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This article describes how Yubico's YubiKey works and how you can use it.

Note: Before you overwrite the initial configuration of slot 1, please be aware of the warning under #Initial configuration.



The YubiKey is a small USB Security token that supports:

  • generating one-time passwords (OTP) - using either AES based Yubico OTP algorithm or OATH-HOTP following RFC 4226
  • outputting a up to 63 char long static password
  • handling challenge-response requests, using either Yubico OTP mode or HMAC-SHA1 mode
  • handling Universal 2nd Factor (U2F) requests (YubiKey 4 and YubiKey NEO)
  • acting as smartcard (using the CCID protocol) (YubiKey 4 and YubiKey NEO) - allowing storage of signing, encrypting, authenticating (RSA) keys to be used for instance for SSH login (authentication), Email signature/encryption, git commit signature, etc.

It is manufactured by Yubico.

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.

While offering a lot of features, newer versions of the YubiKey are not released as open source. An alternative is the Solo.


There are several packages available:

  • Yubico PAM — Module to integrate the YubiKey into PAM.
https://developers.yubico.com/yubico-pam/ || yubico-pam
  • YubiKey Manager — Python library and command-line tool (ykman) for configuring a YubiKey over all USB connections. Has optional GUI.
https://developers.yubico.com/yubikey-manager/ || yubikey-manager, yubikey-manager-qt
Note: After you install the yubikey-manager (which can be called by ykman in CLI), you need to enable pcscd.service to get it running
  • Yubikey Personalization — Library and tool to configure slot features over the OTP USB connection. Has optional GUI.
https://developers.yubico.com/yubikey-personalization/ || yubikey-personalization, yubikey-personalization-gui
  • Yubico Authenticator for Desktop — Lets you read OATH codes from your YubiKey over USB. Support the newer OATH implementation (YubiKey NEO and 4) as well as the older slot-based implementation (YubiKey Standard and Edge).
https://developers.yubico.com/OATH/YubiKey_OATH_software.html || yubioath-desktop

Understanding the YubiKey

The YubiKey is a small USB dongle with one button and an LED to communicate with you.

One of its strengths is that it can emulate a USB keyboard to send a password (OTP or static password) as text, and thus requires only USB HID drivers found on practically all computers (desktop, mobile, tablet, etc.).

This also makes it vulnerable to keyloggers if the static password functionality is used, which is why if possible one should avoid it and try to only use the one-time password (OTP), Challenge-Response and CCID Smartcard functionality.


The YubiKey takes inputs in the form of:

  • API calls sent to it via the USB interface.
  • short button press
  • long button press


The YubiKey transforms these inputs into outputs in the form of:

  • Sending keystroke keycodes (emulating a USB keyboard and typing for you)
    This is used to:
    • type the static password
    • type the OTP
  • Sending back a Response via the API (over the USB interface).
    This is used to send back:
    • the response of a Challenge-Response request (calculated using either Yubico OTP mode or HMAC-SHA1 mode)
    • the response of a U2F Challenge-Response request
    • the response of a CCID Smartcard related request

The button

The button activates by slightly touching it. Sometimes it even reacts when you are very close but are not touching it yet. However, it may, in some cases, only activate when you are touching it and a metal object at the same time.

Depending on the context, touching the button does one of these things:

  • triggering a function (like triggering the output of a static password or of a one-time password (OTP))
  • confirming / allowing a function or access
  • inserting / ejecting the smartcard

In the OTP mode a short press yields slot 1 and a long press yields slot 2.

USB connection modes

Depending on the YubiKey model, the device provides up to three different USB interfaces with their associated protocols. Two of the interfaces implement the USB HID (Human Interface Device) device class; the third is a smart card interface (CCID). The YubiKey is a multi-function USB device, just like a USB printer that can also function as a scanner.

The following table shows which interfaces the different applications use:

Application USB Interface
OTP Keyboard HID

All three interfaces can be independently enabled or disabled. The ykman program uses the term "manage connection modes" and uses OTP, FIDO, and CCID as selectors for which modes should be enabled.

