Why Smart Cards Are Smart
If you use GPG keys, learn about the benefits to storing them on a smart card.
GPG has been around for a long time and is used to secure everything from your email to your software. If you want to send an email to someone and be sure that no one else can read or modify it, GPG signing and encryption are the main method you'd use. Distributions use GPG to sign their packages, so you can feel confident that the ones you download and install from a package mirror have not been modified from their original state. Developers in many organizations follow the best practice of GPG-signing any code they commit to a repository. By signing their commits, other people can confirm that the changes that claim to come from a particular developer truly did. Web-based Git front ends like GitHub and GitLab let users upload their GPG public keys, so when they do commit signed code, the interface can display to everyone else that it has been verified.
Yet, all of the security ultimately comes down to the security of your private key. Once others have access to your private key, they can perform all of the same GPG tasks as though they were you. This is why you are prompted to enter a passphrase when you first set up a GPG key. The idea is that if attackers are able to copy your key, they still would need to guess your password before they could use the key. For all of the importance of GPG key security, many people still just leave their keys in ~/.gnupg directories on their filesystem and copy that directory over to any systems where they need to use GPG.
There is a better way. With OpenPGP smart cards, you can store your keys on a secure device that's protected with a PIN and not only store your keys more securely, but also use them more conveniently. Although some laptops come with integrated smart card readers, most don't. Thankfully, these devices are available as part of multi-function USB security token devices from a number of different vendors, and Linux Journal has published reviews of such products in the past. In this article, I discuss all the reasons OpenPGP smart cards are a better choice for storing your keys than your local filesystem.
Reason 1: Tamper-proof Key Storage
One of the main benefits of a smart card is that it stores your GPG keys securely. When you store your keys on a filesystem, anyone who can access that filesystem can copy off the keys. On a smart card, once keys go in, they never leave, neither accidentally nor from tampering. The smart card chips themselves are designed to be tamper-proof and resist attempts to extract key data even when someone has physical access. By putting keys on a smart card, you can have a reasonable assurance that your keys are safe, even from a determined attacker.
Reason 2: GPG Operations Happen on the Card
The next benefit to smart cards is related to the tamper-proof nature of the key storage. Because the private keys never can leave the smart card, all of your GPG operations happen on the smart card itself! When your GPG keys are on your filesystem, each time you encrypt, decrypt or sign something, your keys are unlocked and are copied to RAM so the CPU can perform the GPG operations. With a smart card, the keys never leave the device, and the smart card itself performs the GPG operations. GPG is smart card-aware, so it sends the payload over USB to the smart card, the smart card encrypts, decrypts or signs it, and then it sends the output back over USB to the computer.
The fact that operations happen on the card is important, because it's further assurance that your private keys aren't exposed, even if you use your smart card on an untrusted machine. Even if attackers had remote access to the untrusted machine and could guess your PIN, they could, at best, temporarily use your keys to encrypt, decrypt or sign something. They still could not extract your keys or use them indefinitely—the moment you unplug your smart card, the keys fall out of their grasp.
Reason 3: Portability
One of the other benefits of smart cards is portability. It's true that laptops are pretty portable, and you could, in theory, take one with you everywhere you go (if you are a site reliability engineer that's on call, this might be the case). The reality is that most people leave their computers unattended at least some of the time. If you travel somewhere, you might bring a laptop, but in many cases, you'll probably also leave it in a hotel.
If you have more than one computer, you are faced with having to copy GPG keys around to each one on which you intend to use those keys. In that case, these limitations on portability become a problem, because you aren't going to have each of those laptops in your possession at all times. Laptops get lost and stolen, and an attacker with physical access to the laptop might be able to get access to your GPG keys. Even if the laptop has disk encryption, a savvy attacker could use a cold boot attack to get copies of disk unlock keys still present in RAM on a suspended machine.
When your GPG keys are on a smart card, you can put the USB security token in your purse or pocket (I've found the watch pocket in jeans to be a great place) and have it with you at all times. This portability means you don't have to worry about copying your GPG keys to each of your machines. Instead, you can just insert your smart card when you need to use the keys and then remove it when you are done. Even if you want to leave your smart card attached to your computer while you use it, you can (and should) still remove it when you step away from the computer so it's always with you. Some USB security tokens even offer an NFC interface so you can use your keys on your smart phone.
Reason 4: Multi-factor Authentication
The final benefit to smart cards is that it enforces multi-factor authentication on your GPG keys. Ideally, GPG keys are protected by two different factors: something you have (the key itself) and something you know (the password to unlock the key). When you store your keys on a filesystem, multi-factor authentication is optional. When you first generate your keys, you are prompted to generate a password to protect them, but you are allowed to skip that step and generate keys without password protection.
If you are in an organization that wants to enforce multi-factor authentication on GPG keys, a smart card is a simple way to do it. Smart cards require users to enter a PIN to unlock the key, and GPG will automatically prompt users to enter their smart card and then type in the PIN whenever that particular key is being used.
I hope you've found this discussion of the benefits of OpenPGP smart cards useful. With the large market of USB security tokens out there (which has grown even larger with the interest in secure cryptocurrency storage), you have a lot of options to choose from in a number of price ranges. Be sure to check which GPG key sizes and algorithms a smart card supports before you buy it, especially if you use newer elliptic curve algorithms or larger (3072- or 4096-bit) RSA keys.
- "WebAuthn Web Authentication with YubiKey 5" by Todd A. Jacobs, LJ, February 2019
- "The Purism Librem Key" by Todd Jacobs, LJ, February 2019