Protocols and Applications

The YubiKey 5 Series provides applications for FIDO2, OATH, OpenPGP, OTP, Smart Card, and U2F. The applications are all separate from each other, with separate storage for keys and credentials.

For information on managing all these applications, see Tools and Troubleshooting.

Note that the OTP and OATH categories overlap. Technically, there are three true OTPs:

Yubico OTP (defined by Yubico)
OATH-HOTP (standard RFC4226)
OATH-TOTP (standard RFC6238)
We support the Yubico OTP and OATH-HOTP directly on the touch-triggered OTP function on the YubiKey.
We support OATH-HOTP and OATH-TOTP directly on the OATH function on the YubiKey (usually called OATH and used with Yubico authenticator).
We support a static password and Challenge-Response with Touch-triggered OTP. Challenge-Response can also be used with software (such as Yubico Authenticator) to act as a single OATH-TOTP credential.

All three of these OTPs are described in more detail below, see OATH and OTP.

FIDO2

The FIDO2 standard offers the same high level of security as FIDO U2F, since it is based on public key cryptography. In addition to providing phishing-resistant two-factor authentication, the FIDO2 application on the YubiKey allows for the storage of resident credentials, also called discoverable credentials. As these credentials can accommodate the username and other data, this enables truly passwordless authentication on sites and applications that support the WebAuthn protocol. YubiKeys in the 5 Series can hold up to 25 resident keys.

Note

FIDO2 support is available to the iPad Pro via the USB-C or Lightning® connectors of the YubiKey 5Ci. FIDO2/WebAuthn can be achieved over USB-C using any of the following options:

ASWebAuthenticationSession
SFSafariViewController
Redirect to Safari browser

For more details on support for the iPad Pro, see iPad and iPad Pro. To see which U2F/FIDO2 security keys currently work with iOS/iPadOS 13.3+ devices using the Safari browser in combination with apps using SFSafariViewController or ASWebAuthenticationSession, see Supporting U2F or FIDO2 Security Keys on iOS or iPadOS | Security Key Compatibility.

Locking FIDO2 Credentials

The resident credentials can be left unlocked and used for strong single-factor authentication, or they can be protected by a PIN for two-factor authentication.

The FIDO2 PIN must be between 4 and 63 characters in length.
Once a FIDO2 PIN is set, it can be changed but it cannot be removed without resetting the FIDO2 application.
If the PIN is entered incorrectly 8 times in a row, the FIDO2 application locks. In order to restore this functionality, reset the FIDO2 application.

Note

Resetting the FIDO2 application also resets the U2F key. This means the YubiKey must be re-registered not only with all the FIDO2 sites, but also with all U2F sites.

Note

The YubiKey 5Ci supports Credential Management to allow for selective deletion of resident keys. See Enhancements to FIDO 2 Support for details.

Default Values

PIN: None set.

AAGUID Values

The FIDO2 specification states that an Authenticator Attestation GUID (AAGUID) must be provided during attestation. An AAGUID is a 128-bit identifier indicating the type of the authenticator.

New AAGUIDs are issued for new YubiKey products that support FIDO2, or when existing YubiKey products have FIDO2 features added or removed.

For the complete list of AAGUIDs, see https://support.yubico.com/hc/en-us/articles/360016648959-YubiKey-Hardware-FIDO2-AAGUIDs.

Supported Extensions

The YubiKey 5 Series supports only the AppID extension (appid) as defined by the W3C Web Authentication API specification. This extension allows U2F credentials registered using the legacy FIDO JavaScript APIs to be used with WebAuthn. That means if you register a YubiKey in the 5 Series on a website that used U2F at that time and later upgrades to FIDO2, your U2F credentials continue to work on the website.

FIDO U2F

FIDO U2F is an open standard that provides strong, phishing-resistant two-factor authentication for web services using public key cryptography. U2F does not require any special drivers or configuration to use, just a compatible web browser. The U2F application on the YubiKey can be associated with an unlimited number of U2F sites.

OATH

The OATH application can store up to 32 OATH credentials, either OATH-TOTP (time-based One-Time Password) or OATH-HOTP (counter-based One-Time Password). These credentials are separate from those stored in the OTP application, and can only be accessed through the CCID channel. In order to manage these credentials and read the OTPs generated by the YubiKey, requires the Yubico Authenticator.

