A YubiKey is a small USB and NFC based device, a so called https://developers.yubico.com/Developer_Program/Guides/YubiKey_Hardware.html[hardware security token], with modules for many security related use-cases. It generates one time passwords (OTPs), stores private keys and in general implements different authentication protocols. They are created and sold via a company called http://yubico.com/[Yubico].
As soon as you start working with security tokens you have to account for the potential to lock yourself out of accounts tied to these tokens. As hardware security tokens are unique and designed to be extremely hard to copy you can't just make a backup of it like you can with software vaults like Keepass or AndOTP. Because of this all registrations you do with your primary key you should immediately do with a second backup key that you store in a secure location like a safe or at least always leave at home.
In practice this means to register both hardware tokens with your linux and web accounts, generate private keys twice and configure both public keys at e.g. github.
For some features private keys and other secrets are stored on the YubiKey. Each feature has it's own storage space and hence https://support.yubico.com/hc/en-us/articles/360013790319-How-many-accounts-can-I-register-my-YubiKey-with-[maximum number of credential slots]:
Local system authentication uses https://www.redhat.com/sysadmin/pluggable-authentication-modules-pam[Pluggable Authentication Modules (PAM)]. You have two options here: pam_yubico and pam_u2f. The former is required for YubiKeys without FIDO2/U2F. If your key supports the FIDO2 standard depends on firmware and hardware model.
The setup is as follows: install the PAM module, register a YubiKey with your user account, create base configuration for either of the two authentication options and then choose the PAM configuration you want to use the YubiKey with.
=== Dependencies
The packages required for both PAM modules are available in the official repositories.
There are two ways to configure the YubiKey PAM module to authenticate users. Either via the YubiCloud or using challenge-response. The YubiCloud is the standard method but depends on Yubico's cloud to validate your OTPs and hence requires constant internet access.
Note that the key is optional but without it there is no TLS verification which makes this susceptible to MitM attacks by default. Obtain a key at https://upgrade.yubico.com/getapikey[Yubico].
If you have SELinux on the enforcing mode (the default mode), you should flip on the allow_ypbind boolean first, because pam_yubico needs to be able to connect to Yubico's online authentication. servers.
If you want to use both methods for different use-cases just create the respective configuration files and use them as includes as described in the next section accordingly.
If you use the online YubiCloud method you need the ID of your YubiKey. For this just enter the key and retrieve an OTP code with a short press on the button and extract the first 12 characters - this is your key ID.
Use the tool pamu2fcfg to retrieve a configuration line that goes into ~/.config/Yubico/u2f_keys. This configuration line consists of a username and a part tied to a key separated by colon.
Next configure PAM to accept a YubiKey as a means of authentication. There are many options in /etc/pam.d to modify and add a YubiKey, but the most common use-cases are:
In a PAM configuration file if using {yubikey,u2f}-sufficient add an include line before or if using {yubikey,u2f}-required add it after a line that reads "auth substack system-auth" or "auth include system-auth". An include of yubikey-sufficient looks like this:
The following example sets a YubiKey OTP as 'sufficient' factor for terminal login. This means that a YubiKey alone is enough to authenticate a user when logging in on a terminal.
Next time you open a console (local, not ssh session) and attempt to login you should be prompted `YubiKey for '<user>':`. Tap your YubiKey to input an OTP and you will be logged without entering a password.
When using the yubikey-required option make sure to test this thoroughly in another session without closing your current one to mitigate locking yourself out of the system.
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To add a YubiKey to more than terminal login, like local sshd servers, sudo or GDM login, add the respective auth include to one of the other configuration files in /etc/pam.d.
A YubiKey comes pre-configured for Yubico OTP, but apart from that it uses default PINs for every other feature which you'll most likely want to change before use. There is software for customizing the YubiKey in the official repositories.
There are essentially two tools to use together with their respective GUI variants. 'yubikey-manager' and 'ykpersonalize'. The former is newer but supports less options than the latter. For all available options install both.
Newer YubiKeys (YubiKey 2+) have the ability to store two separate configurations. The first is generally used for OTPs, the second for a strong, static password. If the button is pressed shortly, something up to 1.5 seconds, the first configuration is triggered. If the button is pressed longer, in the range of 2.5 to 5 seconds, the second configuration is triggered.
This writes a static key to the YubiKey based on the 32-byte AES key specified with the -a option. The -2 option sets the second slot as target. The other two options are a matter of personal taste. The append-cr option sends a carriage return as the last character of the key. That way I do not have to press <ENTER> myself. The -man-update option disables easy updating of the static key in the YubiKey. Enabling this will allow for altering the static password without the use of ykpersonalize.