Note: The old U2F mode has been renamed and is now called FIDO in ykman 0.6.1 and later (released 2018-04-16) https://developers.yubico.com/yubikey-manager/Release_Notes.html

The set of enabled USB interfaces affects which applications on the key can be accessed. As an example, if only the HID interfaces are enabled, the applications dependent on the CCID interface are unavailable.

Which application on the key is chosen is determined by the host application and is a combination of USB interface and an application selection mechanism. For example, if the host application wants to communicate with the PIV application on the key, it accesses the key using the CCID interface and using the protocols defined by CCID sends a 'select application' command to the key selecting the PIV application.

Get enabled modes

ykman mode will tell you what modes are currently activated/enabled/available. This could output something like

Current connection mode is: OTP+FIDO+CCID

Meaning that currently the OTP, FIDO and CCID subsystem of the key are enabled.

Set the enabled modes

ykman mode <MODE> will allow you to define which modes should be activated/enabled/available.

  • <MODE> can be a string, such as OTP+FIDO+CCID, or a shortened form o+f+c.
    With "+" you can combine multiple modes that you wish to be enabled.
  • <MODE> can be a mode-number, which is one number that encodes several enabled modes (like flags) into one value.
    The only valid modes when using numbers is 0 - 6 (see here). The extra flags are not part of the mode in that sense, they just need to be set at the same time as the mode is set.

Usually what you want is to make all functionality available (you will still need to potentially configure stuff, see functionality sections below for more details). In order to do so you can use:

ykman mode c+f+o or ykman mode 6

Note: Using the "80" mode-number or the corresponding --touch-eject parameter of ykman mode can only be used when the device is only in CCID mode (by running ykman mode ccid --touch-eject for instance).

Once the --touch-eject flag is set, you should be able to eject/insert the smartcard by pressing the button. The LED should indicate if the card is inserted or not as well.

Warning: But using the 80 mode-number or the corresponding --touch-eject is not recommend as it would prevent you from using the U2F/FIDO and OTP features of the YubiKey.
Note: The often seen:

ykman mode c+f+o --touch-eject or ykman mode 86

will ignore the --touch-eject and be identical to the above recommended ykman mode 6.

86 is not a valid mode. It might be a bug that the tool accepts higher values (see here).

Two Slots

Only if the OTP mode is activated (see Modes of the YubiKey below), the Yubikey provides 2 slots.

If the transport mode OTP is enabled, the two YubiKey Slots, long press and short press, can be configured and used.

Configuration of the slots

These slots can have one of the following credentials configured: a Yubico OTP (which is what comes preconfigured in the short press slot on a new key), a static password, a challenge-response credential, an OATH-HOTP credential. All this functionality is found in the ykman otp commands.

Note: A slot has either a Yubico OTP or a challenge-response credential configured. More general: One, and only one, type of the above listed possible credential per slot.

There are several flags that can be set during the configuration of the slots. These flags /cannot/ be read from the device once written. However, the behaviour of the device should change when the flags are set ;)

Note: Actually most of (all of??) the parameters and details you use during configuration of the slots, cannot be read back, once written to the YubiKey.


The YubiKey has a small green LED able to communicate with you. Its message to you actually depends on the currently used #USB connection modes of your YubiKey.

The possible messages are:

  • *steady on*: Press now, to allow access. (typically (TODO exclusively?) FIDO mode)
  • *slow blinking*: Power/setting up/ready for use (TODO explain)
  • *rapid blinking*: Error, configuring driver (TODO explain)
Note: If the CCID mode is turned on, then the LED of the key is always shortly flashing every two-three seconds once inserted.
You can turn the blinking off by disabling the CCID mode. This slow blinking just shows that the device has power, alternatively it shows a need for a button press. On Windows this behavior will typically stop once drivers are installed and it is ready for use. Mac and Linux systems will keep blinking; here the best current workaround to get the LED to blink less is to disable CCID.

Initial configuration

On a new YubiKey the Yubico OTP is preconfigured on slot 1.

