GPGKEY
GnuPG allows to encrypt and sign your data and communication, features a versatile key management system as well as access modules for all kinds of public key directories.
Contents
1 Installation
Install gnupg, available in the official repositories.
2 Environment Variables
2.1 GNUPGHOME
$GNUPGHOME is used by GnuPGP to point to the directory where all configuration files are stored. 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"
2.2 GPG_AGENT_INFO
GPG_AGENT_INFO used to locate the pgp-agent. Consists of 3 colon delimited fields:
- path to Unix Domain Socket
- PID of gpg-agent
- protocol version set to 1
E.g : GPG_AGENT_INFO=/tmp/gpg-eFqmSC/S.gpg-agent:7795:1. When starting the gpg-agent, this variable is set to the correct value.
3 Configuration file
Default is ~/.gnupg/gpg.conf. If you want to change the default location, either run gpg this way $ gpg --homedir path/to/file or use $GNUPGHOME variable.
Append in this file any long options you want. Do not write the two dashes, but simply the name of the option and required arguments. You will find a skeleton file usr/share/gnupg/gpg-conf.skel. Following is a basic configuration file:
~/.gnupg/gpg.conf
default-key name # useful in case you manage several keys and want to set a default one keyring file # will add file to the current list of keyrings trustdb-name file # use file instead of the default trustdb homedir dir # set the name of the gnupg home dir to dir instead of ~/.gnupg display-charset utf-8 # bypass all translation and assume that the OS uses native UTF-8 encoding keyserver name # use name as your keyserver no-greeting # suppress the initial copyright message armor # create ASCII armored output. Default is binary OpenPGP format
If you want to set up default options for a multi-user system, the configuration file of defaults is expected in /etc/skel/.gnupg/. With that in place the new user configuration can be created with
# addgnupghome user1 user2
which will add the respective /home/userX/.gnupg/ and copy the files from the skeleton directory to it.
4 Basic keys management
4.1 Create key
- Set stronger algorithms to be used first:
~/.gnupg/gpg.conf
personal-digest-preferences SHA512 cert-digest-algo SHA512 default-preference-list SHA512 SHA384 SHA256 SHA224 AES256 AES192 AES CAST5 ZLIB BZIP2 ZIP Uncompressed
- Generate a private key by typing in a terminal:
$ gpg --full-gen-key
You will be asked several questions. In general, most users will want both a RSA (sign only) and a RSA (encrypt only) key. It is advised to use a keysize of 4096 bits (default is 2048).
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.
4.2 Manage your key
- Running the gpg --edit-key <user-id> command will present a menu which enables you to do most of your key management related tasks. Following is an example to set your expiration date:
$ gpg --edit-key <user-id> > key number > expire yyyy-mm-dd > save > quit
Some useful commands:
> passwd # change the passphrase > clean # compact any user ID that is no longer usable (e.g revoked or expired) > revkey # revoke a key > addkey # add a subkey to this key > expire # change the key expiration time
- Generate an ASCII version of your public key (e.g. to distribute it by e-mail):
$ gpg --armor --output public.key --export <user-id>
- Register your key with a public PGP key server, so that others can retrieve your key without having to contact you directly:
$ gpg --keyserver pgp.mit.edu --send-keys <user-id>
- Sign and encrypt for user Bob
$ gpg se -r Bob file
- make a clear text signature
$ gpg --clearsign file
4.3 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 <user-id> > addkey
And answer the following questions it asks (see previous section for suggested settings).
- Save changes
> save
- Update it to a keyserver.
$ gpg --keyserver pgp.mit.edu --send-keys <user-id>
4.4 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
4.5 List keys
- Keys in your public key ring:
$ gpg --list-keys
- Keys in your secret key ring:
$ gpg --list-secret-keys
5 Encrypt and decrypt
When encrypting or decrypting it is possible to have more than one private key in use. If this occurs you need to select the active key. This can be done by using the option -u UID or by using the option --local-user UID. This causes the default key to use to be replaced by wanted key.
To encrypt a file, use:
$ gpg --encrypt secret.tar -o secret.tar.gpg
- If you want to change recipient this can be done by the option -r or by the option --recipient.
- You can use gnupg to encrypt your sensitive documents, but only individual files at a time. If you want to encrypt directories or a whole file-system you should consider using TrueCrypt or EncFS, though you can always tarball various files and then encrypt them.
To decrypt a file, use:
$ gpg --decrypt secret.tar.gpg
You'll be prompted to enter your passphrase.
6 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
if [ $EUID -ne 0 ] ; then
envfile="$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
fi
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) &
Log out of your Xsession and log back in. Check if gpg-agent is activated
$ pgrep gpg-agent
6.1 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.
7 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.
8 Smartcards
GnuPG uses scdaemon as an interface to your smartcard reader, please refer to scdaemon man page for details.
8.1 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.
8.2 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.
9 Troubleshooting
9.1 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/.
9.2 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 pinentry-qt. See GnuPG#Pinentry.
9.3 KGpg configuration permissions
There have been issues with kdeutils-kgpg being able to access the ~/.gnupg/ options. One issue might be a result of a deprecated options file, see the bug report.
Another user reported that KGpg failed to start until the ~/.gnupg folder is set to drwxr-xr-x permissions. If you require this work-around, ensure that the directory contents retain -rw------- permissions! Further, report it as a bug to the developers.
