Yubikey-SSH

Configuring the YubiKey NEO for SSH authentication in the prod cluster:

Installation

On Debian / Ubuntu:

apt-get install yubikey-personalization  yubico-piv-tool opensc

The yubico-piv-tool package is in the universe repository in Ubuntu 15.10 and later. If you have an earlier version, you can get it from the PPA:

add-apt-repository ppa:yubico/stable
apt-get update

On MacOS:

brew install opensc ykpers yubico-piv-tool

Enabling CCID smartcard mode

The smartcard/CCID feature on the YubiKey NEO is disabled by default. You can change that with the following command:

ykpersonalize -m82

A message will be printed to the console, confirming the action. This enables OTP (keyboard) and CCID. If you also want to enable U2F, you can use -m86, however this causes the device to not be recognised by the version of pcscd in Ubuntu 14.04.

Remember to remove and re-inserte your YubiKey before proceeding, otherwise you will get the error "failed to connect to reader".

Securing physical access to the YubiKey

When accessing the NEO you need to enter a PIN to prevent access for someone who e.g. stole your YubiKey. If that PIN is entered incorrectly thrice, the YubiKey needs to be unlocked with a PUK. If that PUK is also entered incorrectly three time your YubiKey is toast, so make sure to store these in your password manager.

In addition the NEO uses a management key: It secures access when updating the feature applets running on the YubiKey and protects you from someone backdooring your NEO.

The YubiKey uses default values for PIN, PUK and management key, so we need to change all of them:

• First the management key (24 chars hexadecimal):

key=`dd if=/dev/random bs=1 count=24 2>/dev/null | hexdump -v -e '/1 "%02X"'`
yubico-piv-tool -a set-mgm-key -n $key

• Now the PIN (6 digits, 123456 is the shipped default PIN):

pin=`dd if=/dev/random bs=1 count=6 2>/dev/null | hexdump -v -e '/1 "%u"'| cut -c1-6`
yubico-piv-tool -a change-pin -P 123456 -N $pin

• Finally the PUK (8 digits, with the default 12345678)

puk=`dd if=/dev/random bs=1 count=6 2>/dev/null | hexdump -v -e '/1 "%u"'|cut -c1-8`
yubico-piv-tool -a change-puk -P 12345678 -N $puk

• Finally don't forget to keep the content of $key, $pin and $puk in your password manager.

Key/certificate creation

First of all generate a new private key: (9a is a slot number, for others see the introduction of https://www.yubico.com/wp-content/uploads/2015/04/Yubico-PIV-Management-Tools_v1.0.pdf )

 yubico-piv-tool -k $key -s 9a -a generate -o public.pem

Generate a self-signed certificate:

 yubico-piv-tool -a verify-pin -P $pin -a selfsign-certificate -s 9a -S "/CN=SSH key/" -i public.pem -o cert.pem

Import the certificate:

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

Generate an SSH pubkey

On Debian/Ubuntu:

export OPENSC=$(dpkg -L opensc | grep opensc-pkcs11.so\$ )
ssh-keygen -D $OPENSC -e

On MacOS X / Brew:

export OPENSC="/usr/local/Cellar/opensc/0.15.0/lib/opensc-pkcs11.so"
ssh-keygen -D $OPENSC -e

Accessing the key

The key is accessed via the PKCS11 interface (which OpenSSH supports):

ssh -I $OPENSC $HOST
Enter PIN for 'PIV_II (PIV Card Holder pin)':

You don't need to enter the PIN all the time, since ssh-agent supports PKCS11 as well. Run

ssh-add -s $OPENSC

to enter the key to the agent.

Unfortunately, gnome-keyring, which is the default SSH_AUTH_SOCK implementation on many Linux desktops, does not support this. Neither does gpg-agent. You may see an error like:

SSH_AGENT_FAILURE
Could not add card: /usr/lib/x86_64-linux-gnu/opensc-pkcs11.so

So on such systems, you either need to run a separate SSH agent:

eval `ssh-agent -s`
ssh-add -s $OPENSC

Or hack the session startup so that the real OpenSSH ssh-agent is used.

Note that you can identify the current SSH agent with:

sudo fuser -v $SSH_AUTH_SOCK

If you remove the YubiKey from the USB port and then plug it back in, the card needs to be re-added to the agent:

ssh-add -e $OPENSC
ssh-add -s $OPENSC