Post about over-the-top PGPing

site/content/post/2016-08-11-offline-gnupg-master-yubikey-subkeys.md

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+---
+date: 2016-08-11
+strapline: With bonus completely over-the-top security
+thumbnail: /res/images/yubikey/keys.thumb.png
+title: Creating an offline GnuPG master key with Yubikey-stored subkeys
+url: /2016/08/11/offline-gnupg-master-yubikey-subkeys/
+---
+
+<div class="image left">
+ <img src="/res/images/yubikey/keys.png" alt="Two yubikeys">
+</div>
+
+I recently noticed that I'd accidentally lost my previous GPG private key &mdash; whoops. It was on
+a drive that I'd since formatted and used for a fair amount of time, so there's no hope of
+getting it back (but, on the plus side, there's also no risk of anyone else getting their hands on
+it). I could have created a new one in a few seconds and been done with it, but I decided to treat
+it as an exercise in doing things properly.
+
+### Background: GPG? Yubikey?
+
+GPG or GnuPG is short for [Gnu Privacy Guard](https://www.gnupg.org/), which is a suite of
+applications that provide cryptographic privacy and authentication functionality. At a basic level,
+it works in a similar way to HTTPS certificates: each user has a public key which is shared widely,
+and a private key that is unique to them. You can use someone else's public key to encrypt messages
+so only they can see them, and use your own private key to sign content so that others can verify
+it came from you.
+
+A [Yubikey](https://www.yubico.com/faq/yubikey/) is a small hardware device that offers two-factor
+authentication. Most Yubikey models also act as smartcards and allow you to store OpenPGP
+credentials on them.
+
+<!--more-->
+
+### Introducing subkeys
+
+GnuPG supports subkeys, which provide fairly significant security advantages. Instead of just having
+a single public and private key, you have a master pair and then any number of subkey pairs. The
+subkeys are automatically associated with the master key, but they can be revoked independently.
+
+Having a master key fall into the wrong hands is a problem &mdash; you have to revoke the whole
+thing (assuming you have access to a revocation certificate) and start again, convincing everyone
+else that your new key is the "real" you. With subkeys, you can issue a revocation signed with your
+master key and then sign some new subkeys. There's no loss of trust, and as long as people refresh
+your key from a keyserver, everything carries on as normal.
+
+The other advantage to using subkeys is that you can keep the master key elsewhere. It doesn't
+need to be routinely accessible, and using it doesn't require access to the Internet. The master
+key is kept offline, significantly reducing the risk of anything bad happening to it.
+
+### Setting up a secure environment
+
+My main desktop runs Windows, and most of my other devices are work ones which come with automatic
+backups and network mounts that I don't fully grok. Neither of those is a particularly good option
+if I want to do something security sensitive.
+
+[Tails](https://tails.boum.org/) is the defacto choice for a secure, live linux system, so I went
+through their installation process and eventually ended up with a USB drive that can boot into
+Tails. The installation process from Windows is slightly convoluted, as it involves creating a
+bootable Tails image, then booting to that and using the Tails installer to create the real image
+on a different drive. If you're starting on a Linux box, you can just use the Tails installer
+directly instead of doing the two-drive shuffle.
+
+Just to be completely paranoid, I disconnected my PC from the network before booting Tails. This
+is known as [air-gapping](https://en.wikipedia.org/wiki/Air_gap_(networking)), and is done to
+eliminate the possibility of a remote attacker doing something to your system. Ideally the machine
+would never have been connected to the network, but I didn't happen to have an unused machine
+laying around.
+
+The final thing I needed was a secure place to store my master key. I opted for an
+[IronKey](http://www.ironkey.com/en-US/) &mdash; a hardware-encrypted USB drive that self-destructs
+if there are too many unsuccessful attempts to access it. It works out-of-the-box on both Windows
+and Linux, presenting a small unencrypted drive with software to run to interact with the secure
