Yesterday I started putting my GPG setup in order… I should have done it a long time ago, as it was quite messy:
- My main key was still an ancient 1024 bits DSA key from I create in ‘03
- a few others were not used anymore
- a more recent one was a smartcard-generated 2048 bits RSA key, which was way too easy to lose
- I didn’t have a proper backup plan for all this
In addition, I wanted to play with new Yubikeys that support OpenPGP from the get go, and to clean up my approach to development machine bootstrapping, by using said yubikey as well.
So here are the components of this brand new shiny setup
- 2 secure USB keys for backup
- 2 Yubikeys 4
- 1 master key that will be on secure storage only (on a daily basis I’m gonna use only subkeys)
- 1 encryption subkey that’s on secure storage and replicated on each yubikey
- 1 signing and 1 authentication subkey per yubikey, generated on the yubikeys directly (they can die with them)
- of course, revocation certificates for everything, stored on secure storage
- keychain for managing my ssh- and gpg-agent
- a keybase graph centered around the master key
Secure USB keys
I purchased 2 of these bad boys: Apricorn Aegis Secure Key
The idea is that I’m going to keep them in sync as much as possible, with one being available at home and one in a secure location. I’m using them to hold copies of my master key, revocation certificates, and a few other important things. They are pin-protected, FIPS-compliant (military-grade) and will destroy their content after a few (10 I think) attempts (which is why it’s important to keep one of them somewhere safe !) All in all, good enough for me :)
On a daily basis, I use a low-profile Yubikey 4 Nano
As a backup, I also have a regular Yubikey 4
Of course, I don’t use them only for GPG: U2F is super convenient for protecting my Google/GitHub/Dropbox/… accounts. Still, in the context of this post, the built-in support for OpenPGP is what I’m interested in. That and the fact that they finally support 4096 bits RSA keys. Definitely a step up from the 2048 bits max of the previous generation, and most likely sufficient for the foreseeable future.
GPG keys generation
I won’t go into the details of the keys generation, as I pretty much applied methods found elsewhere:
- https://www.esev.com/blog/post/2015-01-pgp-ssh-key-on-yubikey-neo/ provides
a very nice step-by-step tutorial to generate the kind of keys I’m
interested in. Remember that the 2048 bits limitation doesn’t apply to
Yubikey 4, and that there’s no need to run
ykpersonalize -m82to enable OpenPGP on these.
- https://wiki.debian.org/Subkeys describes good practices for keys
- https://blog.josefsson.org/2014/06/23/offline-gnupg-master-key-and-subkeys-on-yubikey-neo-smartcard/ also has great content (and goes a bit further, by using a completely offline machine)
As for key sizes, both my master and my encryption key are 4096 bits RSA keys. All signing and authentication keys are 3072 bits RSA keys.
In the end, my key looks like this:
$ gpg2 --edit-key 2B412392 gpg (GnuPG/MacGPG2) 2.0.28; Copyright (C) 2015 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. Secret key is available. pub 4096R/0xDC8615542B412392 created: 2016-03-11 expires: 2017-03-11 usage: C trust: ultimate validity: ultimate sub 3072R/0x3F0AC4425EAFEC31 created: 2016-03-11 expires: 2017-03-11 usage: S sub 3072R/0x03FCFA93348F287F created: 2016-03-11 expires: 2017-03-11 usage: A sub 3072R/0xA72C149B819C180C created: 2016-03-11 expires: 2017-03-11 usage: S sub 3072R/0x115C66B8C52959D4 created: 2016-03-11 expires: 2017-03-11 usage: A sub 4096R/0xBE7F9034433375D0 created: 2016-03-11 expires: 2017-03-11 usage: E [ultimate] (1). Yann Hodique <email@example.com> [ultimate] (2) Yann Hodique <firstname.lastname@example.org> [ultimate] (3) Yann Hodique <email@example.com> [ultimate] (4) Yann Hodique <firstname.lastname@example.org>
The “usage” column on the far right defines what those (sub-)keys are for: Certification, Signature, Authentication and Encryption. As expected, 2 “S” and 2 “A”.
I use Zsh as my shell of choice. In order to make sure
the right keys are made available, I’m running something like this in my
eval `keychain -q --nogui --eval --agents ssh,gpg --inherit any-once id_rsa`
As I briefly mentioned, each Yubikey is also holding an authentication subkey, which can be used an SSH key. Therefore, the content of my agent looks like this in the end:
$ ssh-add -l 3072 SHA256:... cardno:000604163319 (RSA) 2048 SHA256:... /Users/hodiquey/.ssh/id_rsa (RSA)
The first line corresponds to the yubikey-held key. The public part of it can be obtained with:
$ gpgkey2ssh 433375D0 ssh-rsa AAAA...
That part can then be uploaded to wherever (GitHub for example)
Now, shell integration is good, but most of the time I use all this from Emacs instead.
Just because I’m lazy, I piggy-back on my shell configuration with the following snippet:
(use-package exec-path-from-shell :ensure t :config (when (memq window-system '(mac ns)) (exec-path-from-shell-initialize) (exec-path-from-shell-copy-envs '("PYTHONPATH" "GOPATH" "SSH_AUTH_SOCK" "SSH_AGENT_PID" "GPG_AGENT_INFO"))))
Amongst other things, it makes sure
GPG_AGENT_INFO is properly set, which is
what Emacs will need to communicate with my already-running
The reason this is needed is that on OSX my Emacs is not run from a shell, so
it doesn’t inherit all the right environment variables it should
is a nice module that works around that.
Obviously, GPG keys are meant to be used by others, so publication is key (ok, that was a bad one). I’m using pool.sks-keyservers.net most of the time, so I published my key there as well. For all practical purposes it’s just the right thing to do.
$ gpg2 --keyserver hkps://hkps.pool.sks-keyservers.net --send-key 2B412392
After that, all I needed to do was to revoke the bunch of old GPG keys I had lying around. Good riddance.