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Yubikey, Keybase and SSH

History

About a year ago, I purchased a Yubikey NEO, a hardware-based two-factor authentication keyfob. I’d had an earlier version that could generate tokens upon being pressed, but the NEO included a few things I wanted to test out, including the ability to store gpg keys on it. I was also curious to see how hard it’d be to use the Yubikey with SSH connections. I went through the entire setup a year ago, got frustrated with using GPGTools on OS X and the keychain, and then promptly forgot about it for a year until my key expired and I’d forgotten the passphrase I’d use to encrypt it. I’ve just recently rectified that and went through the whole process again, so I thought I’d document it here for posterity.

Setup

Generating the keys offline

I followed these instructions by Simon Josefsson to create a private key on an offline machine, and then subkeys that I could transfer to my Yubikey for daily use.

Note here that I downloaded a recent ISO installer for Mint Linux. I’m booting it on a 2013 MacBook Air, and the Broadcom drivers are non-free, and don’t come bundled with recent versions of Debian that I’ve seen.

Simon argues for a short key validity period1, and I tend to agree with him. If I somehow lose my keys and can’t revoke them, they’re only good for at most another 100 days. Minimizing blast radius sounds good to me.

SSH Setup

This helped me get my keys usable for SSH in OS X. Like Glenn, the biggest issue I saw while using gpg-agent with my Yubikey was that it wouldn’t recognize my reinserted card until I killed and restarted it. Glenn suggests using ControlPlane for listening for the USB device to be inserted/removed. As I’m already using the awesome Hammerspoon for scripting a lot of things like window resizing and shortcut keys, I added a script for that to my dotfiles2. Now whenever I insert the Yubikey, gpg-agent is restarted. As an added bonus, removing it automatically starts my screensaver and locks the display.

Keybase and some final thoughts

I started this whole mess because I received an invite to Keybase (here’s my profile if you’re curious). I’m not too keen on their suggestion to upload my private key to their servers, so I’m keeping it on the Yubikey for now. I haven’t had too much time to use it, but it’s an interesting idea that I’ll be watching.

One thing that bugs me about the Yubikey NEO is that it only supports RSA keys of up to 2048 bits in length. The newer Yubikey 4 will support 4096, which is what I’m used to using with file-based keys. Plus, there’ve been some speed and security enhancements I’d like to take advantage of. I’d lose out on NFC, but since I’ve got an iPhone now, I can’t use it anyway.