Introduction

"GnuPG uses public-key cryptography so that users may communicate securely. In a public-key system, each user has a pair of keys consisting of a private key and a public key. A user's private key is kept secret; it need never be revealed. The public key may be given to anyone with whom the user wants to communicate." From The GNU Privacy Handbook

This page describes how to use OpenPGP keys. For a brief description of what OpenPGP is see the next section. The package gnupg (including the command gpg) is installed on all Ubuntu systems. The package description says in part: "GnuPG 1.4 is the standalone, non-modularized series. In contrast to the version 2 series, shipped with the gnupg2 package, it comes with no support for S/MIME and some other tools useful for desktop environments, but also with less dependencies." This page mainly describes how to use the gpg command.

Some Terminology

The terms "OpenPGP", "PGP", and "GnuPG / GPG" are often used interchangeably. This is a common mistake, since they are distinctly different.

  • OpenPGP is technically a proposed standard, although it is widely used. OpenPGP is not a program, and shouldn't be referred to as such. PGP and GnuPG are computer programs that implement the OpenPGP standard.

  • PGP is an acronym for Pretty Good Privacy, a computer program which provides cryptographic privacy and authentication. For more information, see this Wikipedia article.

  • GnuPG (aka GPG) is an acronym for GNU Privacy Guard, another computer program which provides cryptographic privacy and authentication. For further information on GnuPG, see this Wikipedia article.

Graphical Interfaces

There are several programs which provide a graphical interface to the GnuPG system.

  • Enigmail, an OpenPGP plugin for Mozilla Thunderbird.

    sudo apt-get install enigmail

  • GNU Privacy Assistant is a graphical user interface for the GnuPG (GNU Privacy Guard).

    sudo apt-get install gpa

  • Seahorse is a GNOME application for managing encryption keys. It also integrates with nautilus, gedit, and in other places for encryption operations. Note: seahorse is installed by default on Ubuntu. It is called "Passwords and Keys" in Ubuntu.

  • KGPG is a simple, free, open source KDE frontend for gpg.

    sudo apt-get install kgpg

  • Kleopatra is another KDE frontend for gpg that is integrated with the KDE PIM (although you need to install it separately for now).

    sudo apt-get install kleopatra

Generating an OpenPGP Key

To generate a key:

  • Open a terminal and enter:

    gpg --gen-key
    • If you are using GnuPG version 1.4.10 or newer, this will lead to a selection screen with the following options:
      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)
      Select (1), which will enable both encryption and signing.
    • If you are using an older version, the selection screen will have the following options:
      Please select what kind of key you want:
         (1) DSA and Elgamal (default)
         (2) DSA (sign only)
         (5) RSA (sign only)
      We suggest you select (5). We will generate an encryption subkey later.
    What keysize do you want? (2048)
  • Enter the keysize. A keysize of 2048 (which is the default) is a good choice.

    Key is valid for? (0)
  • Specify how long the key should be valid for. Most people make their keys valid until infinity, which is the default option. If you do this don't forget to revoke the key when you no longer use it (see below).
  • Hit Y to confirm.

    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>"
  • Enter real name, comment, and email address when prompted. Make sure that the name on the key is not a pseudonym, and that it matches the name in your passport, or other government issued photo-identification! You can add extra e-mail addresses to the key later.
  • Type O to continue.

    You need a Passphrase to protect your secret key.
  • Enter your passphrase twice. Usually, a short sentence or phrase that isn't easy to guess can be used. Note: forgetting your passphrase will result in your key being useless.
  • You will be asked to tap on the keyboard (or do any of the things you normally do) in order for randomization to take place. This is done so that the encryption algorithm has more human-entered elements, which, combined with the passphrase entered above, will result in the user's private key. Please follow the instructions on the screen till you reach a screen similar to the one below.

    gpg: key D8FC66D2 marked as ultimately trusted
    public and secret key created and signed.
    
    pub   1024D/D8FC66D2 2005-09-08
          Key fingerprint = 95BD 8377 2644 DD4F 28B5  2C37 0F6E 4CA6 D8FC 66D2
    uid                  Dennis Kaarsemaker (Tutorial key) <dennis@kaarsemaker.net>
    sub   2048g/389AA63E 2005-09-08

    You have now generated the key. In the above listing the key-id is D8FC66D2 (yours will be different).

Setting the key to be the default

It is probably a good idea to set this key as default in your ~/.bash_profile or ~/.profile. Doing this will allow applications using GPG to automatically use your key.

