Difference between revisions of "GnuPG"

Revision as of 10:21, 27 October 2013

GnuPG can be used to sign and encrypt files or mails.

Installation

Install gnupg, available in the official repositories.

Environment Variables

GnuPGP uses the directory pointed to by $GNUPGHOME to store all of its configuration files. By default $GNUPGHOME isn't set and your $HOME is used instead, thus you will find a ~/.gnupg directory right after the install. You may change this default setting by putting this line in one of your regular startup files

export GNUPGHOME="/path/to/gnupg/directory"

Basic keys management

Create key

• Generate a private key by typing in a terminal:

# gpg --gen-key

You’ll have to answer a bunch of questions but generally, you can accept the defaults.

While having an expiration date for subkeys isn't technically necessary, it is considered good practice. A period of a year is generally good enough for the average user. This way even if you lose access to your keyring, it will allow others to know that it is no longer valid.

• Set expiration date (repeat for both/all subkeys)

# gpg --edit-key 'Your Name'
# key [number]
# expire
# save

• Generate an ASCII version of your public key (e.g. to distribute it by e-mail):

# gpg --armor --output public.key --export 'Your Name'

• Register your key with a public PGP key server, so that others can retrieve your key without having to contact you directly:

# gpg  --keyserver hkp://subkeys.pgp.net --send-keys Key Id

Rotating subkeys

If you have set your subkeys to expire after a set time, you will have to create new ones. Do this a few weeks in advanced to allow others to update their keyring.

• Create new subkey (repeat for both signing and encrypting key)

# gpg --edit-key 'Your Name'
# addkey

And answer the following questions it asks.

• Save changes

# save

• Update it to a keyserver.

# gpg  --keyserver hkp://subkeys.pgp.net --send-keys Key Id

Import key

• Import a public key to your public key ring:

# gpg --import public.key

• Import a private key to your secret key ring:

# gpg --import private.key

List keys

• Keys in your public key ring:

# gpg --list-keys

• Keys in your secret key ring:

# gpg --list-secret-keys

Basic usage

You can use gnupg to encrypt your sensitive documents, but only individual files at a time.

For example, to decrypt a file, use:

# gpg -d secret.tar.gpg

You'll be prompted to enter your passphrase.

If you want to encrypt directories or a whole file-system you should consider using TrueCrypt, though you can always tarball various files and then encrypt them.

Symmetric Encryption

gpg-agent

Gpg-agent is mostly used as daemon to request and cache the password for the keychain. This is useful if GnuPG is used from an external program like a mail client. It can be activated by adding following line in ~/.gnupg/gpg.conf:

use-agent

This tells GnuPG to use the agent whenever it needs the password. However, the agent needs to run already. To autostart it, create the following file and make it executable, and remember to change the envfile path if you changed your $GNUPGHOME:

/etc/profile.d/gpg-agent.sh

envfile="$HOME/.gnupg/gpg-agent.env"
if [[ -e "$envfile" ]] && kill -0 $(grep GPG_AGENT_INFO "$envfile" | cut -d: -f 2) 2>/dev/null; then
    eval "$(cat "$envfile")"
else
    eval "$(gpg-agent --daemon --enable-ssh-support --write-env-file "$envfile")"
fi
export GPG_AGENT_INFO  # the env file does not contain the export statement
export SSH_AUTH_SOCK   # enable gpg-agent for ssh

If you don't want gpg-agent to autostart for all users or just want to keep user daemons in the users own configuration files you can add the following entry to your .xinitrc:

eval $(gpg-agent --daemon) &

Or, if you don't use an X server, you can also put this into your regular Startup_files. Here is an example using Bash syntax with Ssh support:

gpg-agent --daemon --enable-ssh-support --write-env-file "$HOME/.gnupg/.gpg-agent-info"

Log out of your Xsession and log back in. Check if gpg-agent is activated

# pgrep agent

Pinentry

Finally, the agent needs to know how to ask the user for the password. This can be set in ~/.gnupg/gpg-agent.conf

The default uses a gtk dialog. To change it to ncurses or qt, set the following in the above file

pinentry-program /usr/bin/pinentry-curses

or

pinentry-program /usr/bin/pinentry-qt4

For more options see man gpg-agent and info pinentry.

Keysigning Parties

To allow users to validate keys on the keyservers and in their keyrings (i.e. make sure they are from whom they claim to be), PGP/GPG uses a so-called "Web of Trust". To build this Web of Trust, many hacker events include keysigning parties.

The Zimmermann-Sassaman key-signing protocol is a way of making these very effective. Here you'll find a How-To-article.

Caff

For an easier process of signing keys and sending signatures to the owners after a keysigning party, you can use the tool 'caff'. It can be installed from the AUR with the package caff-svn^AUR or bundled together with other useful tools in the package signing-party-svn^AUR. Either way, there will be a lot of dependencies installing from the AUR. Alternatively you can install them with

cpanm Any:Moose
cpanm GnuPG::Interface 

To send the signatures to their owners you need a working MTA. If you don't have already one, install SSMTP.

Smartcards

GnuPG uses scdaemon as an interface to your smartcard reader, please refer to scdaemon man page for details.

GnuPG only setups

If you do not plan to use other cards but those based on GnuPG, you should check the reader-port parameter in ~/.gnupg/scdaemon.conf. The value '0' refers to the first available serial port reader and a value of '32768' (default) refers to the first USB reader.

GnuPG together with OpenSC

If you are using any smartcard with an opensc driver (e.g.: ID cards from some countries) you should pay some attention to GnuPG configuration. Out of the box you might receive a message like this when using gpg --card-status

gpg: selecting openpgp failed: ec=6.108

By default, scdaemon will try to connect directly to the device. This connection will fail if the reader is being used by another process. For example: the pcscd daemon used by OpenSC. To cope with this situation we should use the same underlying driver as opensc so they can work well together. In order to point scdaemon to use pcscd you should remove reader-port from ~/gnupg/scdaemon.conf, specify the location to libpcsclite.so library and disable ccid so we make sure that we use pcscd.

~/scdaemon.conf

pcsc-driver /usr/lib/libpcsclite.so 
card-timeout 5
disable-ccid

Please check man scdaemon if you do not use OpenSC.

Troubleshooting

Su

When using pinentry, you must have the proper permisions of the terminal device (e.g. /dev/tty1) in use. However, with su (or sudo), the ownership stays with the original user, not the new one. This means that pinentry will fail, even as root. The fix is to change the permissions of the device at some point before the use of pinentry (i.e. using gpg with an agent). If doing gpg as root, simply change the ownership to root right before using gpg

chown root /dev/ttyN  # where N is the current tty

and then change it back after using gpg the first time. The equivalent is likely to be true with /dev/pts/.

Agent complains end of file

The default pinentry program is pinentry-gtk-2, which needs a DBus session bus to run properly. See General Troubleshooting#Session permissions for details.

Alternatively you can use the qt pinentry.

# ln -sf /usr/bin/pinentry-qt4 /usr/bin/pinentry

See also

• A more comprehensive gpg Tutorial