To restrict access to the OTPs, set a password for the OATH application.

Note

Developers: Using the OATH application functions on iOS requires the Yubico iOS SDK.

HOTP and TOTP

Both OATH-TOTP and OATH-HOTP credentials are described in detail in the OATH Overview.

OpenPGP

The OpenPGP application provides an OpenPGP-compatible smart card in compliance with version 3.4 of the specification if the YubiKey firmware is 5.2.3 or later. If the firmware is an earlier version, the OpenPGP-compatible smart card is in compliance with version 2.0 of the specification.

OpenPGP-compatible smart card can be used with compatible PGP software such as GnuPG (GPG) and can store one PGP key each for authentication, signing, and encryption. Similar to the PIV / Smart Card touch policy, the OpenPGP application can also be set to require the YubiKey’s metal contact be touched to authorize an operation.

Note

Developers: Using the OpenPGP functions on iOS requires the Yubico iOS SDK.

YubiKey firmware 5.2.3 - 5.2.8 and 5.3.2 - 5.4.3 in combination with OpenPGP 3.4:

Extends existing RSA support for OpenPGP operations to ECC algorithms
Provides the Yubico Attestation feature for verifying keys generated on a YubiKey device
Utilizes separate x.509 cardholder certificates alongside the existing OpenPGP certificates for authentication, signature and encryption/decipher
Bring attestation functionality to OpenPGP keys and certificates generated on a YubiKey
Improves security by supporting Key Derivation Function (KDF) PINs. With KDF enabled, the PIN is stored as a hash on the YubiKey. The OpenPGP client will only pass the hashed value, never the PIN directly.

Elliptic Curve Cryptographic (ECC) Algorithms

The YubiKey 5.2.3 firmware added support for ECC algorithms. These can be used for Signature, Authentication and Decipher keys. The full list of curves supported by OpenPGP 3.4 can be found in section 4.4.3.10 of the OpenPGP Smart Card 3.4 spec (page 35).

In addition to RSA Algorithms, YubiKeys support the following ECC algorithms:

secp256r1
secp256k1
secp384r1
secp521r1
brainpoolP256r1
brainpoolP384r1
brainpoolP512r1
curve25519
- x25519 (decipher only)
- ed25519 (sign / auth only)

For further details on the new features, including key attestation, expanded encryption algorithms and additional cardholder certificates, refer to Enhancements to OpenPGP Support.

RSA Algorithms

RSA 1024
RSA 2048
RSA 3072 (requires GnuPG version 2.0 or higher)
RSA 4096 (requires GnuPG version 2.0 or higher)

Default Values

PIN: 123456
Admin PIN: 12345678

OTP

The OTP application provides two programmable slots, each of which can hold one of the types of credentials listed below. A Yubico OTP credential is programmed to slot 1 during manufacturing. Output is sent as a series of keystrokes from a virtual keyboard.

Trigger the YubiKey to produce the credential in the first slot by briefly touching the metal contact of the YubiKey.
If a credential has been programmed to the second slot, trigger the YubiKey to produce it by touching the contact for 3 seconds.

Yubico OTP

Yubico OTP is a strong authentication mechanism that is supported by the YubiKey 5 Series. Yubico OTP can be used as the second factor in a two-factor authentication (2FA) scheme or on its own, providing single-factor authentication.

The OTP generated by the YubiKey has two parts, with the first 12 characters being the public identity which a validation server can link to a user, while the remaining 32 characters are the unique passcode that is changed each time an OTP is generated.

The character representation of the Yubico OTP is designed to handle a variety of keyboard layouts. It is crucial that the same code is generated if a YubiKey is inserted into a German computer with a QWERTZ layout, a French one with an AZERTY layout, or a US one with a QWERTY layout. The “Modhex”, or Modified Hexadecimal coding, was invented by Yubico to use only specific characters to ensure that the YubiKey works with the maximum number of keyboard layouts. (USB keyboards send their keystrokes by means of “scan codes” rather than the actual character. The translation to keystrokes is done by the device to which the YubiKey is connected).

