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November 17, 2008

Beginners Guide to GnuPG

To download GnuPG check out the download links for your OS. The following is the most complete doc on the web for GnuPG. Here is the link to the full thread. By Dr Small Note: the following posting/how to assumes you are on ubuntu.


Lightbulb Beginners Guide to GnuPG

A Beginner's Guide to GnuPG

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Introduction

I am going to give a basic run down of how to use GnuPG to encrypt files, sign your messages, read encrypted messages from your friends and whatnot, along with some of useful commands and applications you can use in aiding you along the way

First off, incase you don't understand completely (which is absolutely fine, as we are not expected to know everything), GPG is a key-based encryption method. You will be given a public key and a private key. The private key, as indicated, should remain private as to keep the entire idea of encryption secure.

A person who holds your public key and wishes to send you an encrypted message, would encrypt the message with your public key. They can not decrypt their own message after they encrypt it. Only you, who holds the private key can decrypt the message.


Applications

There are two different GUI based applications which can assist you in setting up a GPG key. These two are GPA, which is a very simple application that does everything you need, as far as key managing, deleteing, adding, signing and adding a level of trust to another person's public key.

The second one is seahorse. It is built for Gnome, and uses the gnome-keyring (if I am not mistaken) so it is a little bit more heavy than GPA, which is my favorite. (Note to the reader, I am not a KDE user, but I am sure there is a KDE GnuPG application suited for them. I am in no way discriminating them from this guide for any reason).

Let's begin by installing GPA and seahorse (or you can choose one of the two if you wish).
From the Terminal (Applications > Accessories > Terminal), run the following command:
Code:
sudo apt-get install gpa seahorse
To launch GPA, from run dialog (ALT + F2) or the terminal:
Code:
gpa
To launch seahorse, from the run dialog (ALT + F2) or the terminal:
Code:
seahorse
Both of this GUI applications give you the opportunity to create a GPG key from their menus, and if you wish to do it that way, you may do so. It should be very simple and informational, so I will not explain how to do it from those applications (as it could also be subject to change over time).

Another application worth mentioning would be FireGPG for Firefox. It can encrypt / decrypt / sign / verify / import and export with GPG. To install it for Firefox, please visit their website:
http://firegpg.tuxfamily.org/?page=install&lang=en


Key Generating

As an alternative, you could create a GPG key from the command line of the terminal.
To do so, launch your terminal (Applications > Accessories > Terminal) and run the following command, to get started:
Code:
gpg --gen-key
You will then be prompted back with the following:
Code:
Please select what kind of key you want:   (1) DSA and Elgamal (default)   (2) DSA (sign only)   (5) RSA (sign only)Your selection?
You will want to select number 1, as it can be used for encryption and decryption, whereas the second and third choices are only allowed to sign messages. To do so, press the number 1, and then press enter.

You then will be prompted with the following:
Code:
DSA keypair will have 1024 bits.ELG-E keys may be between 1024 and 4096 bits long.What keysize do you want? (2048)
You will want to enter "2048" here, as recommended by gnupg.
If you don't want your key to expire (for the next prompt, select 0).

Answer yes if the information is correct, when prompted, and then enter your Real Name, Email address, and a comment (which is optional). If everything is correct, press "o" (for Ok) and then enter.

You will then be asked to enter a passphrase. This process will be repeated. As always, make a strong password which would be difficult to crack. Do not enter a name / address / birthdate or word from a dictionary as your password. Take the usual precautions, and make it random and difficult to crack.

After entering your passphrase, follow the instructions in the terminal:
Code:
We need to generate a lot of random bytes. It is a good idea to performsome other action (type on the keyboard, move the mouse, utilize thedisks) during the prime generation; this gives the random numbergenerator a better chance to gain enough entropy.

When you have successfully finished generating your key, you will see a message similar to the following:
Quote:
gpg: key 069C39A4 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: 2 signed: 1 trust: 0-, 0q, 0n, 0m, 0f, 2u
gpg: depth: 1 valid: 1 signed: 0 trust: 0-, 0q, 0n, 0m, 1f, 0u
pub 1024D/069C39A4 2008-01-28
Key fingerprint = 516E E759 94BA 0DC1 37BE 1074 E46C B27D 069C 39A4
uid samplekey <samplekey@linux.org>
sub 2048g/BC9EC4CB 2008-01-28
Your KEY-ID would be the two keys (both identical) above which are in bold.
The Key fingerprint is also listed there.


Key Servers

Key servers are used to distribute your public key to other key servers and so other users can easily look your name (or email up) in the database and find your public key to send encrypted messages to you. This eliminates the process of physically or unsecurely giving your friend your public key, and allows others to be able to find you on an online database.

To upload your public key to the Ubuntu keyserver, there are 2 ways to do this.
  1. By pasting your ASCII Armored Public Key into the submission box at the Ubuntu Keyserver
  2. By using the terminal and gnupg to send your public key to the Ubuntu Keyserver.