  • Set your key as the default key by entering this line in your ~/.bash_profile (along with any other environment variables to be exported):

    export GPGKEY=D8FC66D2
    Please note that this will be sourced only during your next login. To use it straight away, you should run the export command in the current shell.
  • Now restart the gpg-agent and set the relevant environment variable:

    killall -q gpg-agent
    eval $(gpg-agent --daemon)
    export GPGKEY=D8FC66D2

Adding Encryption Capabilities

  • If you created an "RSA (sign only)" key earlier, you will probably want to add encryption capabilities. Assuming you edited ~/.bash_profile as above, open a terminal again and enter:

    gpg --cert-digest-algo=SHA256 --edit-key $GPGKEY
  • This will present a dialog like the following:
    Secret key is available.
    
    pub   2048R/D8FC66D2  created: 2005-09-08  expires: never       usage: SC  
                         trust: ultimate      validity: ultimate
    [ultimate] (1). Dennis Kaarsemaker (Tutorial key) <dennis@kaarsemaker.net>
    
    Command> 
  • To create a subkey, enter 'addkey'. You will have to enter your key's passphrase, and then you'll see a somewhat familiar series of dialogues:
    Please select what kind of key you want:
       (2) DSA (sign only)
       (4) Elgamal (encrypt only)
       (5) RSA (sign only)
       (6) RSA (encrypt only)
  • Choose 6.
    What keysize do you want? (2048)
  • Again, 2048 is a sensible default.
    Key is valid for? (0)
  • Choose whether this encryption subkey is set to expire (default: it doesn't). Then confirm that you want to make this subkey.
    pub   2048R/D8FC66D2  created: 2005-09-08  expires: never       usage: SC  
                         trust: ultimate      validity: ultimate
    sub   2048R/389AA63E created: 2005-09-08  expires: never       usage: E   
    [ultimate] (1). Dennis Kaarsemaker (Tutorial key) <dennis@kaarsemaker.net>
    Command> 
  • Enter 'save', then 'quit.' Your key is now capable of encryption.

Creating a revocation certificate

A revocation certificate must be generated to revoke your public key if your private key has been compromised in any way. It is recommended to create a revocation certificate when you create your key. Keep your revocation certificate on a medium that you can safely secure, like a thumb drive in a locked box.

To create a revocation certificate, enter:

gpg --output revoke.asc --gen-revoke $GPGKEY

and follow the instructions. Note: you will need the passphrase. The revocation certificate may be printed and/or stored as a file.

IconsPage/warning.png Anybody having access to your revocation certificate can revoke your key, rendering it useless.

Making an ASCII armored version of your public key

Some keyservers allow you to paste an ASCII armored version of your public key in order to upload it. This method is often preferred, because the key comes directly from the user and the user can see that the key has been successfully uploaded.

To create an ASCII armored version of your public key, enter:

gpg --output mykey.asc --export -a $GPGKEY

IconsPage/example.png This is the command using our example (key-id = D8FC66D2):

gpg --output mykey.asc --export -a D8FC66D2

Uploading the key to Ubuntu keyserver

This section explains how to upload your public key to a keyserver so that anyone can download it. Once you have uploaded it to one keyserver, it will propagate to the other keyservers. Eventually most of the keyservers will have a copy of your key. You can accelerate the process by uploading your key to several keyservers.

To upload the key, enter:

gpg --send-keys --keyserver keyserver.ubuntu.com $GPGKEY

IconsPage/example.png Using our example (key-id = D8FC66D2), the command is:

gpg --send-keys --keyserver keyserver.ubuntu.com D8FC66D2

To upload the key using a web browser:

  • Export your key by issuing this command (where GPGKEY is defined above) :

    gpg --export -a $GPGKEY > mykey.asc
  • Copy the content of the file mykey.asc.

  • Open http://keyserver.ubuntu.com/ in a browser window.

  • Paste the copied content in the box under the label Submitting a new OpenPGP Key.

  • Click Submit.

Note that keyserver.ubuntu.com is only reachable via IPv4.

Reading OpenPGP E-mail

OpenPGP implementations can be used to digitally sign, encrypt, and decrypt email messages for heightened security. You can validate your keys with Launchpad, and under some situations, Launchpad will send you signed or encrypted email. You would then use OpenPGP support in your mail reader to decrypt these messages or verify a message's digital signature. Of course, you can also use the OpenPGP support in your mail reader to trade encrypted messages with your colleagues, or sign your own messages so that others can have better assurances that the email that appears to come from you actually does come from you.