Warning: Before you overwrite your slot 1, please be aware that one is not able to reconfigure the same trust level see here. Meaning:

One could think that it is a good idea to reset configuration slot 1 to a new OTP. But then a "VV" prefix in your credentials must be used. Whereas the factory credentials on your Yubikey use a "CC" prefix. You can upload a "VV" credential using the Yubico personalization tool GUI or manually upload the new AES key to the yubico.com website in order to regain the same functionality than with the original factory configuration. VV credentials are not less secure than CC. However some services may only trust CC credentials as they believe that the user process is more prone to security vulnerabilities. This is because you could have malware on your machine or someone intercepting your key when sending it to the YubiCloud.

Limitations of the passwords typed by YubiKey via USB-keyboard -- or "Why do my password look so weak ?"

The YubiKey can type passwords (OTP or Static Password) for you by acting as USB keyboard and sending scan-codes like if you would type.

A limitation of the YubiKey, however, prevents you from choosing characters that require a modifier key other than Shift. And in order for the YubiKey to work with all possible keyboard layouts (e.g. the Z on a German keyboard has a different scan-code than the Z on a US keyboard) it is necessary to limit the characters used by YubiKey passwords to the ModHex alphabet + Digits (0-9) (+ optionally "!" as the only available Symbol in static passwords).

The 16 characters used in the ModHex alphabet are: c,b,d,e,f,g,h,i,j,k,l,n,r,t,u,v. These characters share a property that makes them very valuable to a YubiKey: They use the same scan codes across a very large number of keyboard layouts. In other words, the scan code 0x06 maps to the character c for English, Swedish, German, French, and many others.

See here for full info

One-time password

Yubico OTP mode

The Yubico OTP mode is AES symmetric key based. On a new YubiKey the Yubico OTP is preconfigured on slot 1. This initial AES symmetric key is stored in the YubiKey and the same AES key is already stored on the Yubico Authentication server. This allows validating against YubiCloud, meaning the use of Yubico OTP in combination with the Yubico Forum website for instance or on https://demo.yubico.com).

The initial configuration and AES key stored in slot 1 can of course be overwritten.

Warning: Please read #Initial configuration before you overwrite the intial configuration of slot 1 that your YubiKey comes shipped with.

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.

Security risks

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 cryptography, 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.
Note: To authenticate the Yubico validation server, you can:
  • with HMAC: use https://upgrade.yubico.com/getapikey/ to get an HMAC key and ID
  • with HTTPS: the validation server's certificate is signed by GoDaddy, and is thus trusted by default in Arch installs (at least if you have package ca-certificates)

OATH-HOTP mode (RFC 4226)

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Reason: Empty section. (Discuss in Talk:YubiKey#)


Function and Application of Challenge-Response

This technique can be used to authenticate.

A challenge is sent to the YubiKey and a response is (auto-magically) calculated and send back. This calculation needs a secret (e.g. an AES key in case of the Yubico OTP mode) The same challenge always results in the same response. Without the secret this calculation is not meant to be feasable. Even if in the possession of many challenge-response pairs.

This can be used for:

  • true 2-factor (possession and knowledge) authentication:
    If you combine the response (possession factor) with a password (knowledge factor) and to authenticate you need to present the triple (challenge,response, password) to 3rd party. In which case the challenge and the corresponding response can be (publicly) send to a 3rd party to authenticate the possession factor, by redoing basically the same (auto-magical) calculation. The needed secret needs to be shared with 3rd party to allow an authentication.
  • semi 2-factor (possession and knowledge) authentication:
    The challenge can be public. Only with the possession of the YubiKey hardware the response can be generated. This can be used to create a "sort-of" 2-factor authentication (possession and knowledge): If you combine the response (possession factor) with a password (knowledge factor) and use the result for instance as passphrase for cryptsetup.

This functionality will consume one slot. And it is used via API calls to the YubiKey. So you usually use some tool to communicate the challenge to your YubiKey and get back the response.

