Hardening SSH authentication using Yubikey (2/2)

In my previous blog post, I demonstrated how to use a Yubikey to add a 2nd factor (2FA) authentication to SSH using pam_ssh and pam_yubico.

In this article, I will go further and demonstrate another method using Yubikey’s Personal Identity Verification (PIV) capability.

This one has the huge advantage to allow a 2nd factor authentication while using the public key authentication mechanism of OpenSSH and thus does not need any kind of setup on the servers.

Method 2 – SSH using Yubikey and PIV

Yubikey 4 and NEO also act as smartcards supporting the PIV standard which allows you to store a private key on your security key through PKCS#11. This is an amazing feature which is also very good for our use case.

Tools installation

For this to work, we will need some tools on our local machines to setup our Yubikey correctly.

Gentoo users should install those packages:

emerge dev-libs/opensc sys-auth/ykpers sys-auth/yubico-piv-tool sys-apps/pcsc-lite app-crypt/ccid sys-apps/pcsc-tools sys-auth/yubikey-personalization-gui

Gentoo users should also allow the pcscd service to be hotplugged (started automatically upon key insertion) by modifying their /etc/rc.conf and having:

rc_hotplug="pcscd"

Yubikey setup

The idea behind the Yubikey setup is to generate and store a private key in our Yubikey and to secure it via a PIN code.

First, insert your Yubikey and let’s change its USB operating mode to OTP+CCID.

ykpersonalize -m2
Firmware version 4.3.4 Touch level 783 Program sequence 3

The USB mode will be set to: 0x2

Commit? (y/n) [n]: y

Then, we will create a new management key:

key=`dd if=/dev/random bs=1 count=24 2>/dev/null | hexdump -v -e '/1 "%02X"'`
echo $key
D59E46FE263DDC052A409C68EB71941D8DD0C5915B7C143A

Replace the default management key (if prompted, copy/paste the key printed above):

yubico-piv-tool -a set-mgm-key -n $key --key 010203040506070801020304050607080102030405060708

Then change the default PIN code and PUK code of your Yubikey

yubico-piv-tool -a change-pin -P 123456 -N <NEW PIN>

yubico-piv-tool -a change-puk -P 12345678 -N <NEW PUK>

Now that your Yubikey is secure, let’s proceed with the PCKS#11 certificate generation. You will be prompted for your management key that you generated before.

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

Then create a self-signed certificate (only used for libpcks11) and import it in the Yubikey:

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

Here you are! You can now export your public key to use with OpenSSH:

ssh-keygen -D opensc-pkcs11.so -e
ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQCWtqI37jwxYMJ9XLq9VwHgJlhZViPVAGIUfMm8SAlfs6cka4Cj570lkoGK04r8JAVJFy/iKfhGpL9N9XuartfIoq6Cg/6Qvg3REupuqs51V2cBaC/gnWIQ7qZqlzBulvcOvzNfHFD/lX42J58+E8tWnYg6GzIsImFZQVpmI6SxNfSmVQIqxIufInrbQaI+pKXntdTQC9wyNK5FAA8TXAdff5ZDnmetsOTVble9Ia5m6gqM7MnxNZ56uDpn+6lCxRZSW+Ln2PDE7sivVcST4qpfwY4P4Lrb3vrjCGODFg4xmGNKXsLi2+uZbs5rW7bg4HFO50kKDucPV1M+rBWA9999

Copy to your servers your SSH public key to your usual ~/.ssh/authorized_keys file in your $HOME.

Testing PIV secured SSH

Plug-in your Yubikey, and then SSH to your remote server using the opensc-pkcs11 library. You will be prompted for your PIN and then successfully logged in 🙂

ssh -I opensc-pkcs11.so cheetah
Enter PIN for 'PIV_II (PIV Card Holder pin)':

You can then configure SSH to use it by default for all your hosts in your ~/.ssh/config

Host=*
PKCS11Provider /usr/lib/opensc-pkcs11.so

Using PIV with ssh-agent

You can also use ssh-agent to avoid typing your PIN every time.

When asked for the passphrase, enter your PIN:

ssh-add -s /usr/lib/opensc-pkcs11.so
Enter passphrase for PKCS#11: 
Card added: /usr/lib/opensc-pkcs11.so

You can verify that it worked by listing the available keys in your ssh agent:

ssh-add -L
ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQCWtqI37jwxYMJ9XLq9VwHgJlhZViPVAGIUfMm8SAlfs6cka4Cj570lkoGK04r8JAVJFy/iKfhGpL9N9XuartfIoq6Cg/6Qvg3REupuqs51V2cBaC/gnWIQ7qZqlzBulvcOvzNfHFD/lX42J58+E8tWnYg6GzIsImFZQVpmI6SxNfSmVQIqxIufInrbQaI+pKXntdTQC9wyNK5FAA8TXAdff5ZDnmetsOTVble9Ia5m6gqM7MnxNZ56uDpn+6lCxRZSW+Ln2PDE7sivVcST4qpfwY4P4Lrb3vrjCGODFg4xmGNKXsLi2+uZbs5rW7bg4HFO50kKDucPV1M+rBWA9999 /usr/lib64/opensc-pkcs11.so

Enjoy!

Now you have a flexible yet robust way to authenticate your users which you can also extend by adding another type of authentication on your servers using PAM.

8 thoughts on “Hardening SSH authentication using Yubikey (2/2)

  1. Grzegorz Kulewski

    Why not use GPG capability (with auth key + optional cert)? It can secure both email, SSH, VPNs and Web (client certificate)?

    The advantage over PIV is possibility to use 4096 bit keys and integration with GPG.

    Reply
  2. Grzegorz Kulewski

    Additionally I think you are generating the key on yubikey so you are trusting completely their random number generator and can’t have a backup. You can instead (with GPG but with PIV probably too) generate the key on the (offline, secure) computer and then write a backup to some (LUKS encrypted) SD card/USB pendrive and then import to the card.

    Reply
    1. ultrabug Post author

      Thanks for pointing that out Grzegorz, you’re right that the GPG feature is also to be explored and taken into account.

      It looked to me that it was a bit more complex to setup and work with at first glance. The Yubico guys themselves point to an external howto.

      Still, this will surely become a “method 3” blog post since you got me (re)interested in this 😉

      Reply
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