One of its strengths is that it emulates a USB keyboard to send the OTP as text, and thus requires only USB HID drivers found on practically all desktop computers.
- 1 Introduction
- 2 Two-factor authentication with SSH
- 3 Installing the OATH Applet for a Yubikey NEO
- 4 Enabling U2F in the browser
- 5 Enabling OpenPGP smartcard mode
- 6 Troubleshooting
How does it work
Yubikey's authentication protocol is based on symmetric cryptography. More specifically, each Yubikey contains a 128-bit AES key unique to that device. It is used to encrypt a token made of different fields such as the ID of the key, a counter, a random number, etc. The OTP is made from concatenating the ID of the key with this encrypted token.
This OTP is sent to the target system, to which we want to authenticate. This target system asks a validation server if the OTP is good. The validation server has a mapping of Yubikey IDs -> AES key. Using the key ID in the OTP, it can thus retrieve the AES key and decrypt the other part of the OTP. If it looks OK (plain-text ID and encrypted ID are the same, the counter is bigger than the last seen one to prevent replay attacks...), then authentication is successful.
The validation server sends that authentication status back to the target system, which grants access or not based on that response.
AES key compromise
As you can imagine, the AES key should be kept secret. It cannot be retrieved from the Yubikey itself (or it should not, at least not with software). It is present in the validation server though, so the security of this server is very important.
Validation requests/responses tampering
Since the target system relies on the ruling of the validation server, a trivial attack would be to impersonate the validation server. The target system thus needs to authenticate the validation server. 2 methods are available :
- HMAC: This is also symmetric crypto, the target server and validation server share a key that is used to sign requests and responses.
- TLS: Requests and responses travel via HTTP, so TLS (HTTPS) can be used to authenticate and encrypt the connection.
YubiCloud and validation servers
When you buy a Yubikey, it is preloaded with an AES key that is known only to Yubico. They will not even communicate it to you. Yubico provides a validation server with free unlimited access (YubiCloud). It also offers open-source implementations of the server.
So you can either:
- choose to use your Yubikey with its preloaded AES key and validate against Yubico's validation server ;
- or load a new AES key in your Yubikey and run your own validation server.
Two-factor authentication with SSH
This details how to use a Yubikey to have two-factor authentication with SSH, that is, to use both a password and a Yubikey-generated OTP.
You have to edit
/etc/pam.d/sshd, and modify the line that reads :
auth required pam_unix.so
auth required pam_unix.so use_first_pass
Then choose one of the following.
If using HTTPS to authenticate the validation server
Insert the following line before the previously modified
auth required pam_yubico.so id=1 url=https://api.yubico.com/wsapi/2.0/verify?id=%d&otp=%s
The id=1 is of no real use but it is required.
If using HMAC to authenticate the validation server
Insert the following line before the previously modified
auth required pam_yubico.so id=1234 key=YnVubmllcyBhcmUgY29vbAo=
key are your own HMAC ID and key, requested from Yubico as explained above.
You should also disallow unprivileged users to read the file to prevent them from seeing the HMAC credentials:
# chmod o-r /etc/pam.d/sshd
You should check that
/etc/ssh/sshd_config contains these lines and that they are not commented, but I believe this is the default.
ChallengeResponseAuthentication no UsePAM yes
That is it!
You should not need to restart anything if you just touched the PAM config file.
To log in, at the
Password: prompt of SSH, you have to type your password without pressing enter and touch the Yubikey's button.
The Yubikey should send a return at the end of the OTP so you do not need to touch the enter key at all.
This works because the prompt is
pam_yubico.so's one, since this module is before
pam_unix.so, which does basic password authentication.
So, you are giving a string that is the concatenation of your password and the OTP to
Since the OTPs have a fixed length (let us call this size N), it just has to get the last N characters to retrieve the OTP, and it assumes that the other characters at the start are the password.
It tries to validate the OTP, and in case of success, sends the password to the next PAM module,
pam_unix.so, which was instructed not to prompt for the password, but to receive it from the previous module, with
Installing the OATH Applet for a Yubikey NEO
These steps will allow you to install the OATH applet onto your Yubikey NEO. This allows the use of Yubico Authenticator in the Google Play Store.
Configure the NEO as a CCID Device
- Get AUR from the AUR.
- Add the udev rules and reboot so you can manage the YubiKey without needing to be root
ykpersonalize -m82, enter
y, and hit enter.
Install the Applet
- Install AUR, AUR, AUR, and .
sudo systemctl start pcscd.service.
- Download the most recent CAP file from the ykneo-oath site.
- Edit the line in gpinstall.txt beginning with
install -fileto reflect the path where the CAP file is located.
- Open a terminal and run
gpshell <location of gpinstall.txt>
- Ideally, a bunch of text will scroll by and it ends saying something like
Command --> 80E88013D7C000C400BE00C700CA00CA00B400BE00CE00D200D500D700B000DB00C700DF00BEFFFF00BE00E400AC00AE00AE00DB00E700A A00EA00ED00ED00ED00BE00EF00F100F400F100F700FA00FF00BE00F700AA01010103010700CA00C400B400AA00F700B400AA00B600C7010C 010C00AA0140012001B0056810B0013005600000056810E0011006B4B44304B44404B44106B44B4405B443400343B002410636810E06B4B44 407326810B004B43103441003334002B102B404B3B403BB4003B440076820A4100221024405B4341008B44600000231066820A100 Wrapped command --> 84E88013DFC000C400BE00C700CA00CA00B400BE00CE00D200D500D700B000DB00C700DF00BEFFFF00BE00E400AC00AE00AE00DB00E700A A00EA00ED00ED00ED00BE00EF00F100F400F100F700FA00FF00BE00F700AA01010103010700CA00C400B400AA00F700B400AA00B600C7010C 010C00AA0140012001B0056810B0013005600000056810E0011006B4B44304B44404B44106B44B4405B443400343B002410636810E06B4B44 407326810B004B43103441003334002B102B404B3B403BB4003B440076820A4100221024405B4341008B44600000231066820A15D848CB77 27D0EDA00 Response <-- 009000 Command --> 80E60C002107A000000527210108A00000052721010108A000000527210101010003C901000000 Wrapped command --> 84E60C002907A000000527210108A00000052721010108A000000527210101010003C9010000B4648127914A4C7C00 Response <-- 009000 card_disconnect release_context
- Unplug the NEO and try it with the Yubico Authenticator app
(Optional) Install the Yubico Authenticator Desktop client
You can get the desktop version of the Yubico Authenticator by installingAUR.
pcscd.service is running, run
yubioath-gui and insert your Yubikey when prompted.
Enabling U2F in the browser
In order for the U2F functionality to work with Chromium you need to install the udev rules required to enable access to the Yubikey as a user. Yubikey is by default only accessible by root, and without these rules Chromium will give an error.library. This provides the
Enabling OpenPGP smartcard mode
These steps will allow you to use the OpenPGP functionality of your YubiKey.
- Configure your YubiKey as a CCID device as mentioned above.
- Install , and
- Enable and start
sudo systemctl enable pcscd.serviceand
sudo systemctl start pcscd.service
- To verify that your YubiKey is ready to be used run
pcsc_scanwhich will provide some informations about the connected device. Further you can use
gpg --card-statusto verify that GPG can interact with the card.
Restart, especially if you have completed updates since your Yubikey last worked. Do this even if some functions appear to be functioning.