GnuPG
| Related |
|---|
| pacman/Package signing |
| Disk encryption |
| List of applications/Security#Encryption, signing, steganography |
According to the official website:
- GnuPG is a complete and free implementation of the OpenPGP standard as defined by RFC4880 (also known as PGP). 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. GnuPG, also known as GPG, is a command line tool with features for easy integration with other applications. A wealth of frontend applications and libraries are available. Version 2 of GnuPG also provides support for S/MIME and Secure Shell (ssh).
Contents
- 1 Installation
- 2 Configuration
- 3 Usage
- 4 Key maintenance
- 5 Signatures
- 6 gpg-agent
- 7 Smartcards
- 8 Tips and tricks
- 9 Troubleshooting
- 9.1 Not enough random bytes available
- 9.2 su
- 9.3 Agent complains end of file
- 9.4 KGpg configuration permissions
- 9.5 GNOME on Wayland overrides SSH agent socket
- 9.6 mutt and gpg
- 9.7 "Lost" keys, upgrading to gnupg version 2.1
- 9.8 gpg hanged for all keyservers (when trying to receive keys)
- 9.9 Smartcard not detected
- 10 See also
- 11 Acknowledgement
1 Installation
This will also install pinentry, a collection of simple PIN or passphrase entry dialogs which GnuPG uses for passphrase entry. Which pinentry dialog is used is determined by the symbolic link /usr/bin/pinentry, which by default points to /usr/bin/pinentry-gtk-2.
If you want to use a graphical frontend or program that integrates with GnuPG, see List of applications/Security#Encryption, signing, steganography.
2 Configuration
2.1 Directory location
$GNUPGHOME is used by GnuPG to point to the directory where its configuration files are stored. By default $GNUPGHOME is not set and your $HOME is used instead; thus, you will find a ~/.gnupg directory right after installation.
To change the default location, either run gpg this way $ gpg --homedir path/to/file or set GNUPGHOME in one of your regular startup files.
2.2 Configuration files
The default configuration files are ~/.gnupg/gpg.conf and ~/.gnupg/dirmngr.conf.
By default, the gnupg directory has its permissions set to 700 and the files it contains have their permissions set to 600. Only the owner of the directory has permission to read, write, and access the files. This is for security purposes and should not be changed. In case this directory or any file inside it does not follow this security measure, you will get warnings about unsafe file and home directory permissions.
Append to these files any long options you want. Do not write the two dashes, but simply the name of the option and required arguments. You will find skeleton files in /usr/share/gnupg. These files are copied to ~/.gnupg the first time gpg is run if they do not exist there. Other examples are found in #See also.
Additionally, pacman uses a different set of configuration files for package signature verification. See Pacman/Package signing for details.
2.3 Default options for new users
If you want to setup some default options for new users, put configuration files in /etc/skel/.gnupg/. When the new user is added in system, files from here will be copied to its GnuPG home directory. There is also a simple script called addgnupghome which you can use to create new GnuPG home directories for existing users:
# addgnupghome user1 user2
This will add the respective /home/user1/.gnupg and /home/user2/.gnupg and copy the files from the skeleton directory to it. Users with existing GnuPG home directory are simply skipped.
3 Usage
3.1 Create key pair
Generate a key pair by typing in a terminal:
$ gpg --full-gen-key
The command will prompt for answers to several questions. For general use most people will want:
- the RSA (sign only) and a RSA (encrypt only) key.
- a keysize of the default value (2048). A larger keysize of 4096 "gives us almost nothing, while costing us quite a lot"[1].
- an expiration date. A period of a year is good enough for the average user. This way even if access is lost to the keyring, it will allow others to know that it is no longer valid. Later, if necessary, the expiration date can be extended without having to re-issue a new key.
- your name and email address. You can add multiple identities to the same key later (e.g., if you have multiple email addresses you want to associate with this key).
- no optional comment. Since the semantics of the comment field are not well-defined, it has limited value for identification.
- a secure passphrase.
3.2 List keys
To list keys in your public key ring:
$ gpg --list-keys
To list keys in your secret key ring:
$ gpg --list-secret-keys
3.3 Export your public key
gpg's main usage is to ensure confidentiality of exchanged messages via public-key cryptography. With it each user distributes the public key of their keyring, which can be be used by others to encrypt messages to the user. The private key must always be kept private, otherwise confidentiality is broken. See w:Public-key cryptography for examples about the message exchange.
So, in order for others to send encrypted messages to you, they need your public key.
