This is another post in the series of how to protect SSH keys with hardware, making them impossible to steal.

This means that you know that your piece of hardware (e.g. Yubikey or TPM inside your laptop) was actively involved in the transaction, and not, say, turned off and disconnected from the Internet at the time (like in a safe or on an airplane).

What’s new this time is that we can now have a physical presence test on every use of the key. That means that even if someone hacks your workstation completely and installs a keylogger to get your PIN, unless they also break into your home they can’t use the key even while the machine is on and connected. Evil hackers in another country are out of luck.

Intro

Most of this is a repeat of official docs (see references).

If it looks like a command is hanging, check to see if the Yubikey is flashing. If it is, then touch it.

The touch feature is optional. If you don’t want a key to require it, you can chose to generate a key that doesn’t.

Install yubico-c, ykpersonalization, and yubico-piv-tool

sudo apt-get install help2man gengetopt libtool autoconf automake opensc asciidoc libxml2-utils libusb-dev libssl-dev libpcsclite-dev pcscd
git clone https://github.com/yubico/yubico-c
cd yubico-c
autoreconf -i
./configure --prefix=$HOME/opt/yubico
make
make install
cd ..

git clone https://github.com/yubico/yubikey-personalization
cd yubikey-personalization
autoreconf -i
./configure --prefix=$HOME/opt/yubico
make
make install
cd ..

git clone https://github.com/yubico/yubico-piv-tool
cd yubico-piv-tool
autoreconf -i
./configure --prefix=$HOME/opt/yubico
make
make install
cd ..

Turn on CCID only mode

ykpersonalize -m1

Don’t worry about the scary warning. The NEO tool can set it back the way it was. Unplug and plug the Yubikey.

You should now see the Yubikey as a smartcard reader:

opensc-tool --list-readers
# Detected readers (pcsc)
Nr.  Card  Features  Name
0    Yes             Yubico Yubikey 4 CCID 00 00

If you get “no smart card readers found”, open /etc/libccid_Info.plist, and add the vendor (should be 0x1050), product (0x0407 or near there) and “Yubico Yubikey 4 Something” as the first entry in ifdVendorID, ifdProductID and ifdFriendlyName arrays respectively. The actual vendor and product codes you can see by running lsusb. Then restart pcscd and try again.

Change the PIN and the PUK.

They default to 123456 and 12345678 respectively.

The PIN will be asked for by the user, and the PUK is used in case the user locked themselves out by failing to provide the right PIN too many times. Three times by default.

Wikipedia has more on PUK.

yubico-piv-tool -a change-pin
yubico-piv-tool -a change-puk

If you lose your PIN and PUK you can reset the Yubikey by first spending all your PIN/PUK attempts, and then running

yubico-piv-tool -a reset

You may want to set the management key too. See this matrix for details about what PIN is used for what. In short, if you are a hobbyist then I’d say you don’t need to set the management key. If you are the security department for your organization and you’re handing out these Yubikeys, then you probably do. You don’t want your users to change the number of PIN retries they’re allowed, for example.

Generate a key

The second command here requires a touch. Check for the Yubikey blinking. The first command is not waiting for touch. It just takes a while to generate a 2048 bit RSA key.

yubico-piv-tool -a generate --touch-policy=always -s 9a -o public.pem
yubico-piv-tool -a verify-pin -a selfsign-certificate -s 9a -S '/CN=my SSH key/' -i public.pem -o cert.pem

Inspect the certificate with openssl x509 -text -in cert.pem.

Touch policy “always” means that every time the key is used it requires a touch.

The key generation command also takes a --pin-policy that can be set to “once”. I’m not sure what that means. I would have guessed that only once per power-up, but when testing I get asked every time, even if I set it to “never”.

Import certificate onto Yubikey

yubico-piv-tool -a import-certificate -s 9a -i cert.pem

The two generated PEM files can now be discarded.

Use PKCS#11 to get the public key in SSH format

ssh-keygen -D /usr/lib/x86_64-linux-gnu/opensc-pkcs11.so -e
ssh-rsa AAAA...

Use the SSH key

ssh -I /usr/lib/x86_64-linux-gnu/opensc-pkcs11.so [email protected]
Enter PIN:    (and then touch Yubikey when it flashes)

or

ssh-add -s /usr/lib/x86_64-linux-gnu/opensc-pkcs11.so
Enter PIN:

ssh [email protected]
(touch Yubikey when it flashes)

Other commands

yubico-piv-tool -a read-cert -s 9a | openssl x509 -text

Notes

  • The Yubikey 4 does support elliptic curves, but they don’t seem to be compatible with what OpenSSH supports. We’re stuck with RSA2048 for now.

References