To accomplish method 1, you will need to open seahorse, select your key under "My Personal Keys" and click the "Export Public Key" in the toolbar. You can optionally choose the name and location of which it will be saved to. Proceed to open up:
http://keyserver.ubuntu.com:11371
while opening your newly exported public key with a text editor (gedit).

Select and copy the entire contents of your public key file, and paste it into the "Submit a Key" text area on the Ubuntu Keyserver (link provided above). Submit it, and it should then proceed to submit the key to the keyserver.

You should then be able to search for your email or name in the search string box on the Ubuntu Keyserver page, to find your public key on the internet. This is undoubtedly the Graphical way of doing it, but it can be somewhat longer.

To do it by the means of method 2, you would first need to open up a terminal (Applications > Accessories > Terminal) and paste the following into it:
Code:
gpg --send-keys --keyserver keyserver.ubuntu.com <KEY-ID>
Naturally, you would replace <KEY-ID> with your public key id, as stated before. It also helps to memorize it, like I have done
If you forget what your keyid is, just run:
Code:
gpg --list-keys <EMAIL>
That will list the keys registered with your email (and since there should only be one, it will only list your key.) Then you can obtain your KEY-ID and run the command above, to submit it to the keyservers.


Importing Keys

There are four different methods to importing a key, as stated below:
  1. FireGPG
  2. GPA
  3. Seahorse
  4. Terminal

All are quite simple to do, but FireGPG is the easiest of all if you are importing a public key from a keyserver with Firefox. I will briefly explain all four.

FireGPG
If you have somebody's public key on a webpage while in Firefox and have installed FireGPG (as mentioned above under Applications), then simply highlight the Public key from beginning PGP comment to ending PGP comment, right click on it, select FireGPG and click the Import button. It's that simple!

For your information, to solve confusion, the beginning and ending PGP comment tags look like the following:
Code:
-----BEGIN PGP PUBLIC KEY BLOCK----------END PGP PUBLIC KEY BLOCK-----

GPA
If someone has given you their public key as a file, simply launch GPA and select "Import" from the toolbar.


Seahorse
If someone has given you their public key as a file, you can do one of two things. First, you can open up Nautilus and double click this file, and it should automatically import the public key into your GnuPG, or open Seahorse and select "Key" from the menu and click "Import".


Terminal
Open up the terminal (Applications > Accessories > Terminal) and type:
Code:
gpg --import KEYFILE
KEYFILE would be the filename of the public key in your home folder.
(If it is not in your home folder, please cd to the proper directory first, and then run the above command.)


Tips and Tricks

Here is some more valuable information that can be useful when encrypting / decrypting files with GPG from the terminal.

List Keys
If you wish to see all of the keys you have imported into GnuPG, you can issue the following command:
Code:
gpg --list-keys

Encrypt a File
If you wish to encrypt a file for your friend with his Public Key, run the command in the following format:
Code:
gpg -o encrypted_file.gpg --encrypt -r <KEY-ID> original.file
Explanation:
-o encrypted_file.gpg = Output to the following filename.
--encrypt = Duh, that's the encrypting part
-r <KEY-ID> = Recipient. KEY-ID would be your friends KEY-ID here.
original.file = The original file that you will be encrypting.


Decrypt a File
If someone has sent you a file that has been encrypted with your public key, you can decrypt it by the following:
Code:
gpg --decrypt filename.gpg

Clearsign a Document
Clearsigning is very similar to adding your signature to the bottom of a letter or important paper. It signifies that it actually came from you. By clearsigning, it generates a SHA1 hash of the entire file's contents and add's the SHA1 sum to the bottom of the signature. If the file has been tampered with, the signature verification will fail, which can be used to spot forgery.

If the user has edited the file after it has been signed, the verification of the signature will also fail, because the SHA1 sum will not match that of the actual content.

To clearsign a document / file, run the following:
Code:
gpg --clearsign filename.txt

Exporting your Public Key
To export your public key in ASCII Armored fashion, run the following command:
Code:
gpg --export -a <KEY-ID> > publickey.asc
Replace <KEY-ID> with your Public Key ID, and it will create a file called "publickey.asc" which you can distribute to your friends and they can import.


Symmetric Encryption
GPG can also do a symmetric encrytion where you can encrypt a file with a passphrase (this is not keybased encryption). To encrypt a file with a passphrase, use this:
Code:
gpg -c filename.txt
To decrypt this type of file, just use:
Code:
gpg filename.txt
And you will be prompted for the passphrase and it will decrypt the file.


Additional Links:
GnuPG.org
GnuPrivacyGuardHowto
The GNU Privacy Handbook
Wikipedia: GNU Privacy Guard

Related Guides
How To: Use GnuPG along with WHIRLPOOL Hash to Encrypt an Individual File
Advanced GnuPG Concepts - Advanced Key Generation
How To: Install a Port Knocker - FWKNOP