Static Password

A static password can be programmed to the YubiKey so that it will type the password for you when you touch the metal contact.

For managing multiple passwords, see the password managers that the YubiKey can secure with two-factor authentication (2FA).

HMAC-SHA1 Challenge-Response

This type of credential is most often used for offline authentication, as it does not require contacting a server for validation.

An HMAC-SHA1 Challenge-Response credential enables software to send a challenge to the YubiKey and verify that an expected, predetermined response is returned. This credential can also be set to require a touch on the metal contact before the response is sent to the requesting software. This type of credential must be activated by the software sending the challenge; it cannot be activated by touching the metal contact on the YubiKey.

Note

Developers: Because the Challenge-Response function requires two-way communication with the YubiKey, using this feature on iOS requires the Yubico iOS SDK.

Smart Card (PIV Compatible)

The YubiKey 5 Series provides a PIV-compatible smart card application. PIV, or FIPS 201, is a US government standard. It enables RSA or ECC sign/encrypt operations using a private key stored on a smart card through common interfaces like PKCS#11.

On Windows, the smart card functionality can be extended with the YubiKey Smart Card Minidriver.

Note

The YubiKey Smart Card Minidriver is not available for Android, Linux, macOS or iOS.

The YubiKey 5 Series supports extended APDUs, extended Answer To Reset (ATR), and Answer To Select (ATS). Using the PIV APDUs on iOS requires the Yubico iOS SDK.

Default Values

PIN: 123456
PUK: 12345678
Management Key (3DES): 010203040506070801020304050607080102030405060708

Supported Algorithms

The YubiKey 5 Series supports the following algorithms on the PIV smart card application.

RSA 1024
RSA 2048
ECC P-256
ECC P-384

Policies

PIN Policy

To specify how often the PIN needs to be entered for access to the credential in a given slot, set a PIN policy for that slot. This policy must be set upon key generation or import. It cannot be changed later.

Touch Policy

In addition to requiring the PIN, the YubiKey can require a physical touch on the metal contact. Similar to the PIN policy, the touch policy must be set upon key generation or import.

Slot Information

The keys and certificates for the smart card application are stored in slots, which are described below. The PIN policies described below are the defaults, before they are overridden with a custom PIN policy. These slots are separate from the programmable slots in the OTP application.

Slot 9a: PIV Authentication

This certificate and its associated private key is used to authenticate the card and the cardholder. This slot is used for system login, etc. To perform any private key operations, the end user PIN is required. Once the correct PIN has been provided, multiple private key operations may be performed without additional cardholder consent.

Slot 9c: Digital Signature

This certificate and its associated private key is used for digital signatures for the purpose of document, email, file, and executable signing. To perform any private key operations, the end user PIN is required. The PIN must be submitted immediately before each sign operation to ensure cardholder participation for every digital signature generated.

Slot 9d: Key Management

This certificate and its associated private key is used for encryption to assure confidentiality. This slot is used for encrypting emails or files. The end user PIN is required to perform any private key operations. Once the correct PIN has been provided, multiple private key operations may be performed without additional cardholder consent.

Slot 9e: Card Authentication

This certificate and its associated private key is used to support additional physical access applications, such as providing physical access to buildings through PIV-enabled door locks. The end user PIN is NOT required to perform private key operations for this slot.

Slots 82-95: Retired Key Management

These slots are meant for previously used Key Management keys to be able to decrypt earlier encrypted documents or emails.

Slot f9: Attestation

This slot is only used for attestation of other keys generated on the device with instruction f9. This slot is not cleared on reset, but can be overwritten.

Attestation

Attestation enables you to verify that a key on the smart card application was generated on the YubiKey and was not imported. An X.509 certificate for the key to be attested is created if the key has been generated on the YubiKey. Included in the certificate are the following extensions that provide information about the YubiKey.

1.3.6.1.4.1.41482.3.3: Firmware version, encoded as three bytes. For example, 050100 indicates firmware version 5.1.0.
1.3.6.1.4.1.41482.3.7: Serial number of the YubiKey, encoded as an integer.
1.3.6.1.4.1.41482.3.8: Two bytes, the first encoding the PIN policy and the second encoding the touch policy.