To generate an ASCII version of your public key (e.g. to distribute it by e-mail):
$ gpg --output public.key --armor --export <user-id>
Alternatively, or in addition, you can #Use a keyserver to share your key.
3.4 Import a public key
In order to encrypt messages to others, as well as verify their signatures, you need their public key. To import a public key with file name public.key to your public key ring:
$ gpg --import public.key
Alternatively, #Use a keyserver to find a public key.
3.5 Use a keyserver
You can register your key with a public PGP key server, so that others can retrieve your key without having to contact you directly:
$ gpg --send-keys <key-id>
To find out details of a key on the keyserver, without importing it, do:
$ gpg --search-keys <key-id>
To import a key from a key server:
$ gpg --recv-keys <key-id>
- You should verify the authenticity of the retrieved public key by comparing its fingerprint with one that the owner published on an independent source(s) (i.e., contacting the person directly). See Wikipedia:Public key fingerprint for more information.
- Using a short ID may encounter collisions. All keys will be imported that have the short ID. To avoid this, use the full fingerprint or long key ID when receiving a key.[2]
- Adding keyserver-options auto-key-retrieve to gpg.conf will automatically fetch keys from the key server as needed.
- An alternative key server is pool.sks-keyservers.net and can be specified with keyserver in dirmngr.conf.; see also wikipedia:Key server (cryptographic)#Keyserver examples.
- You can connect to the keyserver over Tor using --use-tor. hkp://jijrk5u4osbsr34t5.onion is the onion address for the sks-keyservers pool. See this GnuPG blog post for more information.
- You can connect to a keyserver using a proxy by setting the http_proxy environment variable and setting honor-http-proxy in dirmngr.conf. Alternatively, set http-proxy host[:port] in dirmngr.conf, overriding the http_proxy environment variable.
3.6 Encrypt and decrypt
You need to #Import a public key of a user before encrypting (options --encrypt or -e) a file or message to that recipient (options --recipient or -r).
To encrypt a file with the name doc, use:
$ gpg --recipient <user-id> --encrypt doc
To decrypt (option --decrypt or -d) a file with the name doc.gpg encrypted with your public key, use:
$ gpg --output doc --decrypt doc.gpg
gpg will prompt you for your passphrase and then decrypt and write the data from doc.gpg to doc. If you omit the -o (--output) option, gpg will write the decrypted data to stdout.
- Add --armor to encrypt a file using ASCII armor (suitable for copying and pasting a message in text format)
- Use -R <user-id> or --hidden-recipient <user-id> instead of -r to not put the recipient key IDs in the encrypted message. This helps to hide the receivers of the message and is a limited countermeasure against traffic analysis.
- Add --no-emit-version to avoid printing the version number, or add the corresponding setting to your configuration file.
- You can use gnupg to encrypt your sensitive documents by using your own user-id as recipient, but only individual files at a time, though you can always tarball various files and then encrypt the tarball. See also Disk encryption#Available methods if you want to encrypt directories or a whole file-system.
4 Key maintenance
4.1 Backup your private key
To backup your private key do the following:
$ gpg --export-secret-keys --armor <user-id> > privkey.asc
Note that gpg release 2.1 changed default behaviour so that the above command enforces a passphrase protection, even if you deliberately chose not to use one on key creation. This is because otherwise anyone who gains access to the above exported file would be able to encrypt and sign documents as if they were you without needing to know your passphrase.
To import the backup of your private key:
$ gpg --allow-secret-key-import --import privkey.asc
4.2 Edit 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.
Some useful commands in the edit key sub menu:
> 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
Type help in the edit key sub menu for more commands.
4.3 Exporting subkey
If you plan to use the same key across multiple devices, you may want to strip out your master key and only keep the bare minimum encryption subkey on less secure systems.
First, find out which subkey you want to export.
$ gpg -K
Select only that subkey to export.
$ gpg -a --export-secret-subkeys [subkey id]! > /tmp/subkey.gpg
At this point you could stop, but it is most likely a good idea to change the passphrase as well. Import the key into a temporary folder.
$ gpg --homedir /tmp/gpg --import /tmp/subkey.gpg $ gpg --homedir /tmp/gpg --edit-key <user-id> > passwd > save $ gpg --homedir /tmp/gpg -a --export-secret-subkeys [subkey id]! > /tmp/subkey.altpass.gpg
At this point, you can now use /tmp/subkey.altpass.gpg on your other devices.
4.4 Rotating subkeys
If you have set your subkeys to expire after a set time, you can create new ones. Do this a few weeks in advance 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>
5 Signatures
Signatures certify and timestamp documents. If the document is modified, verification of the signature will fail. Unlike encryption which uses public keys to encrypt a document, signatures are created with the user's private key. The recipient of a signed document then verifies the signature using the sender's public key.
