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GNU Privacy Guard -- Frequently Asked Questions
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=================================================
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This FAQ is partly compiled from messages of the developers mailing list.
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Many thanks to Kirk Fort, Brian Warner, ...
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Q: How does this whole thing work?
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A: To generate a secret/public keypair, run
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and choose the default values.
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Data that is encrypted with a public key can only be decrypted by the
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matching secret key. The secret key is protected by a password, the
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So to send your friend a message, you would encrypt your message with his
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public key, and he would only be able to decrypt it by having the secret
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key and putting in the password to use his secret key.
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GnuPG is also useful for signing things. Things that are encrypted with
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the secret key can be decrypted with the public key. To sign something, a
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hash is taken of the data, and then the hash is in some form encoded with
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the secret key. If someone has your public key, they can verify that it
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is from you and that it hasn't changed by checking the encoded form of
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the hash with the public key.
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A keyring is just a large file that stores keys. You have a public keyring
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where you store yours and your friend's public keys. You have a secret
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keyring that you keep your secret key on, and be very careful with this
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secret keyring: Never ever give anyone else access to it and use a *good*
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passphrase to protect the data in it.
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You can 'conventionally' encrypt something by using the option 'gpg -c'.
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It is encrypted using a passphrase, and does not use public and secret
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keys. If the person you send the data to knows that passphrase, they can
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decrypt it. This is usually most useful for encrypting things to
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yourself, although you can encrypt things to your own public key in the
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same way. It should be used for communication with partners you know and
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where it is easy to exchange the passphrases (e.g. with your boy friend or
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your wife). The advantage is that you can change the passphrase from time
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to time and decrease the risk, that many old messages may be decrypted by
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people who accidently got your passphrase.
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You can add and copy keys to and from your keyring with the 'gpg --import'
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and 'gpg --export' option. 'gpg --export-secret-keys' will export secret
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keys. This is normally not useful, but you can generate the key on one
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machine then move it to another machine.
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Keys can be signed under the 'gpg --edit-key' option. When you sign a
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key, you are saying that you are certain that the key belongs to the
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person it says it comes from. You should be very sure that is really
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that person: You should verify the key fingerprint
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gpg --fingerprint user-id
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over phone (if you really know the voice of the other person) or at
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a key signing party (which are often held at computer conferences)
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or at a meeting of your local GNU/Linux User Group.
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Hmm, what else. You may use the option "-o filename" to force output
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to this filename (use "-" to force output to stdout). "-r" just lets you
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specify the recipient (which public key you encrypt with) on the command
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line instead of typing it interactively.
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Oh yeah, this is important. By default all data is encrypted in some weird
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binary format. If you want to have things appear in ASCII text that is
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readable, just add the '-a' option. But the preferred method is to use
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a MIME aware mail reader (Mutt, Pine and many more).
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There is a small security glitch in the OpenPGP (and therefore GnuPG) system;
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to avoid this you should always sign and encrypt a message instead of only
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Q: What is the recommended key size?
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A: 1024 bit for DSA signatures; even for plain ElGamal
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signatures this is sufficient as the size of the hash
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is probably the weakest link if the keysize is larger
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than 1024 bits. Encryption keys may have greater sizes,
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but you should than check the fingerprint of this key:
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"gpg --fingerprint --fingerprint <user ID>".
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Q: Why are some signatures with an ELG-E key valid?
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A: These are ElGamal Key generated by GnuPG in v3 (rfc1991)
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packets. The OpenPGP draft later changed the algorithm
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identifier for ElGamal keys which are usable for signatures
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and encryption from 16 to 20. GnuPG now uses 20 when it
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generates new ElGamal keys but still accept 16 (which is
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according to OpenPGP "encryption only") if this key is in
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a v3 packet. GnuPG is the only program which had used
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these v3 ElGamal keys - so this assumption is quite safe.
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Q: Why is PGP 5.x not able to encrypt messages with some keys?
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A: PGP Inc refuses to accept ElGamal keys of type 20 even for
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encryption. They only support type 16 (which is identical
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at least for decryption). To be more inter-operable, GnuPG
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(starting with version 0.3.3) now also uses type 16 for the
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ElGamal subkey which is created if the default key algorithm
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is chosen. You may add an type 16 ElGamal key to your public
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key which is easy as your key signatures are still valid.
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Q: Why is PGP 5.x not able to verify my messages?
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A: PGP 5.x does not accept V4 signatures for data material but
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OpenPGP requires generation of V4 signatures for all kind of
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data. Use the option "--force-v3-sigs" to generate V3 signatures
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Q: I can't delete an user id because it is already deleted on my
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A: Because you can only select from the public key ring, there is
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no direct way to do this. However it is not very complicated
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to do it anyway. Create a new user id with exactly the same name
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and you will see that there are now two identical user ids on the
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secret ring. Now select this user id and delete it. Both user
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ids will be removed from the secret ring.
