Smart Cards and Secret Agents

Update, 2016-11: The following information is fairly out of date, six years later, as now GnuPG uses stable socket names, which is good. Please see this newer post which includes some information on setting up agent forwarding.

I’ve been meaning to write about my adventure to properly set up authentication using the Fellowship of FSFe smartcard for quite a while, and since Markos actually brought the subject up earlier tonight I guess today is the right time. Incidentally, earlier in my “morning” I had to fight with getting it working correctly on Yamato so it might be useful after all…

First of all, what is the card and what is needed to use it… the FSFe Fellowship card is a smartcard with the OpenPGP application on it; smartcards can have different applications installed, quite a few are designed to support PKCS#11 and PKCS#15, but those are used by the S/MIME signature and encryption framework; the OpenPGP application instead is designed to work with GnuPG. When I went to FOSDEM, I set up my new key using the card itself.

The card provides three keys: a signing key, an encryption key, and an authentication key; the first two are used for GnuPG, as usual; the third instead is something that you usually don’t handle with GnuPG… SSH authentication. The gpg-agent program can actually handle your standard RSA/DSA keys for SSH, but that’s generally not very useful; if combined with the OpenPGP smartcard, this comes very useful.

So first of all you need a compatible smartcard reader; thankfully the CCID protocol is pretty standard and should work fine; I’ve got luck and three out of three smartcard readers I have work fine; one is from an Italian brand (but most likely built in Taiwan or China), the other is a GemAlto PinPad, and the third is the one integrated in my Dell laptop, Broadcom BCM5880v3. The last one requires an updated firmware and a ccid package capable of recognizing it… the one in Gentoo ~arch is already patched so that it works out of the box. I got mine at Cryptoshop which seems a decent place to get them in Europe.

Out of experience, at least GnuPG seems to have problems dealing with pinpads, and quite a few pinpad-provided readers seem to have driver problems; so get a cheaper, but just as valid, non-pinpad reader.

On the software side, there isn’t much to need: GnuPG itself could use the CCID readers directly, but my best luck has been using pcsc-lite; just make sure your pcsc-lite does not use HAL but rather has libusb support directly, by setting -hal usb as USE flags for it. GnuPG has to be built with the smartcard USE flag; pcsc-lite USE flag will give you the dependency as well, but it does not change the build at all. Update: Matija noted that there is also the need to install app-crypt/ccid (which is the userspace driver of the CCID-based smartcard readers); for whatever reason I assumed it was already a dependency of the whole set but that is not the case.

Make sure the pcscd service is started with the system, you’re gonna need it.

To actually make use of the key properly you’re going to need to replace ssh-agent with gnupg-agent…. more interesting, GNOME-Keyring also replaces ssh-agent, but if you let it do so, it won’t handler your OpenPGP card auth key! So you’re going to override that. Since using keyring with this setup seem to be impossible, my solution is to use a simple wrapper which I now release under CC-BY license.

You got run this script on every shell and your X session as well, for this to work as intended (it is needed in X session so that it works with libvirt over SSH otherwise virt-manager will still try to get the key from gnome-keyring). To do so I added a source of that script from both my ~/.shrc file and my ~/.xsession file, and make sure the latter is called; to do so I have this:

# in both ~/.shrc and ~/.xsession:
. /path/to/gpg-agent-wrapper

# in /etc/X11/xinit/xinitrc.d/01-xsession
[ -f ${HOME}/.xsession ] && . ${HOME}/.xsession

The trick of the script is making sure that gpg-agent is not already running, that it does not collide with the current information, but also it takes care of overriding gnome-keyring (it could be also done by changing the priority of ~/.xsession to be higher than gnome-keyring), and ensures that the SSH Agent Forwarding works… and yes it works even if on the client there is gpg-agent used for SSH, which means it can forward the card’s authentication credentials over a network connection.

So here it is, should be easy enough to set up for anybody interested.

2 thoughts on “Smart Cards and Secret Agents

  1. Sweet, thanks works for my tiny Gemalto as well :]Although it would be nice if you would point out a bit more that app-crypt/ccid is also a package that needs to be emerged.

    Like

  2. I’m not sure why but I was assuming ccid was a dependency of pcsc-lite itself… I’ll correct the article, thanks!

    Like

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