The instructions below are not intended to provide you with detailed information on OpenPGP, its various implementations, or its use. These instructions simply provide links that can help you set up your mail reader to be compatible with OpenPGP signed and/or encrypted email.

We need your help to flesh out these instructions!

Linux mail readers

This section is not all inclusive. Please feel free to add additional mail clients.

Evolution

Evolution has built-in support for OpenPGP. Look under the Security tab when you edit accounts.

  • Open Evolution and go to Edit->Preferences.

    • Choose your email account, click on it, and then click Edit.

    • Click on the security tab.

    • In the PGP/GPG Key ID: box, paste your KEY-ID.

    • Click OK. Click Close.

  • If you want to use your key in any new email, simply click on the Security menu item in your new mail message, and then click on PGP Sign.

KMail

Kmail / Kontact has built-in support For Gutsy, and later releases, everything required is installed by default. See the Kmail GPG page for details.

Claws Mail

Claws Mail supports OpenPGP through the plugin claws-mail-pgpinline

  • claws-mail-pgpinline is available in the "Universe" repository.

  • sudo apt-get install claws-mail-pgpinline
  • The plugin may have to be loaded manually after installing it. Open Claws Mail and select Configuration -> Plugins

    • If PGP/Core and PGP/inline are in the Plugins dialogue box, the plugins are loaded correctly.

    • Otherwise, click on the Load Plugin button towards the bottom of the window. In the file selection dialogue, select pgpinline.so and click the Open button.

  • When Claws Mail tries to open encrypted e-mail, the program will prompt for your key's passphrase and then show the e-mail with the decrypted message.

Thunderbird

  • Thunderbird supports OpenPGP through the enigmail plugin.
  • Enigmail is available in the "Main" repository.
  • sudo apt-get install enigmail
  • Configure OpenPGP support in Thunderbird under Enigmail->Preferences and add under GnuPG executable path. The path for GnuPG is /usr/bin/gpg.

Mutt

  • Create a ~/.mutt directory and copy this file into it: /usr/share/doc/mutt/examples/gpg.rc

  • Append this line to the muttrc configuration file.

    source ~/.mutt/gpg.rc                           # Use GPG
  • If you're using Mutt 1.5.13, you'll need to fix the paths to pgpewrap as detailed in this post

Miscellaneous/all platforms (web mail)

This section in need of expansion. Please feel free to add any additional plugins for Firefox or other browsers.

Gmail

  • If you use the Chrome or the Chromium browser you can use Goopg to sign and verify emails directly from the Gmail web interface. For details click here).

It's All Text!

  • It's All Text! is a Firefox extension which allows you to edit your mail in your preferred local text editor.

  • If your editor supports it, this can make handling of encrypted mail easier.
  • For example, you could use gnupg.vim and a local Vim instance.

Validation with Launchpad

You need to validate a key with Launchpad (that is, import it into Launchpad) in order to be able to sign the Ubuntu Code of Conduct (and thus become an Ubuntero) and to build packages using HCT.

OpenPGP keys and Launchpad

To import a key you need the key fingerprint. To list all keys and their fingerprints, enter:

gpg --fingerprint

A fingerprint will look something like this:

95BD 8377 2644 DD4F 28B5  2C37 0F6E 4CA6 D8FC 66D2

To import a key:

  • Log into Launchpad if necessary.
  • Visit the "Change your OpenPGP keys" page.
  • Paste the fingerprint into the textbox and click "Import Key". Launchpad will send you an encrypted email. The text of the email will look similar to this:

    -----BEGIN PGP MESSAGE-----
    Version: GnuPG v1.4.3 (GNU/Linux)
    
    hQIOA0THhKozD+K5EAf9F3PcOL2iU6onH2YsvB6IKDXNxbK0NBVy6ppxcNq8hoTe
    cuHvzWLFfh1ehhSNe1V6xpuFnt5sJoeA4qEEOxez3HmY80tKIKMPLyhC/8JiSIW9
    
    [Lines deleted]
    
    V9E+hW6Qehx5DbsIXEvfeaBBHOfAHHOhUH14WK4bsJWm8wZ50XiYBZrNFOqzsm13
    2STcY4VIoJp3Uw2qNyvZXQUhpndlfgQGO14CMSadzDn6Vts=
    =hTe6
    -----END PGP MESSAGE-----
  • Save the text of the email to a file.
  • Decrypt the text:

    gpg --decrypt file.txt
  • Enter your passphrase. The message will be displayed along with the link you must follow to validate your key with Launchpad.
  • Follow the link, enter your Launchpad password when prompted, and you are done!