+partition.
+
+### Creating the keys
+
+Now I had a nice over-the-top setup it was time to actually the keys. There is [an excellent
+guide by Simon Josefsson](https://blog.josefsson.org/2014/06/23/offline-gnupg-master-key-and-subkeys-on-yubikey-neo-smartcard/)
+that walks through the entire process of creating the master key, creating three subkeys, and
+then transferring them to a Yubikey.
+
+The only point where I had to deviate from Simon's guide was setting the machine up to work with
+the Yubikeys. I was setting up two keys (a nano and a neo), and one just worked out of the box
+with the version of `libykpers-1-1` that was in Tails' apt repository. The other needed a slightly
+newer version but that was also available in apt and can be selected by specifying the version
+manually as pointed out in [this GitHub issue](https://github.com/freedomofpress/securedrop/issues/1035#issuecomment-140172267).
+The version numbers have since changed, but `apt-policy` makes it easy to figure out what's needed.
+As I was using an air-gapped system this process was a bit more complicated than it sounds,
+involving several USB drive transfers.
+
+After finishing the guide, I had my master key and a pre-generated revocation certificate stored
+securely on my IronKey, and the three subkeys stored on each Yubikey. Time to go back to Windows.
+
+### GPG, Windows and SSH
+
+Now with the IronKey disconnected and the master key out of harms way, it's time to go back to
+Windows. I downloaded the [GnuPG Modern](https://www.gnupg.org/download/) distribution and
+followed the instructions at the end of Simon's guide to import my public key and make GPG aware
+of the subkeys on the Yubikey. After that [Enigmail](https://www.enigmail.net/index.php/en/)
+was able to sign and encrypt e-mail in Thunderbird.
+
+<figure class="image left">
+  <img src="/res/images/yubikey/wisdom_of_the_ancients.png" alt="XKCD: Wisdom of the ancients">
+  <figcaption><a href="https://xkcd.com/979/">XKCD #979: Wisdom of the ancients</a></figcaption>
+</figure>
+
+Next up, I enabled PuTTy support and started the GPG agent, as documented over on
+[Yubico's site](https://developers.yubico.com/PGP/SSH_authentication/Windows.html). This allows
+you to use the authentication GPG key to authenticate SSH sessions from PuTTy. To find the
+SSH key you need to add to `.authorized_keys`, simply run `gpg --export-ssh-key`. At first I
+could SSH into a host but not use agent forwarding. After lots of unsuccessful Googling, I realised
+that GPG couldn't access the key anymore locally. Another quick search and I found a
+[forum thread](http://forum.yubico.com/viewtopic.php?f=35&t=2231) where someone had the same
+issue and found it was a problem with exclusive access to the card. They even passed on their
+wisdom and updated the thread with a solution, which got everything working for me.
+
+### VMWare
+
+I have a Ubuntu image running inside VMWare on my desktop that I use for most development
+activities. I'm unlikely to want to sign e-mail, but I probably want to sign commits (especially
+now that [GitHub exposes verified signatures](https://github.com/blog/2144-gpg-signature-verification)).
+
+To do that I need to pass the Yubikey through to the virtual machine. In its default configuration,
+VMWare recognises the Yubikey device but doesn't pass it through correctly. You need to configure it
+to [allow HIDs](http://www.timothysalmon.com/2014/12/vmware-workstation-connect-yubikey-to.html),
+after which `gpg --card-status` starts working from the VM.
+
+Unfortunately passing through the device makes it exclusively available to the VM so the host OS
+can no longer use it. As I have two Yubikeys, I just configured VMWare to ignore the nano key that's
+always plugged in, and pass through the neo when I plug that in. While swapping the configuration
+around, I found out that GPG remembers the ID of the smartcard that store credentials - if you
+swap the two keys with identical subkeys it demands the other one is reinserted. You can get rid of
+the references to the previous card using `gpg --delete-secret-keys`.
+
+### In conclusion...
+
+... I have a <a href="/16402FE2.txt">new PGP key</a> you can use to verify things I sign or encrypt
+messages to me.

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