PIN policy:

01 - never require PIN

02 - require PIN once per session

03 - always require PIN.

Touch policy:

01 - never require touch

02 - always require touch

03 - cache touch for 15 seconds.

1.3.6.1.4.1.41482.3.9: YubiKey’s form factor, encoded as a one-byte octet-string.

USB-A Keychain: 0x01

USB-A Nano: 0x02

USB-C Keychain: 0x03

USB-C Nano: 0x04

USB-C and Lightning®: 0x05

Undefined: 0x00

PIV Metadata

Background: How PIV Attestation Works

A technical description of YubiKey PIV attestation is available at the Yubico developer website.

Attestation is performed on a public key that has been generated on the YubiKey. For example, consider an asymmetric key-pair that is generated on the YubiKey with the following ykman (YubiKey Manager) command:

ykman piv generate-key 9c -

This command generates an asymmetric key-pair, and stores the private key in the specified slot (9c in this example). The public key that has been generated is returned as output.

The ykman attestation command can be executed for the key-pair at the slot (9c):

ykman piv attest 9c C:\Test\attestation-cert-9c.cer

The generated certificate is generated in real time at the YubiKey. The attestation certificate and private key, which are stored in slot f9, are used for signing the generated certificate for the slot (9c). The attestation certificate is used as template when creating the generated certificate for the slot (9c). In addition to the template attestation certificate, the extensions and subject details are appended to the generated certificate.

However, the generated certificate is not the same as the X.509 certificate that may be issued by an external CA or self-signed on the YubiKey. For example, the X.509 certificate could be issued by the Microsoft ADCS and written to the YubiKey. The YubiKey Manager GUI can be used to generate a key-pair and self-sign the public key at the YubiKey.

The public key at slot 9a can be attested (signed in real time by the CA attestation certificate) with the same ykman command as above:

ykman piv attest 9a C:\Test\attestation-cert-9a.cer

And the X.509 self-signed certificate can be exported from the YubiKey with the following ykman command:

ykman piv export-certificate 9a C:\Test\self-signed-9a.cer

The Shortcomings of PIV Attestation

PIV attestation only works for asymmetric keys that have been generated on the YubiKey. It does not work for asymmetric keys that have been imported into the YubiKey.

For example, the following ykman command imports a PKCS #12 file into the YubiKey at slot 9e:

ykman piv import-key 9e C:\\Test\\TestUser1.p12 -P 123456

ykman piv import-certificate 9e C:\\Test\\TestUser1.p12 -P 123456

These ykman commands unpack the PKCS #12 file, store the private key in the private key slot (9e), and store the X.509 certificate in the corresponding certificate slot.

Now, if one tries to attest the public key at slot 9e with the ykman attestation command, the operation fails:

ykman piv attest 9e C:\\Test\\attestation-9e.cer

Error: Attestation failed.

One more drawback with PIV attestation is performance, since generation of multiple PIV attestation certificates can be time-consuming.

When To Use PIV Metadata

PIV metadata should be used for the following cases:

If PIV attestation cannot be used (for imported keys),
If an attestation certificate is not required, PIV metadata can be used for achieving higher performance.

Yubico PIV Library and Metadata API

PIV metadata is supported by YubiKey v5.3.0 firmware and above. YubiKey PIV metadata can be accessed by using the libykpiv library.

The Yubico PIV Tool contains the library

libykpiv.so (for Linux),
libykpiv.dylib (for MacOS),
libykpiv.dll (for Windows).

The source code of the libykpiv library is published at the Yubico GitHub repo.

libykpiv

The libykpiv library exposes a C API in the header file ykpiv.h, which includes the functions ykpiv_get_metadata() and ykpiv_util_parse_metadata(). The source code of these functions is available in the file ykpiv.c.

In particular, the function ykpiv_get_metadata() calls the underlying function _ykpiv_transfer_data(), which transfers APDUs to the YubiKey PIV applet over the CCID interface.

The function _ykpiv_transfer_data() takes the input parameter templ, which is populated with the APDUs (CLA, INS, P1, P2) that are specified at the Yubico developer website for PIV extensions under the section GET METADATA. The YubiKey returns the tag length values (TLVs) (Algorithm, Policy, Origin, etc.) that are specified in the same PIV extensions section, and the TLV-encoded output is returned in the ykpiv_get_metadata() parameter data.