There are two Challenge-Response modes:

  • Yubico OTP mode
  • HMAC-SHA1 mode

Setup the slot

One way to setup slot 2 in challenge-response mode (-ochal-resp) is with ykpersonalize:

ykpersonalize -v -2 -ochal-resp -ochal-hmac -ohmac-lt64 -oserial-api-visible -ochal-btn-trig

Check with ykpersonalize(1) to make sure that the options used above are right for you. You may also enable challenge-response mode using graphical interface through yubikey-personalization-gui.

Note: Before you overwrite the initial configuration of slot 1, please be aware of the Warning under #Initial configuration.

Use the slot - get a response for a challenge

To use a Challenge-Response slot (no matter which mode):

ykchalresp -2 <CHALLENGE>


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Reason: please use the first argument of the template to provide a brief explanation. (Discuss in Talk:YubiKey#)

Enabling U2F in the browser


Chromium/Tips and tricks#U2F authentication


Firefox/Tweaks#Fido U2F authentication


OpenSSH supports FIDO/U2F hardware tokens natively since 8.2 [1]. Generate a security key backed key pair with:

$ ssh-keygen -t ecdsa-sk
Note: firmware version 5.2.3 is needed for the ed25519-sk key type

Both client and server needs to be up to date to use the new key types.

CCID Smartcard

CCID (Chip Card Interface Device) is a USB standard device class for use by USB devices that act as smart card readers or with security tokens that connect directly via USB, like the Yubikey. HID (Human Interface Device) and CCID are both USB device classes, i.e. they are in the same category of USB specifications. HID is a specification for computer peripherals, like keyboards. The Yubikey works like a USB (HID) keyboard when used in the OTP and FIDO modes, but switches to the CCID protocol when using the PIV application, or as an OpenPGP device.

CCID mode should be enabled by default on all YubiKeys shipped since November 2015 [2]. Enable at least the CCID mode. Please see #Set the enabled modes.


Starting with the YubiKey NEO, the Yubikeys contain a PIV (Personal Identity Verification) application on the chip. PIV is a US government standard (FIPS 201) that specifies how a token using RSA or ECC (Elliptic Curve Cryptography) is used for personal electronic identification. The YubiKey NEO only supports RSA encryption, later models (YubiKey 4 and 5) support both RSA and ECC. The distinguishing characteristic of a PIV token is that it is built to protect private keys and operate on-chip. A private key never leaves the token after it has been installed on it. Optionally, the private key can even be generated on-chip with the aid of an on-chip random number generator. If generated on-chip, the private key is never handled outside of the chip, and there is no way to recover it from the token. When using the PIV mechanism, the Yubikey functions as a CCID device.

Use OpenPGP smartcard mode

See GnuPG#Smartcards

Using a YubiKey with SSH

The following example describes how to use a YubiKey for SSH keys. A YubiKey with the PIV (Personal Identification Verification) application is required; this means you need a YubiKey NEO or YubiKey 4 or later.

Generating a key pair on the YubiKey

A private key and associated certificate need to be either generated on the YubiKey or imported to it. Install the yubikey-manager package. Insert the YubiKey in a USB port and check that it is recognized:

$ ykman list
YubiKey 4 [OTP+FIDO+CCID] Serial: 1234567

The following two commands (generate-key and generate-certificate) require providing the PIV application's 24-byte management key, if it has been changed from its default value (recommended). The examples below assume the shell variable MK has been set in advance to the 48 character hex string representation of the management key. For example:

$ MK=AB019982CA48BDC6776B1F9A0A3E129FDE0705D219AF0037

Generate a 2048-bit RSA key pair on the YubiKey. The private key will be stored in key slot 9a on the YubiKey, and the public key will be written to the file pubkey.pem.

$ ykman piv generate-key -m $MK -a RSA2048 9a pubkey.pem

Next, use the YubiKey to generate a self-signed certificate for the public key:

$ ykman piv generate-certificate -m $MK -d 1826 -s "SSH Key" 9a pubkey.pem

The Subject field in the certificate will be set to "SSH Key" with the -s option. This field will be included in the prompt for PIN to help identify which key is used for authentication. The example command also specifies the -d option to set the number of days until the certificate expires. If the -d option is omitted, a default value of 365 days is used.

