First steps with smartcards under Linux and Android — hard, but it works

Well this has been an interesting project.

It all started with a need to get better password storage at work. We wound up looking heavily at a GPG-based solution. This prompted the question: how can we make it even more secure?

Well, perhaps, smartcards. The theory is this: a smartcard holds your private keys in a highly-secure piece of hardware. The PC can never actually access the private keys. Signing and decrypting operations are done directly on the card to prevent the need to export the private key material to the PC. There are lots of “standards” to choose from (PKCS#11, PKCS#15, and OpenPGP card specs) that are relevant here. And there are ways to use SSH and OpenVPN with some of these keys too. Access to the card is protected by a passphrase (called a “PIN” in smartcard lingo, even though it need not be numeric). These smartcards might be USB sticks, or cards you pop into a reader. In any case, you can pop them out when not needed, pop them in to use them, and… well, pretty nice, eh?

So that’s the theory. Let’s talk a bit of reality.

First of all, it is hard for a person like me to evaluate how secure my data is in hardware. There was a high-profile bug in the OpenPGP JavaCard applet used by Yubico that caused the potential to use keys without a PIN, for instance. And how well protected is the key in the physical hardware? Granted, in most of these cards you’re talking serious hardware skill to compromise them, but still, this is unknown in absolute terms.

Here’s the bigger problem: compatibility. There are all sorts of card readers, but compatibility with pcsc-tools and pcscd on Linux seems pretty good. But the cards themselves — oh my. PKCS#11 defines an interface API, but each vendor would provide their own .so or .dll file to interface. Some cards (for instance, the ACOS5-64 mentioned on the Debian wiki!) are made by vendors that charge $50 for the privilege of getting the drivers needed to make them work… and they’re closed-source proprietary drivers at that.

Some attempts

I ordered several cards to evaluate: the OpenPGP card, specifically designed to support GPG; the ACOS5-64 card, the JavaCOS A22, the Yubikey Neo, and a simple reader listed on the GPG smartcard howto.

The OpenPGP card and ACOS5-64 are the only ones in the list that support 4096-bit RSA keys due to the computational demands of them. The others all support 2048-bit RSA keys.

The JavaCOS requires the user to install a JavaCard applet to the card to make it useable. The Yubico OpenPGP applet works here, along with GlobalPlatform to install it. I am not sure just how solid it is. The Yubikey Neo has yet to arrive; it integrates some interesting OAUTH and TOTP capabilities as well.

I found that Debian’s wiki page for smartcards lists a bunch of them that are not really useable using the tools in main. The ACOS5-64 was such a dud. But I got the JavaCOS A22 working quite nicely. It’s also NFC-enabled and works perfectly with OpenKeyChain on Android (looking like a “Yubikey Neo” to it, once the OpenPGP applet is installed). I’m impressed! Here’s a way to be secure with my smartphone without revealing everything all the time.

Really the large amount of time is put into figuring out how all this stuff fits together. I’m getting there, but I’ve got a ways to go yet.

Update: Corrected to read “signing and decrypting” rather than “signing and encrypting” operations are being done on the card. Thanks to Benoît Allard for catching this error.

3 thoughts on “First steps with smartcards under Linux and Android — hard, but it works

  1. I am currently going through the same journey. Your blog really helped a lot. I live on an island so I have to be cautious what I order. Returning it is generally not an option.

    I had my mind set to buy a couple of ACOS5-64 and a card reader to test. But you said: “The ACOS5-64 was such a dud”. Can you expand on that some more please?


    John Goerzen Reply:

    Sure. The ACOS5-64 isn’t supported by OpenSC or any other drivers on Linux. It comes with no drivers at all, in fact, and AFAICT the only way to make it work is to pay $50 for their client software kit.

    It’s not worth that kind of hassle.


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