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GNU Privacy Guard -- Frequently Asked Questions
=================================================
This FAQ is partly compiled from messages of the developers mailing list.
Many thanks to Kirk Fort, Brian Warner, ...
Q: How does this whole thing work?
A: 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.
Q: What is the recommended key size?
A: 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 keysize 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>".
Q: Why are some signatures with an ELG-E key valid?
A: 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.
Q: Why is PGP 5.x not able to encrypt messages with some keys?
A: 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.
Q: Why is PGP 5.x not able to verify my messages?
A: 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.
Q: I can't delete an user id because it is already deleted on my
public keyring?
A: 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.
Q: How can I encrypt a message so that pgp 2.x is able to decrypt it?
A: 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;
other wise, pgp 2 will not be able to handle it.
Q: How can I conventional encrypt a message, so that PGP can decrypt it?
A: 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.
Q: Why does it sometimes take so long to create keys?
A: 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 it's 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).
Q: And it really takes long when I work on a remote system. Why?
A: 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 keyring (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.
Q: How does the whole trust thing work?
A: 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).
Q: What is the difference between options and commands?
A: 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
Q: What kind of output is this: "key C26EE891.298, uid 09FB: ...."?
A: 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.
Q: What is trust, validity and ownertrust?
A: "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
Q: How do I interpret some of the informational outputs?
A: 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 (..//..).
Q: How do I sign a patch file?
A: 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.
Q: Where is the "encrypt-to-self" option?
A: 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".
Q: How can I get rid of the Version and Comment headers in
armored messages?
A: Use "--no-version --comment ''". Note that the left over blank line
is required by the protocol.
Q: What does the "You are using the xxxx character set." mean?
A: 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.
Q: How do I transfer owner trust values from PGP to GnuPG?
A: 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.
Q: Are the headerlines of a cleartext signater part of the signed
material?
A: 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 modern signatures, to tell the parser which
hash algorithm to use.