GNUPG FREQUENTLY ASKED QUESTIONS

Version: 0.1
Last-Modified: Sep 14, 2000
Maintained-by: Nils Ellmenreich <nils 'at' infosun.fmi.uni-passau.de>

This is the GnuPG FAQ. The latest HTML version is available here.

The index is generated automatically, so there may be errors here. 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 maintainer. 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. Otherwise, please provide the answer to be included here.


1. GENERAL

  • 1.1) What is GnuPG?
  • 1.2) Is GnuPG compatible with PGP?

    2. SOURCES OF INFORMATION

  • 2.1) Where can I find more information?
  • 2.2) Where do I get GnuPG?

    3. INSTALLATION

  • 3.1) Which OSes does GnuPG run on?
  • 3.2) Which random gatherer should I use?
  • 3.3) How do I include support for RSA and IDEA?

    4. USAGE

  • 4.1) What is the recommended key size?
  • 4.2) Why does it sometimes take so long to create keys?
  • 4.3) And it really takes long when I work on a remote system. Why?
  • 4.4) What is the difference between options and commands?
  • 4.5) I can't delete an user id because it is already deleted on my public keying?
  • 4.6) What are trust, validity and ownertrust?
  • 4.7) How do I sign a patch file?
  • 4.8) Where is the "encrypt-to-self" option?
  • 4.9) How can I get rid of the Version and Comment headers in armored messages?
  • 4.10) What does the "You are using the xxxx character set." mean?
  • 4.11) How can a get list of key IDs used to encrypt a message?
  • 4.12) I can't decrypt my symmetrical only (-c) encrypted message with a new version of GnuPG.
  • 4.13) How can I used GnuPG in an automated environment?

    5. COMPATIBILITY ISSUES

  • 5.1) How can I encrypt a message so that pgp 2.x is able to decrypt it?
  • 5.2) How can I conventional encrypt a message, so that PGP can decrypt it?
  • 5.3) Why is PGP 5.x not able to encrypt messages with some keys?
  • 5.4) Why is PGP 5.x not able to verify my messages?
  • 5.5) How do I transfer owner trust values from PGP to GnuPG?
  • 5.6) PGP 5.x, 6.x do not like my secret key.

    6. PROBLEMS and ERROR MESSAGES

  • 6.1) Why do I get "gpg: Warning: using insecure memory!"
  • 6.2) In the edit menu the trust values is not displayed correctly after signing uids - why?
  • 6.3) An ElGamal signature does not verify anymore since version 1.0.2 ...
  • 6.4) Old versions of GnuPG can't verify ElGamal signatures
  • 6.5) When I use --clearsign, the plain text has sometimes extra dashes in it - why?

    7. ADVANCED TOPICS

  • 7.1) How does this whole thing work?
  • 7.2) Why are some signatures with an ELG-E key valid?
  • 7.3) How does the whole trust thing work?
  • 7.4) What kind of output is this: "key C26EE891.298, uid 09FB: ...."?
  • 7.5) How do I interpret some of the informational outputs?
  • 7.6) Are the header lines of a cleartext signature part of the signed material?

    8. ACKNOWLEDGEMENTS


  • 1. GENERAL

    1.1) 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 2440. As such, it is aimed to be compatible with PGP from NAI Inc.

    1.2) 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 questions 5.1ff. for details.

    2. SOURCES OF INFORMATION

    2.1) Where can I find more information?

    Here's a list of on-line resources:

    2.2) Where do I get GnuPG?

    You can download the GNU Privacy Guard from it's primary FTP server ftp.gnupg.org or from one of the mirrors: <http://www.gnupg.org/mirror.html>

    3. INSTALLATION

    3.1) Which OSes does GnuPG run on?

    It should run on most Unices as well as Windows 95 and Windows NT. A list of OSes reported to be OK is presented at http://www.gnupg.org/gnupg.html#supsys .

    3.2) Which random 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
    .

    On other systems, the Entropy Gathering Daemon (EGD) is a good choice. It is a perl-daemon that monitors system activity nad hashes it into random data. See the download page <http://www.gnupg.org/download.html> how 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.

    3.3) How do I include support for RSA and IDEA?

    The official GnuPG distribution (as of 1.0.2) does not contain either of them due to patents restriction. The RSA patent expires Sept 20, 2000. A new GnuPG release is then scheduled to include it. The IDEA patent does not expire before 2007 so don't expect official support before then.

    However, there are unofficial modules to include both of them even in earlier version of GnuPG. They're available from <ftp://ftp.gnupg.org/pub/gcrypt/contrib/>. Look for

    idea.c
    and
    rsa.c
    . Compilation directives are in the headers of these files. Then add the following lines to your ~/.gnupg/options:
    load-extension idea 
    load-extension rsa
    

    These extensions are not available for the Windows version of GnuPG.

    4. USAGE

    4.1) 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 than check the fingerprint of this key: "gpg --fingerprint --fingerprint <user ID>".

    4.2) 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 capslock 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/[u]random).

    4.3) 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 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.

    4.4) 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 -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 go 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 argument. The --encrypt (or -e) command comes after all the options followed by the file you wish to encrypt. So use

     
    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 saving it in a file called ".txt" then you'd probably expect to see ASCII-armored text in there, 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 either to use "./-a.txt" or stop the option and command processing with two hyphens: "-- -a.txt". The exception: signing and encrypting at the same time. Use

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

    4.5) I can't delete an user id because it is already deleted on my public keying?

    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 it 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.