Note that the last parameter in the generate-key command is the file name where the public key is written to, whereas the last parameter in the generate-certificate command specifies where the public key is read from. The certificate is constructed and signed on the YubiKey and stored alongside the private key; the command does not output the certificate.

At this point the YubiKey is ready for authenticating to a SSH server. For this to happen, some additional configuration on both the client and the server is required.

Client configuration

The standard API used to communicate with cryptographic tokens is defined by PKCS#11. Install the opensc package which provides this API in the form of the shared library /usr/lib/opensc-pkcs11.so. The ssh client should be configured to use this library with the directive PKCS11Provider in ~/.ssh/config:

PKCS11Provider /usr/lib/opensc-pkcs11.so

Public key conversion

The pubkey.pem file contains the public key in PEM (Privacy Enhanced Mail) format. OpenSSH uses a different format defined in RFC 4253, section 6.6, so the PEM formatted key should be converted to the format OpenSSH understands. This can be done using ssh-keygen:

$ ssh-keygen -i -m PKCS8 -f pubkey.pem > pubkey.txt

This command uses the import (-i) function of the ssh-keygen, specifies PKCS8 as the input file format (-m), and reads the input from the (-f) file pubkey.pem. The converted key is written on standard output, which is the example is redirected to the file pubkey.txt.

The converted public key should now be copied to the remote server as described in SSH keys#Copying the public key to the remote server.

Initiating an SSH session with the YubiKey

To authenticate a SSH connection using the YubiKey, just use ssh normally. You will be prompted for the PIN instead of a password:

$ ssh remote
Enter PIN for 'SSH Key' 
[user@remote ~]$ 

Using ssh-agent to cache the PIN

ssh-agent (see SSH keys#SSH agents) can also be used with the PKCS#11 library; in this case the PIN code is cached instead of the private key.

$ ssh-add -s /usr/lib/opensc-pkcs11.so

As long as the PIN is cached in by the agent, the cached value is used and the user is not prompted for it.

Further reading

The default PIN code of the PIV application on the YubiKey is 123456; you may want to change it. The YubiKey also requires a 24-byte management key for administrative functions like key generation. If the management key has been changed from its default value, the new value needs to be specified using the -m option on the command line for certain commands. See What is PIV?

Tips and tricks

YubiKey and LUKS encrypted partition/disk

YubiKey can be used to strengthen the security of your LUKS encrypted partition/disk. There are multiple ways to achieve it. But before enabling Yubikey as a 2FA device it is recommended to setup plain LUKS encryption first and make sure it works correctly.

Challenge-Response mode for LUKS passphrase (udev/encrypt)

One way to do it is to use a Challenge-Response mode for creating strong LUKS passphrases. First, install yubikey-full-disk-encryption package. Using this tool you can add/modify/remove Yubikey-protected passphrases.

Make changes to /etc/ykfde.conf configuration file as as:

  • choose Yubikey slot to use for LUKS (YKFDE_CHALLENGE_SLOT)
  • whether you want to type a password at boot (YKFDE_CHALLENGE_PASSWORD_NEEDED) or use predefined password (YKFDE_CHALLENGE) which essentially means you check for Yubikey hardware presence only.

LUKS supports multiple password so you need to select a slot that is going to store Yubikey-protected passphrase. Inspect existing slots using command:

# cryptsetup luksDump /dev/<DISK>

Check entries under Keyslots: section. And then pick an unused slot.

Enroll your new passphrase:

# ykfde-enroll -d /dev/<DISK> -s <LUKS_SLOT>

It will require the new and existing passphrases.

And the last step is to add ykfde hook to /etc/mkinitcpio.conf file before or instead of encrypt hook. Then regenerate initramfs with mkinitcpio -P.

Warning: Plymouth Users: Replace your plymouth-encrypt hook with the ykfde hook. Failure to do so may result in you being locked out of your system.
Warning: As of December 2019 `sd-encrypt` enabled boot is not supported by yubikey-full-disk-encryption.

