About me: My name is Solène Rapenne. I like learning and sharing experiences about IT stuff. Hobbies: '(BSD OpenBSD h+ Lisp cmdline gaming internet-stuff Crossbow). I love percent and lambda characters. OpenBSD developer solene@.

Contact me: solene on Freenode, solene+www at dataswamp dot org or solene@bsd.network (mastodon)

GPG2 cheatsheet

Written by Solène, on 06 September 2019.
Tags: #security

Introduction

I don’t use gpg a lot but it seems the only tool out there for encrypting data which “works” and widely used.

So this is my personal cheatsheet for everyday use of gpg.

In this post, I use the command gpg2 which is the binary to GPG version 2. On your system, “gpg” command could be gpg2 or gpg1. You can use gpg --versionif you want to check the real version behind gpg binary.

In your ~/.profile file you may need the following line:

export GPG_TTY=$(tty)

Install GPG

The real name of GPG is GnuPG, so depending on your system the package can be either gpg2, gpg, gnupg, gnugp2 etc…

On OpenBSD, you can install it with: pkg_add gnupg--%gnupg2

GPG Principle using private/public keys

  • YOU make a private and a public key (associated with a mail)
  • YOU give the public key to people
  • PEOPLE import your public key into they keyring
  • PEOPLE use your public key from the keyring
  • YOU will need your password everytime

I think gpg can do much more, but read the manual for that :)

Initialization

We need to create a public and a private key.

solene$ gpg2 --gen-key
gpg (GnuPG) 2.2.12; Copyright (C) 2018 Free Software Foundation, Inc.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Note: Use "gpg2 --full-generate-key" for a full featured key generation dialog.

GnuPG needs to construct a user ID to identify your key.

In this part, you should put your real name and your email address and validate with “O” if you are okay with the input. You will get ask for a passphrase after.

Real name: Solene
Email address: solene@domain.example
You selected this USER-ID:
    "Solene <solene@domain.example>"

Change (N)ame, (E)mail, or (O)kay/(Q)uit? o
We need to generate a lot of random bytes. It is a good idea to perform
some other action (type on the keyboard, move the mouse, utilize the
disks) during the prime generation; this gives the random number
generator a better chance to gain enough entropy.
We need to generate a lot of random bytes. It is a good idea to perform
some other action (type on the keyboard, move the mouse, utilize the
disks) during the prime generation; this gives the random number
generator a better chance to gain enough entropy.
gpg: key 368E580748D5CA75 marked as ultimately trusted
gpg: revocation certificate stored as '/home/solene/.gnupg/openpgp-revocs.d/7914C6A7439EADA52643933B368E580748D5CA75.rev'
public and secret key created and signed.

pub   rsa2048 2019-09-06 [SC] [expires: 2021-09-05]
      7914C6A7439EADA52643933B368E580748D5CA75
uid                    Solene <solene@domain.example>
sub   rsa2048 2019-09-06 [E] [expires: 2021-09-05]

The key will expire in 2 years, but this is okay. This is a good thing, if you stop using the key, it will die silently at it expiration time. If you still use it, you will be able to extend the expiracy time and people will be able to notice you still use that key.

Export the public key

If someone asks your GPG key, this is what they want:

gpg2 --armor --export solene@domain.example > solene.asc

Import a public key

Import the public key:

gpg2 --import solene.asc

Encrypt a file for someone

If you want to send file picture.jpg to remote@mail then use the command:

gpg2 --encrypt --recipient remote@domain.example picture.jpg > picture.jpg.gpg

You can now send picture.jpg.gpg to remote@mail who will be able to read the file with his/her private key.

You can use `–armor`` parameter to make the output plaintext, so you can put it into a mail or a text file.

Decrypt a file

Easy!

gpg2 --decrypt image.jpg.gpg > image.jpg

Get public key fingerprint

The fingerprint is a short string made out of your public key and can be embedded in a mail (often as a signature) or anywhere.

It allows comparing a public key you received from someone with the fingerprint that you may find in mailing list archives, twitter, a html page etc.. if the person spreaded it somewhere. This allow to multiple check the authenticity of the public key you received.

it looks like:

4398 3BAD 3EDC B35C 9B8F  2442 8CD4 2DFD 57F0 A909

This is my real key fingerprint, so if I send you my public key, you can use the fingerprint from this page to check it matches the key you received!

You can obtain your fingerprint using the following command:

solene@t480 ~ $ gpg2 --fingerprint
pub   rsa4096 2018-06-08 [SC]
      4398 3BAD 3EDC B35C 9B8F  2442 8CD4 2DFD 57F0 A909
uid          [  ultime ] XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
sub   rsa4096 2018-06-08 [E]

Add a new mail / identity

If for some reason, you need to add another mail to your GPG key (like personal/work keys) you can create a new identity with the new mail.

Type gpg2 --edit-key solene@domain.example and then in the prompt, type adduid and answer questions.

You can now export the public key with a different identity.

List known keys

If you want to get the list of keys you imported, you can use

gpg2 -k

Testing

If you want to do some tests, I’d recommend making new users on your system, exchanges their keys and try to encrypt a message from one user to another.

I have a few spare users on my system on which I can ssh locally for various tests, it is always useful.