    4.6) What are trust, validity and ownertrust?

    "ownertrust" is used instead of "trust" to make clear 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 so introduce) other keys. "validity", or calculated trust, is a value which says how much GnuPG thinks a key is valid (that it really belongs to the one who claims to be the owner of the key). For more see the chapter "The Web of Trust" in the Manual.

    4.7) 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 of lines starting with a dash and these are then quoted and that is not good for 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.

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

    Use "--encrypt-to your_keyid". You can use more than one of these options. To temporary override the use of this additional keys, you can use the option "--no-encrypt-to".

    4.9) 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.

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

    This note is printed when UTF8 mapping has to be done. Make sure that the displayed charset is the one you have activated on your system "iso-8859-1" is the most used one, so this is the default. You can change the charset with the option "--charset". It is important that you active character set matches the one displayed - if not, restrict yourself to plain 7 bit ASCII and no mapping has to be done.

    4.11) How can a 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 }' 

    4.12) I can't decrypt my symmetrical only (-c) encrypted message with a new version of GnuPG.

    There used to be a bug in GnuPG < 1.0.1 which happens only if 3DES or Twofish has been used for symmetric only encryption (this has never been the default). The bug has been fixed but to enable you to decrypt old messages, you should run gpg with the option "--emulate-3des-s2k-bug", decrypt the message and encrypt it again without this option. The option will be removed in 1.1, so better re-encrypt your message now.

    4.13) How can I used GnuPG in an automated environment?

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

    On a secure machine:

    1. If you want to do automatic signing, create a signing subkey for your key (edit menu, choose "addkey" and the DSA). [H LI] Make sure that you use a passphrase (Needed by the current implementation)
    2. gpg --export-secret-subkeys --no-comment foo >secring.auto
    3. Copy secring.auto and the public keyring to a test directory.
    4. Cd to this directory.
    5. gpg --homedir . --edit foo and use "passwd" to remove the pass-phrase from the subkeys. You may also want to remove all unused subkeys.
    6. copy secring.auto to a floppy and carry it to the target box
    On the target machine:
    1. Install secring.auto as secret keyring.
    2. Now you can start your new service. It is a good idea to install some 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.

    5. COMPATIBILITY ISSUES

    5.1) How can I encrypt a message so that pgp 2.x is able to decrypt it?

    You can't do that because pgp 2.x normally uses IDEA which is not supported by GnuPG because it is patented, 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 give it as a filename; otherwise, pgp 2 will not be able to handle it.

    5.2) How can I conventional encrypt a message, so that PGP can decrypt it?

    You can't do this for PGP 2. For PGP 5 you should use this:

    gpg -c --cipher-algo 3des --compress-algo 1 myfile 
    

    You may replace "3des" by "cast5". "blowfish" does not work with all versions of pgp5. You may also want to put

     compress-algo 1
    
    into your ~/.gnupg/options file - this does not affect normal gnupg operation.

    5.3) 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 an type 16 ElGamal key to your public key which is easy as your key signatures are still valid.

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

    PGP 5.x does not accept V4 signatures for data material but OpenPGP requires generation of V4 signatures for all kind of data. Use the option "--force-v3-sigs" to generate V3 signatures for data.

    5.5) 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 one you might have created in the first step.

    5.6) PGP 5.x, 6.x do not like my secret key.

    PGP probably bails 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-key-id 

    6. PROBLEMS and ERROR MESSAGES

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

    On many 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 memory pages 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.

    If you can't or don't want to install GnuPG setuid(root), you can use the option "--no-secmem-warning" or put

    no-secmem-warning 
    in your ~/.gnupg/options file.

    6.2) In the edit menu the trust values is not displayed correctly after signing uids - why?

    This happens because the some informations are 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.

    6.3) An ElGamal signature does not verify anymore since version 1.0.2 ...

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

    6.4) Old versions of GnuPG can't verify ElGamal signatures

    Update to GnuPG 1.0.2 or newer.

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

    This is called dash-escaped text and required by OpenPGP. It always happens when a line starts with a dash ("-") and is needed to distinguish those lines from the thos lines which make up such a clearsigned message.

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

    7. ADVANCED TOPICS

    7.1) How does this whole thing work?

    To generate a secret/public keypair, run

     gpg --gen-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. Things 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 be very careful with this secret keyring: 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 accidently got your passphrase. You can add and copy keys to and from your keyring with the 'gpg --import' and 'gpg --export' option. '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

        gpg --fingerprint user-id
    
    over phone (if you really know the voice of the other person) or 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.

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

    These are ElGamal Key generated by GnuPG in v3 (rfc1991) 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 accept 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.

    7.3) 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)

     gpg --list-ownertrust 
    The first field is the fingerprint of the primary key, the second field is the assigned value:

     
     - = No Ownertrust value yet assigned.  
     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 trust-DB so it is okay to give a gpg keyring away (but we have a --export command too).

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

    This is the internal representation of an 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.

    7.5) 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 (..//..).

    7.6) 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.

    8. ACKNOWLEDGEMENTS

    Many thanks to Werner Koch for the original FAQ file and to all posters to gnupg-users and gnupg-devel. They all provided most of the answers.

    Also thanks to Casper Dik for providing me with a script to generate this FAQ (he uses it for the excellent Solaris2 FAQ).


    Copyright (C) 2000 Free Software Foundation, Inc. , 59 Temple Place - Suite 330, Boston, MA 02111, USA

    Verbatim copying and distribution of this entire article is permitted in any medium, provided this notice is preserved.