Challenge-Response mode for LUKS passphrase (systemd/sd-encrypt)

On systems which use systemd instead of udev hook in initramfs, full disk encryption can be served by the mkinitcpio-ykfdeAUR package. Using this tool you can add Yubikey-protected luks slots. Note that this is similar but different project than one mentioned in paragraph above and they aren't compatible with each other.

Make changes to /etc/ykfde.conf configuration file as as:

  • general/device name: devicename (rd.luks.name) defined in /etc/default/grub
  • key specific sections: map individual YubiKeys / slots to luks slots.

Add ykfde challenges file to grub initrd /etc/default/grub:


Add ykfde hook to /etc/mkinitcpio.conf file before sd-encrypt hook.

Enroll new luks slot:

# ykfde --ask-2nd-factor

Push challenges to /boot partition:

# ykfde-cpio

Rebuild initrd and update grub config:

 # mkinitcpio -P
 # grub-mkconfig -o /boot/grub/grub.cfg

If you want to unlock one single luks slot using multiple YubiKeys sharing the same challenge response configuration you have to enure ykfde uses the same challenge (stored in /etc/ykfde.d/).

For further details please see the project documentation.

OpenPGP applet

Another way of using YubiKey for full disk encryption is to utilize its OpenPGP applet to decrypt the LUKS keyfile during boot. initramfs-scencrypt is a set of hooks for initramfs that automate this process.

HMAC Secret Extension of FIDO2 protocol

Yet another way of using YubiKey for full disk encryption is to utilize HMAC Secret Extension to retrieve the LUKS password from YubiKey which is protected by passphrase supplied by user. This functionality requires at least YubiKey 5 with firmware 5.2.3+.

In order to use this install fido2-hmac-secretAUR and follow instructions available in project documentation.

Yubikey and KeePass

Tango-edit-clear.pngThis article or section needs language, wiki syntax or style improvements. See Help:Style for reference.Tango-edit-clear.png

Reason: Duplicates KeePass. (Discuss in Talk:YubiKey#)

Yubikey can be integrated with KeePass using plugins.

For a native open-source implementation of KeePass have a look at:


keepassx2AUR (see keepassx.org) a keepass QT FOSS reimplementation, extremely stable and available for Windows, MacOSX and Linux.


keepassxc (see keepassxc.org) a KeePassX fork that integrated YubiKey into KeePassX v2.
The integration covers Challenge-Response as security factor to open the database, but also the generation of OTP using the YubiKey.

In order to have a KeePassXC database work on Android (using the Keepass2Android app), you need to use version 1.06 of the app. You also need to save the database file in the KDBX 4 format, since Keepass2Android do not support the KDBX 3 format.

YubiKey support in Keepass2Android (which is compatible with KeePassXC) is tracked on GitHub.

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 OTP.


Install yubico-pam.

Note: If you are configuring a distant server to use YubiKey, you should open at least one additional, rescue SSH session, so that you are not locked out of your server if the configuration does not work and you exit your main session inadvertently
Note: The following assumes you are using the default Yubico servers. See the yubico-pam documentation for options relevant to using your own server.


Authorization Mapping Files

A mapping must be made between the YubiKey token ID and the user ID it is attached to. There are two ways to do this, either centrally in one file, or individually, where users can create the mapping in their home directories. If the central authorization mapping file is being used, user home directory mappings will not be used and vice versa.

Central authorization mapping

Create a file /etc/yubico/authorized_yubikeys, the file must contain a user name and the YubiKey token ID separated by colons (same format as the passwd file) for each user you want to allow onto the system using a YubiKey.

The mappings should look like this, one per line:

<first user name>:<Yubikey token ID1>:<Yubikey token ID2>:...
<second user name>:<Yubikey token ID3>:<Yubikey token ID4>:...

You can specify multiple key tokens to correspond to one user, but only one is required.

Per-user authorization mapping

Each user creates a ~/.yubico/authorized_yubikeys file inside of their home directory and places the mapping in that file, the file must have only one line:

<user name>:<Yubikey token ID1>:<Yubikey token ID2>

This is much the same concept as the SSH authorized_keys file.

Note that this file must be readable by the pam_yubico module when the user is authenticated, otherwise login will fail. If this is not possible or desired, use the global mapping file instead.