TLVs Returned
Key	TLV	Description
Algorithm	0x01	Algorithm/type of the key
Policy	0x02	PIN and Touch policy of the key (keys only)
Origin	0x03	Origin of the key: imported or generated
Public key	0x04	Public key associated with the private key
Default value	0x05	Whether the PIN/key has a default value (PIN and PUK and Mgmt key only)
Retries	0x06	Number of retries left (PIN and PUK only)

It is even possible to invoke the function ykpiv_transfer_data() directly for low-level APDU communication with the YubiKey’s PIV applet.

The function ykpiv_util_parse_metadata() can be used for parsing the returned TLV-encoded object.

Therefore, the developer can integrate the libykpiv library for low level programming with YubiKey PIV metadata.

Using PIV Metadata with YKCS11

The YKCS11 library is part of the Yubico PIV Tool. YKCS11 is a PKCS#11 module that allows external applications to communicate with the PIV application running on a YubiKey.

When the PKCS #11 function C_OpenSession() is called for a YubiKey PKCS #11 slot (which is a YubiKey PIV application in a PC/SC reader), then the YKCS11 library parses out the public keys for all PIV key slots. If PIV attestation is supported, the PIV attestation certificate is used for parsing out the public key.

If PIV attestation is not supported, that is, if the key-pair has been imported into a YubiKey, then the YKCS11 library calls the functions ykpiv_get_metadata() and ykpiv_util_parse_metadata() to parse out the requested public key.

If both attestation and PIV metadata fail, in that order, YKCS11 falls back to parse the public key from the X.509 certificate.

Note

The X.509 certificate’s public key may not match the private key in the YubiKey PIV slot.

Changes

Answer to Reset (ATR) and Answer to Select (ATS)

The ATR has been changed from Yubikey 4 to YubiKey and adds support for ATS.

PIV Attestation Root CA

YubiKeys 5 Series have a PIV attestation root certificate authority different than previous YubiKeys. Download the certificate of the new root certificate authority on the PIV attestation page.

Easier Identification

The YubiKey 5 Series devices can report their form factor through the PIV application whether or not they have an NFC interface. This enables easier, programmatic identification of the physical attributes of the YubiKey. For more information about how to query this information, see the YubiKey 5 Series Configuration Reference Guide.

PIV AES Management Key

Historically, the YubiKey PIV management key is a 3DES key. With the release of the YubiKey firmware version 5.4.2, the YubiKey PIV management key can also be an AES key. For more details, see the article on our Developer site, YubiKey and PIV.

Technically speaking, this feature expands the management key type held in PIV slot 9b to include AES keys (128, 192 and 256) as defined in the PIV specification (SP800-78-4, section 5). PIV management key in AES format renders the YubiKey compatible with current or future FIPS-compliant CMS services.

YubiHSM Auth

Introduction

YubiHSM Auth is a YubiKey CCID application that stores the long-lived credentials used to establish secure sessions to a YubiHSM 2. The secure session protocol is based on Secure Channel Protocol 3 (SCP03), see Yubico Secure Channel Technical Description. YubiHSM Auth is supported by YubiKey firmware version 5.4.3.

YubiHSM Auth uses hardware to protect the long-lived credentials for accessing a YubiHSM 2. This increases the security of the authentication credentials, as compared to the authentication solution for the YubiHSM 2 based on software credentials derived from the Password-Based Key Derivation Function 2 (PBKDF2) algorithm with a password as input.

Credentials and PIN Codes

Each YubiHSM Auth credential is comprised of two AES-128 keys which are used to derive the three session-specific AES-128 keys. The YubiHSM Auth application can store up to 32 YubiHSM Auth credentials in the YubiKey.

Each YubiHSM Auth credential is protected by a 16-byte user access code provided to the YubiKey for each YubiHSM Auth operation. The access code is used to access the YubiHSM Auth Credential to derive the session-specific AES-128 keys.

Storing or deleting YubiHSM Auth credentials requires a separate 16-byte admin access code.

Each access code has a limit of eight retries and optionally, verification of user presence (touch).

