path: root/doc/FAQ

	    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 usefull 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 usefull 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 chnage the passphrase from time
  to time and decrease the risk, that many old messages may be decryptted 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 usefull, 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
  taht 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 methos is to use
  a MIME aware mail reader (Mutt, Pine and many more).

  There is a small security glitch in the OpenPGP (and therefor 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 keyssize is larger
    than 1024 bits.  Encryption keys may have greater sizes,
    but you should than check the fingerprint of this key.

 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 my public key.
 A: PGP Inc refuses to accept ElGamal keys of type 20 even for
    encryption.  They only supports type 16 (which are identical
    at least for decryption).  To be better interoperable, 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 choosen.  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 V3 signatures for all kind of
    data.  Use the option "--force-v3-sigs" to generate V3 signatures
    for data.

 Q: I can't delete a 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 so complicated
    do to it anyway:  Create a new user id with exactly the same name,
    you will notice that there are two identical user ids on the
    secret ring now.  Now select this user id and delete it; both
    user ids from the secret ring will be remoed.

 Q: How can I encrypt a message in way pgp 2.x is able to decrypt it later?
 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 ...

 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 --no-comment myfile

    You may replace "3des" by "cast5"; "blowfish" does not work with
    all versions of pgp5.  You may also want to put
       no-comment
       compress-algo 1
    into your ~/.gnupg/options file - this does not affect the 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 commited 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,
  capslock keys, as these keys do not produce any 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, your secret key and trust your Root.

  When I check gnupg on a remote system via ssh (I have no Alpha here ;-)
  I have the same problem too: it takes *very* long 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.  I have plans to use a cache for calculated
  trust values to speed up calcualtion.

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

      gpgm --list-keys --with-colons

  If the first field is "pub", 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.

  You can get a list of the assigned trust values (how far you trust
  the owner to correctly sign another one's key)

      gpgm --list-ownertrust

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

      - = No Ownertrust value yet assigned.
      n = Never trust this keyholder to correctly verifiy 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.

  Please keep these values confidential, as they express some opiones of
  you about others. PGP does store these information with the keyring, so
  it is not a good idea to publish the keyring instead of exporting the
  keyring - gnupg stores the trust in the trust-DB and therefor it is okay
  to give the keyring away (but we have a --export command too).


  Q: What is the differenc 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 which 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 --remote-user (-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 --remote-user 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 --remote-user alice --output secret.txt --encrypt test.txt

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

   gpg [--armor] [--remote-user alice] [--output secret.txt] --encrypt test.txt

  The optional parts can be rearranged any way you want:

   gpg --output secret.txt --remote-user 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 representaion of a user id in the trustdb.
     "C26EE891" is the keyid, "298" is the local id (a record number
     in the trustdb) and "09FB" are 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 key to express, how far 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 far the 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 [gpg: Oops: Internal error: manual not found - sorry]

  Q: How do interpret some of the informational outputs:
  A: While checking the validness of a key, GnuPG sometimes print
     some informations which are 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
     tring 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 ;-).  In order 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.