Slot 1 is special as it contains a factory credential already uploaded to YubiCloud. Deleting and recreating a Yubico OTP secret and uploading it to YubiCloud yourself will put a special mark on it which has consequences: service providers might not trust such a key and Yubico might delete those secrets at anytime for practically any reason.
If we want to write a new configuration to the first slot of the key, we need to specify some more options. If you want to be able to upload you key to Yubico, in order to authenticate against their servers, remember what the values are that you use below. You will need them later on.
The -1 option tells ykpersonalize to use the first configuration. The fixed option specifies the public ID of the YubiKey. This is referred to as the 'prefix' later on, when we go uploading it. The value you use here has to start with 'ff' in hex or 'vv' in modhex (xref:#_what_is_modhex[see below]). Yubico enforces this when you try to upload your key to their servers. The value for the fixed option can be up to 16 characters in length.
As part of the OTP, you can specify an internal identifier for your key. This is what the uid option does. The value is in plain hex, not modhex and ''exactly'' 12 character long.
The -a option, again, is the 32-byte AES key and append-cr appends a carriage return to my key as the last character.
When plugged in, the operating system treats the YubiKey as a USB keyboard. USB keyboards send scancodes to the operating system, which the operating system then interprets as keystrokes. The YubiKey has to make sure no ambiguity arises: there are many different kinds of keyboard layouts and the scancodes have to be interpreted as the same character on machines using every random keyboard layout out there. To fix this, the people of Yubico have created 'modhex', which is a modified representation of hexadecimal characters that uses only 'safe' characters. 'Safe' characters are basically characters which have the same scancode on all keyboard layouts.
If you want to customize your YubiKey's AES key but still want to use it to authenticate through Yubico's servers, you can upload the key through https://upgrade.yubico.com/getapikey/. You will need to enter your email address and YubiKey's OTP.
=== Update the PINs of the PIV module
The https://www.yubico.com/authentication-standards/smart-card/[Personal Identitiy Verification (PIV)] module stores private keys and corresponding certificate files for purposes such as encryption, authentication and signatures. If your YubiKey supports this you want to change the PIN and PUK as well as the Management Key.
Enter the current management key [blank to use default key]:
Enter PIN: ********
You can now safely use the PIV module to generate private keys and store certificates.
=== Change the PIN of the FIDO2 module
https://www.yubico.com/authentication-standards/fido2/[FIDO2] is an open authentication standard and encompasses sub-standards and protocols to either provide two-factor or even passwordless authentication methods.
One interesting use case of the FIDO module to note is storing OpenSSH public-key identities, which modern OpenSSH agents can pick up right away and use. This makes ssh keys quite portable.
The OATH feature provides TOTP and HOTP authentication protocols. It can be protected with a passphrase to access and generate OTP codes. This is different from the Yubico OTP feature, which uses a single stored secret on the YubiKey for challenge-response.
Change the OATH password with:
[source, bash]
[…]$ ykman oath access change
Enter the new password:
Repeat for confirmation:
Configure your device to remember this password so you don't have to re-enter it anymore.
Using FIDO2 and OpenSSH 8.2+ you can generate OpenSSH keys that are only usable if the YubiKey is connected. It's possible to protect the key usage by either presence or presence + pin-entry.
Generate a public key on every host you intend to use the private key, so an OpenSSH agent may discover it:
[source, bash]
[…]$ ssh-keygen -t ed25519-sk
Generate the public key and store its identity in the FIDO2 module to make the private-public key-pair portable:
The slot 9a on the key is dedicated to authentication. There are https://docs.yubico.com/yesdk/users-manual/application-piv/slots.html[more slots] for features like encryption or signing.
Create a certificate in this same slot for the PIV/PKCS#11 library:
Now generate a public key from the X.509 certificate stored on the YubiKey. Other features like resident keys work the same as with the FIDO2 approach, but you have to add the additional option as shown below.
As of 2019, there is work in place to attempt to standardize using a YubiKey on the web. The new standard is called WebAuthn, and you can learn more about it here: https://www.yubico.com/solutions/webauthn/. For now, the easiest way to see which platforms support the YubiKey is by browsing https://www.yubico.com/works-with-yubikey/catalog/[yubico's catalog].
As an alternative to Yubico OTP or WebAuthn, neither of which require storage of credentials on the YubiKey by default, you may also use plain old TOTP like employed in most websites today. There are desktop and at least android apps to work with this conveniently. You may store up to 32 TOTP credentials on a YubiKey 5.