YubiHSM 2 Secure Channel

Use the YubiKey YubiHSM Auth application to establish an encrypted and authenticated session to a YubiHSM 2. Although the YubiHSM 2 secure channel is based on the protocol Global Platform Secure Channel Protocol ‘03’ (SCP03), there are two important differences:

The YubiHSM 2 secure channel protocol does not use APDUs, so the commands and possible options are not those of the complete SCP03 specification.
SCP03 uses key sets with three long-lived AES keys, while the YubiHSM 2 secure channel uses key sets with two long-lived AES keys.

The YubiHSM 2 authentication protocol uses a set of static credentials called a long-lived key set. This consists of two AES-128 keys:

ENC: Used for deriving keys for command and response encryption, as specified in SCP03.
MAC: Used for deriving keys for command and response authentication, as specified in SCP03.

The identical long-lived keyset is protected in the YubiHSM 2 and in the YubiKey YubiHSM Auth application.

Those long-lived key sets are used by the YubiHSM Auth application to derive a set of three session-specific AES-128 keys using the challenge-response protocol as defined in SCP03:

Session Secure Channel Encryption Key (S-ENC): Used for data confidentiality.
Secure Channel Message Authentication Code Key for Command (S-MAC): Used for data and protocol integrity.
Secure Channel Message Authentication Code Key for Response (S-RMAC): Used for data and protocol integrity.

The YubiHSM Auth session-specific keys are output from the YubiKey to the calling library, which uses the session keys to encrypt and authenticate commands and responses during a single session. The session keys are discarded afterwards.

Architecture Overview

The figure below shows how the YubiHSM Auth application fits in to the YubiHSM 2 architecture.

The identical long-lived credentials (key sets) are protected in both the YubiKey YubiHSM Auth application and in the YubiHSM 2. The YubiHSM-Shell software tool can be used for generating the key sets in the YubiHSM 2, and the YubiHSM-Auth software tool can be used for importing the same key sets to the YubiKey YubiHSM Auth application.

At the client, the YubiHSM authentication protocol is implemented in the libykhsmauth library, which derives the three session AES-keys by calling the YubiKey YubiHSM Auth CCID application. The session objects that are created can be used by the libyubihsm in the communication with YubiHSM.

The YubiHSM session keys are therefore generated on the basis of the long-lived credentials that are protected in the YubiHSM 2 and YubiKey YubiHSM Auth in conjunction with the SCP03 derivation scheme.

YubiHSM Auth Flowchart

The flowchart below illustrates the authentication protocol communication with YubiHSM using the static keys on YubiHSM Auth. It is assumed that the YubiHSM and YubiHSM Auth application share the same static keyset. The steps are explained below.

The user launches YubiHSM-Shell and enters the commands connect and session open, with the flag ykopen that indicates that the YubiKey with YubiHSM Auth shall be used.
The YubiHSM-Shell invokes the libyubihsm library, with a request to open a session to the YubiHSM 2.
The libyubihsm library generates a host challenge, and opens a session to the YubiHSM 2 device.
The YubiHSM 2 device generates an HSM challenge, and generates the session keys based on the HSM challenge, the host challenge, and the static key set in the YubiHSM 2 device. The YubiHSM 2 returns the HSM challenge in an HSM response to the libyubihsm library.
The libyubihsm library propagates the host challenge and HSM challenge to the YubiHSM Shell.
The user enters the Credential password for unlocking the static keyset in the YubiHSM Auth application in the YubiKey. The YubiHSM Shell invokes the libykhsmauth library, with a request to generate session keys.
The libykhsmauth library invokes the YubiHSM Auth application in the YubiKey with the Credential password, the HSM challenge and host challenge are used as input parameters.
The Credential password unlocks the static keyset in the YubiHSM Auth application, and the YubiHSM Auth application generates the session keys based on the static keys, HSM challenge, and host challenge.
The libykhsmauth library returns the session keys to YubiHSM Shell.
The YubiHSM Shell acknowledges the protocol handshake to libyubihsm.
The libyubihsm sends the host response to the YubiHSM 2 device. The session keys can now be used for secure channel communication between YubiHSM-Shell/libyubihsm in the host and the YubiHSM device.