Validating using Firefox and FireGPG

  • If you are on gmail, using the FireGPG addon, simply scroll down and click "decrypt this mail". You will now see the decrypted message with a link and a token. Copy that URL:

    https://launchpad.net/token/somealphanumerictoken

    • Follow the link and click on "Confirm". Please note that validation does take some time. If you run into an internal 500 server, simply try again with the same token.
  • A confirming page should appear once the validation is successfully completed.

Signing Data

Signing data is helpful in verifying if the data from a person is indeed from that person. A typical scenario is described below.

Signing the Ubuntu Code Of Conduct

When you've generated a key and imported it, it is time to sign the Ubuntu Code Of Conduct if you want to become an Ubuntu member or Ubuntero:

  1. Download the code of conduct from https://launchpad.net/codeofconduct/2.0/+download.

  2. Enter:

    gpg --clearsign UbuntuCodeofConduct-2.0.txt
  3. Upload the contents of UbuntuCodeofConduct-2.0.txt.asc on https://launchpad.net/codeofconduct/2.0/+sign

Getting your key signed

The whole point of all this is to create a web of trust. By signing someone's public key, you state that you have checked that the person that uses a certain keypair, is who they says they are and really is in control of the private key. This way a complete network of people who trust each other can be created. This network is called the Strongly connected set. Information about it can be found at http://pgp.cs.uu.nl/

In summary,

  1. Locate someone that lives near you and can meet with you to verify your ID. Sites like http://www.biglumber.com/ are useful for this purpose

  2. Arrange for a meeting. Bring at least one ID with photo and printed fingerprint of your OpenPGP key, ask the same from the person you will be meeting with.
  3. Print copies of your public key
    • get the last eight digits of your fingerprint: 0995 ECD6 3843 CBB3 C050 28CA E103 6EED 0123 4567

    • terminal: gpg --fingerprint 01234567 >> key.txt

    • print the resulting key.txt file and bring as many copies to the meeting as you expect to have people sign
  4. Meet, verify your IDs and exchange OpenPGP key fingerprints
  5. Sign the key of the person you've just met. Send him/her the key you've just signed.
  6. Update your keys on the keyserver, the signature you've just created will be uploaded.

Keysigning Guidelines

Since a signature means that you checked and verified that a certain public key belongs to a certain person who is in control of the accompanying private key, you need to follow these guidelines when signing peoples keys:

During the Event

  1. Keysigning is always done after meeting in person
  2. During this meeting you hand each other your OpenPGP key fingerprint and at least one government issued ID with a photograph. These key fingerprints are usually distributed as key fingerprint slips, created by a script such as gpg-key2ps (package: signing-party)

  3. You check whether the name on the key corresponds with the name on the ID and whether the person in front of you is indeed who they say they are.

After the Event

You now have the printed public key information from the other participants.

Example key IDs for the other participants will be E4758D1D, C27659A2, and 09026E7B. Replace these IDs with the key IDs you received from the other participants.

  1. Retrieve the keys:

    gpg --recv-keys E4758D1D C27659A2 09026E7B
  2. Sign the keys:

    gpg --sign-key E4758D1D
    gpg --sign-key C27659A2
    gpg --sign-key 09026E7B
  3. Export the keys:

    gpg --armor --export E4758D1D --output E4758D1D.signed-by.01234567.asc
    gpg --armor --export C27659A2 --output C27659A2.signed-by.01234567.asc
    gpg --armor --export 09026E7B --output 09026E7B.signed-by.01234567.asc
  4. Email the key users (use the email address that was part of the key's user ID) and attach the corresponding signature file - or - send their signed key to the key server:

    gpg --send-keys --keyserver keyserver.ubuntu.com E4758D1D
  5. Once you receive your signed key import them to your keyring:

    gpg --import 01234567.signed-by.E4758D1D.asc
    gpg --import 01234567.signed-by.C27659A2.asc
    gpg --import 01234567.signed-by.09026E7B.asc
  6. You should see your keys:

    gpg --list-sigs 01234567
  7. Send your keys to the keyserver:

    gpg --send-keys 01234567

Congrats - you have now entered a web of trust or enlarged an existing one.

