Using the gpg command line tool

Generating a key

0 guest@animal:~$ gpg --gen-key
gpg (GnuPG) 1.4.12; Copyright (C) 2012 Free Software Foundation, Inc.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

gpg: directory `/home/guest/.gnupg' created
gpg: new configuration file `/home/guest/.gnupg/gpg.conf' created
gpg: WARNING: options in `/home/guest/.gnupg/gpg.conf' are not yet active during this run
gpg: keyring `/home/guest/.gnupg/secring.gpg' created
gpg: keyring `/home/guest/.gnupg/pubring.gpg' created
Please select what kind of key you want:
   (1) RSA and RSA (default)
   (2) DSA and Elgamal
   (3) DSA (sign only)
   (4) RSA (sign only)
Your selection? 1
RSA keys may be between 1024 and 4096 bits long.
What keysize do you want? (2048)
Requested keysize is 2048 bits
Please specify how long the key should be valid.
         0 = key does not expire
      <n>  = key expires in n days
      <n>w = key expires in n weeks
      <n>m = key expires in n months
      <n>y = key expires in n years
Key is valid for? (0) 1y
Key expires at Tue 25 Jun 2013 04:15:11 PM EDT
Is this correct? (y/N) y

You need a user ID to identify your key; the software constructs the user ID
from the Real Name, Comment and Email Address in this form:
    "Heinrich Heine (Der Dichter) <heinrichh@duesseldorf.de>"

Real name: Test User
Email address: test@example.org
Comment:
You selected this USER-ID:
    "Test User <test@example.org>"

Change (N)ame, (C)omment, (E)mail or (O)kay/(Q)uit? O
You need a Passphrase to protect your secret key.

We need to generate a lot of random bytes. It is a good idea to perform
some other action (type on the keyboard, move the mouse, utilize the
disks) during the prime generation; this gives the random number
generator a better chance to gain enough entropy.
..+++++
+++++
We need to generate a lot of random bytes. It is a good idea to perform
some other action (type on the keyboard, move the mouse, utilize the
disks) during the prime generation; this gives the random number
generator a better chance to gain enough entropy.
.+++++
....+++++
gpg: /home/guest/.gnupg/trustdb.gpg: trustdb created
gpg: key CCFAE189 marked as ultimately trusted
public and secret key created and signed.

gpg: checking the trustdb
gpg: 3 marginal(s) needed, 1 complete(s) needed, PGP trust model
gpg: depth: 0  valid:   1  signed:   0  trust: 0-, 0q, 0n, 0m, 0f, 1u
gpg: next trustdb check due at 2013-06-25
pub   2048R/CCFAE189 2012-06-25 [expires: 2013-06-25]
      Key fingerprint = 7C3C D023 3427 8195 4CD6  F59E 8ADA A534 CCFA E189
uid                  Test User <test@example.org>
sub   2048R/E37D6467 2012-06-25 [expires: 2013-06-25]

0 guest@animal:~$

Publish the key

gpg --send-key CCFAE189

Find a public key

0 guest@animal:~$ gpg --search jamie@mayfirst.org
gpg: searching for "jamie@mayfirst.org" from hkp server keys.gnupg.net
(1) Jamie McClelland <jm@mayfirst.org>
  Jamie McClelland <jamie@mayfirst.org>
  Jamie McClelland <jamie@progressivetech.org>
    4096 bit RSA key 5F2E4935, created: 2009-05-10
(2) Jamie McClelland <jm@mayfirst.org>
  Jamie McClelland <jamie@mayfirst.org>
  Jamie McClelland <jamie@mediajumpstart.org>
    1024 bit DSA key 76CC057D, created: 2004-01-23
Keys 1-2 of 2 for "jamie@mayfirst.org".  Enter number(s), N)ext, or Q)uit > 1
gpg: requesting key 5F2E4935 from hkp server keys.gnupg.net
gpg: key 5F2E4935: public key "Jamie McClelland <jamie@mayfirst.org>" imported
gpg: 3 marginal(s) needed, 1 complete(s) needed, PGP trust model
gpg: depth: 0  valid:   1  signed:   0  trust: 0-, 0q, 0n, 0m, 0f, 1u
gpg: next trustdb check due at 2013-06-25
gpg: Total number processed: 1
gpg:               imported: 1  (RSA: 1)
0 guest@animal:~$

Signing a key

There are at least two ways to sign a key.

Simple Method

gpg --edit-key jamie@mayfirst.org

That will put you in the gpg edit mode.

Next type:

sign

And follow the prompts.

When you are done, type:

quit

Now you have a signature in your local key ring. At this point, you can choose to either:

• Publish the key directly:
  • First, lookup the key id:

    pg --list-key jamie@mayfirst.org
    

  • Next, send the key, referencing the key id:

    gpg --send-key EEC168BA
    

This method has a draw back. If you are not certain that the owner of the key really does control the email address of the User ID that you just signed, you could be tricked into signing (and publishing publicly) and false key.

The following method is safer:

• Email the signature (and entire key) to the recient
  • First, export the key to a text file:

    gpg --export --armour jamie@mayfirst.org > jamie.asc
    

  • Next, send jamie.asc as an attachment to the key owner.

There is still a small risk with this method - since the recipient may get a validly signed key that they could accidentally publish.

Secure Method

caff is a command line tool to help verify and sign keys in a more secure manner. With caff, the each user id on the key is individually signed, encrypted to that key and emailed to the email address associated with the key id. This ensures that it will only be published if the owner of the email address is also the owner of the corresponding private key.

You can access it by installing the signing-party debian package:

apt-get install signing-party

caff depends on a working mail transport agent, which you can setup by following our esmtp faq.

Then, lookup the keyid of the person whose key you would like to send, and type:

caff <KEYID>

And follow all prompts. Near the end you may be left with a gpg prompt and no instruction. At this point, simply type: save to save your signature and go to the next step.