Software and Tools

YubiHSM-Auth Software Tool

The YubiHSM-Auth software tool is part of the YubiHSM Shell, which is installed with the YubiHSM SDK. YubiHSM-Auth tool can be used for:

Storing the YubiHSM Auth credentials on a YubiKey
Deleting the YubiHSM Auth credentials on a YubiKey
Listing the YubiHSM Auth credentials on a YubiKey
Changing the YubiHSM Auth management key on a YubiKey
Checking the number of retries of the YubiHSM Auth credential password
Checking the version of the YubiHSM Auth application
Calculating session keys, mainly for debugging and test purposes
Resetting the YubiHSM Auth application on a YubiKey

First, the YubiHSM 2 device needs to be configured with an authentication key. The default authentication key password on KeyID=1 is set to password, and this should be changed or replaced with other authentication keys. For the examples in this section, however, it is assumed that the default authentication key is still present on the YubiHSM 2.

To generate and store the equivalent YubiHSM Auth credentials on the YubiKey, use the yubihsm-auth command line tool. To invoke YubiHSM-Auth, simply run yubihsm-auth with the required commands and parameters.

To get a list of available commands, parameters and their syntax, run: yubihsm-auth --help.

An example of how to use yubihsm-auth for storing YubiHSM Auth credentials on a YubiKey is shown below:

$ yubihsm-auth -a put --label="default key" --derivation-password="password" --credpwd="MyPassword" --touch=on --mgmkey="00000000000000000000000000000000" --verbose=5
Credential successfully stored

Where:

-a put is the action to insert a YubiHSM Auth credential on the YubiKey
--label is the label of the YubiHSM Auth credential on the YubiKey
--derivation-password is used as input to the PBKDF2 algorithm, which is used for generating the two AES-128 keys that constitute the YubiHSM Auth credentials to be stored on the YubiKey
--credpwd is the password protecting the YubiHSM Auth credentials on the YubiKey
--touch is set to on. This requires the user touch the YubiKey when accessing the YubiHSM Auth credential
--mgmkey is the management key that is needed for writing the YubiHSM Auth credentials on the YubiKey
--verbose is used to print more information as output

Note

We recommend using an offline air-gapped computer when storing the YubiHSM Auth credentials on the YubiKey.

Now, the YubiKey YubiHSM Auth application can be used with YubiHSM Shell for authentication to the YubiHSM 2.

Using YubiHSM-Auth with YubiHSM Shell

It is possible to authenticate to the YubiHSM 2 device with static credentials that are protected in the YubiKey application called YubiHSM Auth. For more information on this YubiKey feature and how to configure it, see the YubiHSM User Guide, section YubiHSM Auth.

The YubiHSM Shell tool supports authentication with YubiHSM Auth credentials in both interactive mode and command-line mode.

To use yubihsm-shell with the YubiHSM Auth-enabled YubiKey in interactive mode, open a session by executing the following yubihsm-shell command:

yubihsm> session ykopen <authkey> <label> <password>

where, in the context of using YubiHSM-Shell with the YubiHSM Auth application, the following parameters are used:

authkey is the identifier of the authentication key in the YubiHSM 2
label is the label of the YubiHSM-Auth credentials stored in the YubiKey
password is the password that protects the YubiHSM-Auth credentials stored in the YubiKey.

Below is an example of an interactive command with YubiHSM Shell:

yubihsm> session ykopen 1 "default key" "MyPassword"
trying to connect to reader 'Yubico YubiKey OTP+FIDO+CCID 0'
Created session 0

To use yubihsm-shell with YubiHSM Auth in command-line mode, add the parameter --ykhsmauth-label that implicitly invokes the YubiHSM Auth application at the YubiKey. Below is an example of how to use YubiHSM Shell in command-line mode:

$ yubihsm-shell --ykhsmauth-label "default key" -p "MyPassword" -a generate-asymmetric -A rsa2048 -i 11 -c sign-pss -l Signature_Key

If the YubiKey is configured to require touch when accessing the YubiHSM-Auth credentials, the user needs to touch the YubiKey sensor in addition to entering the credential password.

Once the user is authenticated with YubiHSM Auth, all YubiHSM-Shell commands can be used.

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