2022-12-27 22:04:45 +01:00

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GnuPG Quickstart

I love GPG and the way it works. I know there are many that complain about it because it has flaws. My stance on this is that I prefer battle-tested software with known flaws to something with unknown flaws.

Anyway, this should get you started with GnuPG

Prerequisites

Install gpg and pinentry.

# pkg_add gnupg pinentry

You need a Key

If you want to lock and unlock stuff, you need a key. This is how you get to one:

$ gpg --generate-key

Hop through the wizard until you see these lines:

pub   rsa3072 2021-05-19 [SC] [expires: 2023-05-19]
      BA696588D9A04AD9F70DA33EC54733F6DBECC2C1
uid                      John Doe <j.doe@example.com>
sub   rsa3072 2021-05-19 [E] [expires: 2023-05-19]

If you see an error like: gpg: agent_genkey failed: Permission denied

Add the following entry and try again.

$ echo "allow-loopback-pinentry" >> ~/.gnupg/gpg-agent.conf

Congratulations, you got yourself a GPG Key. This long gibberish is your full GPG Key ID. Most of the time, you can simply use the last 8 characters. So the short version of this GPG Key is DBECC2C1.

You can set it as default key, so it's used to encrypt stuff when no explicit key is given.

$ echo "default-key DBECC2C1" >> ~/.gnupg/gpg.conf

If you want someone to be able to encrypt something for you, send him or her the output of:

$ gpg --export -a DBECC2C1

You can also use your email address instead of the Key ID, if you have only one key with it. This key is public. So put it on some webspace and add a link to your email header or signature.

Upload the key so people can find it (optional)

You can also upload your key to a key server. For this, configure a keyserver:

$ echo "keyserver hkps://keys.openpgp.org" >>  ~/.gnupg/gpg.conf

Then send your key to it:

$ gpg --send-keys DBECC2C1

You got a key from someone

Add a key from someone else to gnupg, so you can use it to encrypt data for this person. If the key is on your harddrive, use:

$ gpg --import <pubkeyfile.asc>

The file ending here is kind of undefined. Some call it .asc, .gpg, .pub or .key. If the key is on a key server, you can import it like so:

$ gpg --recv-key 52BE43BA

This would import my key. You can look at it now with:

$ gpg --list-keys 52BE43BA

Encrypt a file

This encrypts the file plain.txt with the public key DBECC2C1.

$ gpg --encrypt -r DBECC2C1 file.txt

Now you have file.txt.gpg, which is the encrypted version

Decrypt a file

GnuPG automaticall figures out what key it can use to decrypt a file. So this will output the content of file.txt on the terminal. If you want to save the output in a file, add -o file.txt.

$ gpg -d file.txt.gpg 
$ gpg -d file.txt.gpg -o file.txt

Choose a better password prompt (optional)

You can change the way gpg asks for the password:

$ cat ~/.gnupg/gpg-agent.conf
[...]
pinentry-program /usr/local/bin/pinentry-curses
[...]

Options are:

pinentry (sometimes also called pinentry-tty)
pinentry-curses
pinentry-gtk2: pkg_add pinentry-gtk2
pinentry-gnome3: pkg_add pinentry-gnome3
pinentry-dmenu: https://github.com/ritze/pinentry-dmenu

Note: If you use a console pinentry program and want to use gpg with a GUI tool (like thunderbird), the password prompt will be invisible and gpg/thunderbird will freeze.

Makes sense, doesn't it?

Start GPG Agent for password caching (optional)

Put this in your .kshrc or .bashrc:

export GPG_TTY=$(tty)
gpg-connect-agent /bye

Make a Backup (not so optional)

There is no handholding cloud or support team you can call when you messed up or deleted your key. So back it up safely.

Either you backup your ~/.gnugp directory, or you export the secret keys and backup them safely.

$ gpg --export-secret-keys -a DBECC2C1 > gpg_key_backup.sec

Seriously, don't skip this step.

Configure Mutt (optional)

Install mutt with the gpgme flavor. Gpgme is the "new way" of handling gpg in mutt.

# pkg_add mutt--gpgme

If you're not on OpenBSD, check with mutt -v if it was compiled with the --enable-gpgme option. Then enable it in mutt.

$ cat ~/.muttrc
[...]
crypt_use_gpgme = yes
[...]

In the mutt compose view, you can now select Security Options.

        From: c0dev0id <c0@example.com>
          To: j.doe@example.com
          Cc:
         Bcc:
     Subject: Hello my friend
    Reply-To:
         Fcc: =Sent
    Security: Sign, Encrypt (PGP/MIME)
     Sign as: <default>

You can change the setting with the key "p", which should bring up a selection menu.

PGP (e)ncrypt, (s)ign, sign (a)s, (b)oth, s/(m)ime or (c)lear?

That's it! GPG is not difficult. You need to know a few bits, but these are not more difficult than many other things we do on a daily basis.

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