Backing up and restoring your keypair

Why should you back up your keypair? If you lose your keypair:

  • Any files encrypted with the lost keypair will be unrecoverable.
  • You will not be able to decrypt emails sent to you. Decrypting emails sent to you requires your private key, this key is not stored on the keyservers.

If you lose your keypair you should revoke your key. This cannot be done without a revocation certificate.

Backing up your public key

  • List your public keys:

    gpg --list-keys
  • Look for the line that starts something like "pub 1024D/". The part after the 1024D is the key-id. To export the key:

    gpg -ao _something_-public.key --export key-id

Backing up your private key

  • List your private keys:

    gpg --list-secret-keys
  • Look for the line that starts something like "sec 1024D/". The part after the 1024D is the key-id. To export the private key:

    gpg -ao _something_-private.key --export-secret-keys key-id

Restoring your keypair

To restore your keypair:

  • Copy the two files created above to the machine.
  • Enter the following commands:

    gpg --import _something_-public.key
    gpg --import _something_-private.key

Make sure you protect these files!

Revoking a keypair

In the event your keys are lost or compromised, you should revoke your keypair. This tells other users that your key is no longer reliable.

IconsPage/warning.png For security purposes, there is no mechanism in place to revoke a key without a revocation certificate. As much as you might want to revoke a key, the revocation certificate prevents malicious revocations. Guard your revocation certificate with the same care you would use for your private key.

  • To revoke your key you need to first create a revocation certificate. Indicate the key to be revoked and direct the output to a file. Continuing with the example (key-id = D8FC66D2):

    gpg --gen-revoke D8FC66D2 >revoke.asc
  • Import your revocation certificate:

    gpg --import revoke.asc
  • Upload the key to your keyserver of choice. Here, the example uses Ubuntu's key server and key-id = D8FC66D2:

    gpg --keyserver keyserver.ubuntu.com --send-key D8FC66D2

Un-revoking a keypair

If you unintentionally revoke a key, or find that your key has in fact not been lost or compromised, it is possible to un-revoke your key. First and foremost, ensure that you do not distribute the key, or send it to the keyserver.

  • Export the key:

    gpg --export key-id > key.gpg
  • Split the key into multiple parts. This breaks the key down into multiple parts:

    gpgsplit key.gpg
  • Find which file contains the revocation certificate. In most cases, it is 000002-002.sig, however you should make sure by using the following. If the sigclass is 0x20, you have the right file. Delete it.

    gpg --list-packets 000002-002.sig
  • Put the key back together:

    cat 0000* > fixedkey.gpg
  • Remove the old key:

    gpg --expert --delete-key key-id
  • Import the new key:

    gpg --import fixedkey.gpg

Changing your Passphrase

To change your passphrase:

  • Enter:

    gpg --edit-key key-id
  • At the gpg prompt enter:

    passwd
  • Enter the current passphrase when prompted.
  • Enter the new passphrase twice when prompted.
  • Enter:

    save

You have now changed the passphrase.

Tips and Tricks

  • Add your key to ~/.bash_profile by adding a line similiar to export GPGKEY=YOUR-KEY-ID

  • gnupg-agent and pinentry-gtk2 are packages that facilitate not having to enter the password for your key every time you want to use it. Open the file ~/.gnupg/gpg.conf in your favorite editor. Browse through it and change what you like. A few useful things to change are:

    • keyserver-options auto-key-retrieve
    • use-agent (the Ubuntu default for Gutsy and later releases.)
    The former makes gpg automatically retrieve GPG keys when verifying signatures. The latter makes you use gpg-agent, which is very useful if you use GPG a lot but don't like typing your password all the time. It is also required for some programs (such a Kmail) to sign or encrypt messages). Gnupg-agent and pinentry are in Main for Gutsy and automatically installed/configured in Kubuntu.

    Now create the file ~/.gnupg/gpg-agent.conf with the following content:

    pinentry-program /usr/bin/pinentry-gtk-2
    default-cache-ttl 86400
    max-cache-ttl 86400
    This will make gpg-agent use pinentry-gtk2 and it will remember your password for 24 hours (please consider the security implications for doing this - anyone gaining access to your computer for 24 hours would then be able to sign anything with your key). For Kubuntu, use pinentry-qt4 instead.

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GnuPrivacyGuardHowto (last edited 2016-12-02 17:44:55 by jpdonnelly)