Obtaining the YubiKey token ID (a.k.a. public ID)

You can obtain the YubiKey token ID in several ways. One is by removing the last 32 characters of any OTP (One Time Password) generated with your YubiKey. Another is by using the modhex calculator.

Enter your YubiKey OTP and convert it, your Yubikey token ID is 12 characters and listed as:

Modhex encoded: XXXXXXX
PAM configuration

Having set up the pam_yubico module, you next need to tell PAM to use it when logging in via SSH. There are several ways of doing this.

The default way

Obtain HMAC credentials from Yubico as described in #YubiCloud and validation servers. You will receive a Client ID and a secret key.

Add one of the two following lines to the beginnning of /etc/pam.d/sshd:

auth            required      pam_yubico.so id=CLIENTID authfile=/etc/yubico/authorized_yubikeys

if you are using a central authorization mapping file, or

auth            required      pam_yubico.so id=CLIENTID

if you are using per-user authorization mapping, where CLIENTID is your Client ID. This method utilizes your ID and the server's certificate to authenticate the connection.

Note: This will authenticate via Yubico's free YubiCloud servers. If you want to use a different server, add it via the urllist parameter.
Using pure HMAC to authenticate the validation server

Add key to the above lines in /etc/pam.d/sshd:

auth            required      pam_yubico.so id=CLIENTID key=SECRETKEY ...

where CLIENTID and SECRETKEY are your HMAC ID and key.

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
Note: HMAC credentials should be unique to a single target server. That way, if an attacker finds them, he will not be able to craft responses to authenticate to other target servers you own
Using pure HTTPS to authenticate the validation server
Warning: While this "old" method of using a dummy id still works, it is unknown how secure and/or future-proof it is, as Yubico no longer describes it in their documentation. Proceed at your own risk. At the very least you should ensure that only HTTPS servers with valid certificates are used for authentication.

If you do not want to use HMAC credentials from Yubico, it is still possible to authenticate via the Yubico server by setting CLIENTID=1 instead of your own ID. Although pam_yubico's default server uses HTTPS already, for security reasons you should specify it manually via the urllist parameter, as the servers certificate is the only way in which the connection is authenticated. You can find the keyserver URL by adding the debug parameter to the auth line.

SSHD configuration

You should check that /etc/ssh/sshd_config contains these lines and that they are not commented. The sshd_config shipped with openssh has these set correctly by default.

ChallengeResponseAuthentication no
UsePAM yes

That is it!

You should not need to restart anything if you did not change the SSHD 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.

You can display information about the login data generated by pam_yubico by adding the debug option to the auth line in/etc/pam.d/sshd. However, if you are using a central authorization file, you should remove that option once finished testing, as it causes pam_yubico to display the entire content of the central file to every user who logs in using a YubiKey.


This works because the prompt is pam_yubico.so's one, since this module is before pam_unix.so, which normally does basic password authentication. So, you are giving a string that is the concatenation of your password and the OTP to pam_yubico.so. 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. In Archlinux' default PAM stack, the authenticator pam_unix.so is instructed to try receiving a password from the previous module with try_first_pass, so it automatically uses the password sent by pam_yubico.so.

Executing actions on insertion/removal of YubiKey device

For example, you want to perform an action when you pull your YubiKey out of the USB slot, create /etc/udev/rules.d/80-yubikey-actions.rules and add the following contents:

ACTION=="remove", ENV{ID_VENDOR}=="Yubico", ENV{ID_VENDOR_ID}=="1050", ENV{ID_MODEL_ID}=="0010|0111|0112|0113|0114|0115|0116|0401|0402|0403|0404|0405|0406|0407|0410", RUN+="/usr/local/bin/script args"

Please note, most keys are covered within this example but it may not work for all versions of YubiKey. You will have to look at the output of lsusb to get the vendor and model ID's, along with the description of the device or you could use udevadm to get information. Of course, to execute a script on insertion, you would change the action to 'add' instead of remove.

Maintenance / Upgrades

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.