5.1 Sign a file
To sign a file use the --sign or -s flag:
$ gpg --output doc.sig --sign doc
The above also encrypts the file and stores it in binary format.
5.2 Clearsign a file or message
To sign a file without compressing it into binary format use:
$ gpg --clearsign doc
This wraps the document into an ASCII-armored signature, but does not modify the document.
5.3 Make a detached signature
To create a separate signature file to be distributed separately from the document or file itself, use the --detach-sig flag:
$ gpg --output doc.sig --detach-sig doc
This method is often used in distributing software projects to allow users to verify that the program has not been modified by a third party.
5.4 Verify a signature
To verify a signature use the --verify flag:
$ gpg --verify doc.sig
where doc.sig is the signature you wish to verify.
To verify and decrypt a file at the same time, use the --decrypt flag as you normally would in decrypting a file.
If you are verifying a detached signature, both the file and the signature must be present when verifying. For example, to verify Parabola's latest SHA512 iso checksums you would do:
$ gpg --verify SHA512SUMS.sig
where SHA512SUMS must be located in the same directory.
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.
Starting with GnuPG 2.1.0 the use of gpg-agent is required. gpg-agent is started on-demand by the GnuPG tools, so there is usually no reason to start it manually.
6.1 Configuration
gpg-agent can be configured via ~/.gnupg/gpg-agent.conf file. The configuration options are listed in man gpg-agent. For example you can change cache ttl for unused keys:
~/.gnupg/gpg-agent.conf
default-cache-ttl 3600
$ /usr/lib/gnupg/gpg-preset-passphrase --preset XXXXXX
where XXXX is the keygrip. You can get its value when running gpg --with-keygrip -K. Passphrase will be stored until gpg-agent is restarted. If you set up default-cache-ttl value, it will take precedence.
6.2 Reload the agent
After changing the configuration, reload the agent using gpg-connect-agent:
$ gpg-connect-agent reloadagent /bye
The command should print OK.
6.3 pinentry
Finally, the agent needs to know how to ask the user for the password. This can be set in the gpg-agent configuration file.
The default uses a gtk dialog. There are other options - see info pinentry. To change the dialog implementation set pinentry-program configuration option:
~/.gnupg/gpg-agent.conf
# PIN entry program # pinentry-program /usr/bin/pinentry-curses # pinentry-program /usr/bin/pinentry-qt # pinentry-program /usr/bin/pinentry-kwallet pinentry-program /usr/bin/pinentry-gtk-2
{{Tip|For using /usr/bin/pinentry-kwallet you have to install the kwalletcli
After making this change, reload the gpg-agent.
6.4 Start gpg-agent with systemd user
It is possible to use the Systemd/User facilities to start the agent.
Create a systemd unit file:
~/.config/systemd/user/gpg-agent.service
[Unit] Description=GnuPG private key agent IgnoreOnIsolate=true [Service] Type=forking ExecStart=/usr/bin/gpg-agent --daemon Restart=on-abort [Install] WantedBy=default.target
6.5 Unattended passphrase
Starting with GnuPG 2.1.0 the use of gpg-agent and pinentry is required, which may break backwards compatibility for passphrases piped in from STDIN using the --passphrase-fd 0 commandline option. In order to have the same type of functionality as the older releases two things must be done:
First, edit the gpg-agent configuration to allow loopback pinentry mode:
~/.gnupg/gpg-agent.conf
allow-loopback-pinentry
Restart the gpg-agent process if it is running to let the change take effect.
Second, either the application needs to be updated to include a commandline parameter to use loopback mode like so:
$ gpg --pinentry-mode loopback ...
...or if this is not possible, add the option to the configuration:
~/.gnupg/gpg.conf
pinentry-mode loopback
6.6 SSH agent
gpg-agent has OpenSSH agent emulation. If you already use the GnuPG suite, you might consider using its agent to also cache your SSH keys. Additionally, some users may prefer the PIN entry dialog GnuPG agent provides as part of its passphrase management.
To start using GnuPG agent for your SSH keys, enable SSH support in the ~/.gnupg/gpg-agent.conf file:
~/.gnupg/gpg-agent.conf
enable-ssh-support
Next, make sure that gpg-agent is always started. Either follow #Start gpg-agent with systemd user, or add the following to your .bashrc file:
~/.bashrc
# Start the gpg-agent if not already running
if ! pgrep -x -u "${USER}" gpg-agent >/dev/null 2>&1; then
gpg-connect-agent /bye >/dev/null 2>&1
fi
Then set SSH_AUTH_SOCK so that SSH will use gpg-agent instead of ssh-agent. To make sure each process can find your gpg-agent instance regardless of e.g. the type of shell it is child of use pam_env.