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Q: How can I encrypt a message so that pgp 2.x is able to decrypt it?
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A: You can't do that because pgp 2.x normally uses IDEA which is not
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supported by GnuPG because it is patented, but if you have a modified
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version of PGP you can try this:
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gpg --rfc1991 --cipher-algo 3des ...
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Please don't pipe the data to encrypt to gpg but give it as a filename;
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other wise, pgp 2 will not be able to handle it.
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Q: How can I conventional encrypt a message, so that PGP can decrypt it?
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A: You can't do this for PGP 2. For PGP 5 you should use this:
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gpg -c --cipher-algo 3des --compress-algo 1 myfile
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You may replace "3des" by "cast5". "blowfish" does not work with
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all versions of pgp5. You may also want to put
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into your ~/.gnupg/options file - this does not affect normal
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Q: Why does it sometimes take so long to create keys?
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A: The problem here is that we need a lot of random bytes and for that
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we (on Linux the /dev/random device) must collect some random data.
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It is really not easy to fill the Linux internal entropy buffer; I
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talked to Ted Ts'o and he commented that the best way to fill the buffer
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is to play with your keyboard. Good security has it's price. What I do
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is to hit several times on the shift, control, alternate, and capslock
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keys, because these keys do not produce output to the screen. This way
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you get your keys really fast (it's the same thing pgp2 does).
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Another problem might be another program which eats up your random bytes
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(a program (look at your daemons) that reads from /dev/[u]random).
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Q: And it really takes long when I work on a remote system. Why?
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A: Don't do this at all! You should never create keys or even use GnuPG
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on a remote system because you normally have no physical control over
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your secret keyring (which is in most cases vulnerable to advanced
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dictionary attacks) - I strongly encourage everyone to only create keys
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on a local computer (a disconnected laptop is probably the best choice)
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and if you need it on your connected box (I know: We all do this) be
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sure to have a strong password for your account and for your secret key
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and that you can trust your system administrator.
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When I check GnuPG on a remote system via ssh (I have no Alpha here ;-)
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I have the same problem. It takes a *very* long time to create the
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keys, so I use a special option, --quick-random, to generate insecure
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keys which are only good for some tests.
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Q: How does the whole trust thing work?
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A: It works more or less like PGP. The difference is that the trust is
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computed at the time it is needed. This is one of the reasons for the
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trustdb which holds a list of valid key signatures. If you are not
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running in batch mode you will be asked to assign a trust parameter
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(ownertrust) to a key.
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You can see the validity (calculated trust value) using this command.
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gpg --list-keys --with-colons
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If the first field is "pub" or "uid", the second field shows you the trust:
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o = Unknown (this key is new to the system)
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e = The key has expired
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q = Undefined (no value assigned)
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n = Don't trust this key at all
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m = There is marginal trust in this key
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f = The key is full trusted.
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u = The key is ultimately trusted; this
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is only used for keys for which
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the secret key is also available.
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r = The key has been revoked
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d = The key has been disabled
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The value in the "pub" record is the best one of all "uid" records.
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You can get a list of the assigned trust values (how much you trust
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the owner to correctly sign another person's key)
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gpg --list-ownertrust
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The first field is the fingerprint of the primary key, the second field
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is the assigned value:
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- = No Ownertrust value yet assigned.
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n = Never trust this keyholder to correctly verify others signatures.
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m = Have marginal trust in the keyholders capability to sign other keys.
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f = Assume that the key holder really knows how to sign keys.
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u = No need to trust ourself because we have the secret key.
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Keep these values confidential because they express your opinions
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about others. PGP stores this information with the keyring thus
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it is not a good idea to publish a PGP keyring instead of exporting the
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keyring. gnupg stores the trust in the trust-DB so it is okay
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to give a gpg keyring away (but we have a --export command too).
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Q: What is the difference between options and commands?
