq h (replace-match "" t t h)))
      (while (string-match "[ \t]+" h)
        (setq h (replace-match "-" t t h)))
      (setq h (downcase h))
      (org-entry-put nil "CUSTOM_ID" h)))