~/.pam_environment
SSH_AGENT_PID DEFAULT=
SSH_AUTH_SOCK DEFAULT="${XDG_RUNTIME_DIR}/gnupg/S.gpg-agent.ssh"
Alternatively, depend on Bash:
~/.bashrc
# Set SSH to use gpg-agent
unset SSH_AGENT_PID
if [ "${gnupg_SSH_AUTH_SOCK_by:-0}" -ne $$ ]; then
export SSH_AUTH_SOCK="/run/user/$UID/gnupg/S.gpg-agent.ssh"
fi
- If you use non-default GnuPG #Directory location, run gpgconf --create-socketdir to create a socket directory under /run/user/$UID/gnupg/. Otherwise the socket will be placed in the GnuPG home directory.
- The test involving the gnupg_SSH_AUTH_SOCK_by variable is for the case where the agent is started as gpg-agent --daemon /bin/sh, in which case the shell inherits the SSH_AUTH_SOCK variable from the parent, gpg-agent [4].
Also set the GPG_TTY and refresh the TTY in case user has switched into an X session as stated in man gpg-agent. For example:
~/.bashrc
# Set GPG TTY export GPG_TTY=$(tty) # Refresh gpg-agent tty in case user switches into an X session gpg-connect-agent updatestartuptty /bye >/dev/null
Once gpg-agent is running you can use ssh-add to approve keys, following the same steps as for ssh-agent. The list of approved keys is stored in the ~/.gnupg/sshcontrol file. Once your key is approved, you will get a pinentry dialog every time your passphrase is needed. You can control passphrase caching in the ~/.gnupg/gpg-agent.conf file. The following example would have gpg-agent cache your keys for 3 hours:
~/.gnupg/gpg-agent.conf
default-cache-ttl-ssh 10800 max-cache-ttl-ssh 10800
7 Smartcards
GnuPG uses scdaemon as an interface to your smartcard reader, please refer to the man page for details.
7.1 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.
7.2 GnuPG with PSCD-Lite
PSCD-Lite is a daemon which handles access to smartcard (SCard API). If GnuPG's scdaemon fails to connect the smartcard directly (e.g. by using its integrated CCID support), it will fallback and try to find a smartcard using the PSCD-Lite driver.
7.2.1 Always use PSCD-Light
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:
~/.gnupg/scdaemon.conf
pcsc-driver /usr/lib/libpcsclite.so card-timeout 5 disable-ccid
Please check man scdaemon if you do not use OpenSC.
8 Tips and tricks
8.1 Different algorithm
You may want to use stronger algorithms:
~/.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 personal-cipher-preferences TWOFISH CAMELLIA256 AES 3DES
In the latest version of GnuPG, the default algorithms used are SHA256 and AES, both of which are secure enough for most people. However, if you are using a version of GnuPG older than 2.1, or if you want an even higher level of security, then you should follow the above step.
8.2 Encrypt a password
It can be useful to encrypt some password, so it will not be written in clear on a configuration file. A good example is your email password.
First create a file with your password. You need to leave one empty line after the password, otherwise gpg will return an error message when evaluating the file.
Then run:
$ gpg -e -a -r <user-id> your_password_file
-e is for encrypt, -a for armor (ASCII output), -r for recipient user ID.
You will be left with a new your_password_file.asc file.
8.3 Revoking a key
- Anybody having access to your revocation certificate can revoke your key, rendering it useless.
- Key revocation should only be performed if your key is compromised or lost, or you forget your passphrase.
Revocation certificates are automatically generated for newly generated keys, although one can be generated manually by the user later. These are located at ~/.gnupg/openpgp-revocs.d/. The filename of the certificate is the fingerprint of the key it will revoke.
To revoke your key, simply import the revocation certificate:
$ gpg --import <fingerprint>.rev
Now update the keyserver:
$ gpg --keyserver subkeys.pgp.net --send <userid>
8.4 Change trust model
By default GnuPG uses the Web of Trust as the trust model. You can change this to Trust on first use by adding --trust-model=tofu when adding a key or adding this option to your GnuPG configuration file. More details are in this email to the GnuPG list.