Note: These steps are only for NEOs with a firmware version <= 3.1.2. The current generation NEOs (with U2F) come with the OpenPGP applet already installed)

Configure the NEO as a CCID Device

  1. Install yubikey-personalization-gui (yubikey-personalization-gui-gitAUR).
  2. Add the udev rules and reboot so you can manage the YubiKey without needing to be root
  3. Run ykpersonalize -m82, enter y, and hit enter.

Install the Applet

  1. Install gpshellAUR, gppcscconnectionpluginAUR, globalplatformAUR, and pcsclite.
  2. Start pcscd.service.
  3. Download the most recent CAP file from the ykneo-oath site.
  4. Download gpinstall.txt from GitHub.
  5. Edit the line in gpinstall.txt beginning with install -file to reflect the path where the CAP file is located.
  6. Open a terminal and run gpshell <location of gpinstall.txt>
  7. Ideally, a bunch of text will scroll by and it ends saying something like
 Command --> 80E88013D7C000C400BE00C700CA00CA00B400BE00CE00D200D500D700B000DB00C700DF00BEFFFF00BE00E400AC00AE00AE00DB00E700A
Wrapped command --> 84E88013DFC000C400BE00C700CA00CA00B400BE00CE00D200D500D700B000DB00C700DF00BEFFFF00BE00E400AC00AE00AE00DB00E700A
Response <-- 009000
Command --> 80E60C002107A000000527210108A00000052721010108A000000527210101010003C901000000
Wrapped command --> 84E60C002907A000000527210108A00000052721010108A000000527210101010003C9010000B4648127914A4C7C00
Response <-- 009000
  1. 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 installing yubioath-desktop.

While pcscd.service is running, run yubioath-desktop and insert your Yubikey when prompted.


Restart, especially if you have completed updates since your YubiKey last worked. Do this even if some functions appear to be functioning.

YubiKey not acting as HID device

Add udev rule as described in this article:

KERNEL=="hidraw*", SUBSYSTEM=="hidraw", MODE="0664", GROUP="users", ATTRS{idVendor}=="2581", ATTRS{idProduct}=="f1d0"

Run udevadm trigger afterwards.

You may also need to install the package libu2f-host if you want support in chrome.

ykman fails to connect to the YubiKey

If the manager fails to connect to the YubiKey, make sure you have started pcscd.service or pcscd.socket.

Error: Failed connecting to YubiKey 5 [OTP+FIDO+CCID]. Make sure the application have the required permissions.

This can occur when using ykman to access the oath credentials on the device if scdaemon has already taken exclusive control of the device. [3]

To fix this you can set the reader-port option with the correct value for your device in ~/.gnupg/scdaemon.conf. [4]

Note: This will cause the gpgagent to re-prompt you to unlock the YubiKey after each time you access the YubiKey through ykman.

For YubiKey NEO and YubiKey 4:

reader-port Yubico Yubikey

or for YubiKey 5

reader-port Yubico Yubi

YubiKey fails to bind within a guest VM

Assuming the YubiKey is available to the guest, the issue results from a driver binding to the device on the host. To unbind the device, the bus and port information is needed from dmesg on the host:

$ dmesg | grep -B1 Yubico | tail -n 2 | head -n 1 | sed -E 's/^\[[^]]+\] usb ([^:]*):.*/\1/'

The resulting USB id should be of the form X-Y.Z or X-Y. Then, on the host, use find to search /sys/bus/usb/drivers for which driver the YubiKey is binded to (e.g. usbhid or usbfs).

$ find /sys/bus/usb/drivers -name "*X-Y.Z*"

To unbind the device, use the result from the previous command (i.e. /sys/bus/usb/drivers/<DRIVER>/X-Y.Z:1.0):

# echo 'X-Y.Z:1.0' > /sys/bus/usb/drivers/<DRIVER>/unbind

Error: [key] could not be locally signed or gpg: No default secret key: No public key

Occurs when attempting to sign keys on a non-standard keyring while a YubiKey is plugged in, e.g. as Pacman does in pacman-key --populate archlinux. The solution is to remove the offending YubiKey and start over.