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A: If you do a "gpg --help", you will get two separate lists. The first is
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a list of commands. The second is a list of options. Whenever you run GPG,
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you *must* pick exactly one command (**with one exception, see below). You
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*may* pick one or more options. The command should, just by convention,
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come at the end of the argument list, after all the options. If the
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command takes a file (all the basic ones do), the filename comes at the
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very end. So the basic way to run gpg is:
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gpg [--option something] [--option2] [--option3 something] --command file
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Some options take arguments, for example the --output option (which can be
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abbreviated -o) is an option that takes a filename. The option's argument
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must follow immediately after the option itself, otherwise gpg doesn't know
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which option the argument is supposed to go with. As an option, --output and
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its filename must come before the command. The --recipient (-r) option takes
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a name or keyid to encrypt the message to, which must come right after the -r
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argument. The --encrypt (or -e) command comes after all the options followed
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by the file you wish to encrypt. So use
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gpg -r alice -o secret.txt -e test.txt
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If you write the options out in full, it is easier to read
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gpg --recipient alice --output secret.txt --encrypt test.txt
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If you're saving it in a file called ".txt" then you'd probably expect to see
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ASCII-armored text in there, so you need to add the --armor (-a) option,
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which doesn't take any arguments.
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gpg --armor --recipient alice --output secret.txt --encrypt test.txt
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If you imagine square brackets around the optional parts, it becomes a bit
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gpg [--armor] [--recipient alice] [--output secret.txt] --encrypt test.txt
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The optional parts can be rearranged any way you want.
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gpg --output secret.txt --recipient alice --armor --encrypt test.txt
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If your filename begins with a hyphen (e.g. "-a.txt"), gnupg assumes this is
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an option and may complain. To avoid this you have either to use
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"./-a.txt" or stop the option and command processing with two hyphens:
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** the exception: signing and encrypting at the same time. Use
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gpg [--options] --sign --encrypt foo.txt
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Q: What kind of output is this: "key C26EE891.298, uid 09FB: ...."?
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A: This is the internal representation of an user id in the trustdb.
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"C26EE891" is the keyid, "298" is the local id (a record number
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in the trustdb) and "09FB" is the last two bytes of a ripe-md-160
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hash of the user id for this key.
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Q: What is trust, validity and ownertrust?
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A: "ownertrust" is used instead of "trust" to make clear that
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this is the value you have assigned to a key to express how much you
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trust the owner of this key to correctly sign (and so introduce)
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other keys. "validity", or calculated trust, is a value which
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says how much GnuPG thinks a key is valid (that it really belongs
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to the one who claims to be the owner of the key).
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For more see the chapter "The Web of Trust" in the Manual
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Q: How do I interpret some of the informational outputs?
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A: While checking the validity of a key, GnuPG sometimes prints
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some information which is prefixed with information about
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This is about the key with key ID 12345678 and the internal
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number 3456, which is the record number of the so called
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directory record in the trustdb.
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"uid 12345678.3456/ACDE"
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This is about the user ID for the same key. To identify the
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user ID the last two bytes of a ripe-md-160 over the user ID
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"sig 12345678.3456/ACDE/9A8B7C6D"
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This is about the signature with key ID 9A8B7C6D for the
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above key and user ID, if it is a signature which is direct
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on a key, the user ID part is empty (..//..).
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Q: How do I sign a patch file?
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A: Use "gpg --clearsign --not-dash-escaped ...".
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The problem with --clearsign is that all lines starting with a dash are
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quoted with "- "; obviously diff produces many of lines starting with a
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dash and these are then quoted and that is not good for patch ;-). To
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use a patch file without removing the cleartext signature, the special
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option --not-dash-escaped may be used to suppress generation of these
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escape sequences. You should not mail such a patch because spaces and
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line endings are also subject to the signature and a mailer may not
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preserve these. If you want to mail a file you can simply sign it
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Q: Where is the "encrypt-to-self" option?
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A: Use "--encrypt-to your_keyid". You can use more than one
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of these options. To temporary override the use of this additional
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keys, you can use the option "--no-encrypt-to".
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Q: How can I get rid of the Version and Comment headers in
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A: Use "--no-version --comment ''". Note that the left over blank line
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is required by the protocol.
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Q: What does the "You are using the xxxx character set." mean?
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A: This note is printed when UTF8 mapping has to be done. Make sure that
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the displayed charset is the one you have activated on your system
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"iso-8859-1" is the most used one, so this is the default. You can
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change the charset with the option "--charset". It is important that
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you active character set matches the one displayed - if not, restrict
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yourself to plain 7 bit ASCII and no mapping has to be done.
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Q: How do I transfer owner trust values from PGP to GnuPG?
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A: There is a script in the tools directory to help you:
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After you have imported the PGP keyring you can give this command:
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$ lspgpot pgpkeyring | gpg --import-ownertrust
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where pgpkeyring is the original keyring and not the GnuPG one you
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might have created in the first step.
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Q: Are the headerlines of a cleartext signater part of the signed
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A: No. For example you can add or remove "Comment:" lines. They
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have a purpose like the mail header lines. However a "Hash:"
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line is needed for modern signatures, to tell the parser which
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hash algorithm to use.
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