8.5 Hide all recipient id's
By default the recipient's key ID is in the encrypted message. This can be removed at encryption time for a recipient by using hidden-recipient <user-id>. To remove it for all recipients add throw-keyids to your configuration file. This helps to hide the receivers of the message and is a limited countermeasure against traffic analysis. (Using a little social engineering anyone who is able to decrypt the message can check whether one of the other recipients is the one he suspects.) On the receiving side, it may slow down the decryption process because all available secret keys must be tried (e.g. with --try-secret-key <user-id>).
8.6 Using caff for 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 he Web of Trust. Keysigning parties allow users to get together in physical location to validate keys. The Zimmermann-Sassaman key-signing protocol is a way of making these very effective. Here you will find a how-to article.
To send the signatures to their owners you need a working MTA. If you do not have already one, install msmtp.
8.7 Always show long ID's and fingerprints
To always show long key ID's add keyid-format 0xlong to your configuration file. To always show full fingerprints of keys, add with-fingerprint to your configuration file.
9 Troubleshooting
9.1 Not enough random bytes available
When generating a key, gpg can run into this error:
Not enough random bytes available. Please do some other work to give the OS a chance to collect more entropy!
To check the available entropy, check the kernel parameters:
cat /proc/sys/kernel/random/entropy_avail
A healthy Linux system with a lot of entropy available will have return close to the full 4,096 bits of entropy. If the value returned is less than 200, the system is running low on entropy.
To solve it, remember you do not often need to create keys and best just do what the message suggests (e.g. create disk activity, move the mouse, edit the wiki - all will create entropy). If that does not help, check which service is using up the entropy and consider stopping it for the time. If that is no alternative, see Random number generation#Faster alternatives.
9.2 su
When using pinentry, you must have the proper permissions 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/.
# script -q -c "gpg --gen-key" /dev/null
9.3 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 #pinentry.
9.4 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.
9.5 GNOME on Wayland overrides SSH agent socket
For Wayland sessions, gnome-session sets SSH_AUTH_SOCK to the standard gnome-keyring socket, $XDG_RUNTIME_DIR/keyring/ssh. This overrides any value set in ~/.pam_environmment or systemd unit files.
To disable this behavior, set the GSM_SKIP_AGENT_WORKAROUND variable:
~/.pam_environment
SSH_AGENT_PID DEFAULT=
SSH_AUTH_SOCK DEFAULT="${XDG_RUNTIME_DIR}/gnupg/S.gpg-agent.ssh"
GSM_SKIP_SSH_AGENT_WORKAROUND DEFAULT="true"
9.6 mutt and gpg
To be asked for your GnuPG password only once per session as of GnuPG 2.1, see this forum thread.
9.7 "Lost" keys, upgrading to gnupg version 2.1
When gpg --list-keys fails to show keys that used to be there, and applications complain about missing or invalid keys, some keys may not have been migrated to the new format.
Please read GnuPG invalid packet workaround. Basically, it says that there is a bug with keys in the old pubring.gpg and secring.gpg files, which have now been superseded by the new pubring.kbx file and the private-keys-v1.d/ subdirectory and files. Your missing keys can be recovered with the following commnads:
$ cd $ cp -r .gnupg gnupgOLD $ gpg --export-ownertrust > otrust.txt $ gpg --import .gnupg/pubring.gpg $ gpg --import-ownertrust otrust.txt $ gpg --list-keys
9.8 gpg hanged for all keyservers (when trying to receive keys)
If gpg hanged with a certain keyserver when trying to receive keys, you might need to kill dirmngr in order to get access to other keyservers which are actually working, otherwise it might keeping hanging for all of them.
9.9 Smartcard not detected
Your user might not have the permission to access the smartcard which results in a card error to be thrown, even though the card is correctly set up and inserted.
One possible solution is to add a new group scard including the users who need access to the smartcard.
Then use an udev rule, similar to the following:
/etc/udev/rules.d/71-gnupg-ccid.rules
ACTION=="add", SUBSYSTEM=="usb", ENV{ID_VENDOR_ID}=="1050", ENV{ID_MODEL_ID}=="0116|0111", MODE="660", GROUP="scard"
One needs to adapt VENDOR and MODEL according to the lsusb output, the above example is for a YubikeyNEO.
10 See also
- GNU Privacy Guard Homepage
- RFC4880 "OpenPGP Message Format"
- Creating GPG Keys (Fedora)
- OpenPGP subkeys in Debian
- A more comprehensive gpg Tutorial
- gpg.conf recommendations and best practices
- Torbirdy gpg.conf
- /r/GPGpractice - a subreddit to practice using GnuPG.
